the nutrient cycle: closing the loop

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the nutrient cycle: closing the loopEdited by Hannah Hislop

Green Alliance is an independent charity. For 28 years we have worked with businesses and otherenvironmental charities to make environmental solutions a priority in British politics. We work withrepresentatives of all three of the main political parties to encourage new ideas, facilitate dialogue and securenew commitments to action and progress on the environment.

Designed and printed by Seacourt www.seacourt.net on paper with at least 75 per cent recycled content.© Green Alliance 2007 £5ISBN 978-1-905869-03-9

Green Alliance36 Buckingham Palace Road, SW1W 0REtel: 020 7233 7433 fax: 020 7233 9033email: ga@green-alliance.org.ukwebsite: www.green-alliance.org.uk

Green Alliance is a registered charity number 1045395Company Limited by guarantee, registered number 3037633

Our thanks go to the Association of Rivers Trusts, the Environment Agency, Natural England, NorthumbrianWater, the Pennon Group,Thames Water and Water UK for their support of this project. Thanks also to CliveBates, Bob Breach, Jim Densham, Jane James and Helen Wakeham for their advice.The recommendationspresented in this report are put forward by Green Alliance and do not necessarily represent the position of allproject partners.

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1contents

foreword _02Ian Christie, Green Alliance Associate

the nutrient cycle: closing the loop _04Jiggy Lloyd, Green Alliance Associate

the water industry’s role in today’s nutrient cycle _13Dr Stephen Bolt, ADAS

recycling biosolids to land _19Dr Stephen R Smith, Imperial College London

implications of farm management on the nutrients cycle _26Professor Mark Kibblewhite, Cranfield University

the role of composting in closing the nutrients loop _31Dr Jane Gilbert, the Composting Association

too much of a good thing? the impacts of nutrients on birds _35and other biodiversityMA MacDonald, JM Densham, R Davis and S Armstrong-Brown, RSPB

energy and cost implications of today’s nutrient cycle _39Rob Lillywhite, Warwick HRI, Warwick University

notes and references _43

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Many of the environmental problems now seen ascontributing to unsustainable human developmenthave this in common: they involve a relatively rapidand large disturbance to the huge geochemicalcycles that help regulate the biosphere. Humansources of carbon dioxide and other greenhousegases may not seem significant in the context of thewhole Earth system or of geological time; but theyhave been sending a large enough volume ofemissions in a relatively short period into theatmosphere such that the overall natural balance ofthe carbon cycle is being disrupted.

The potential dangers fromclimate change as a result ofthis recent disruption of thecarbon cycle are nowdominating political debate.There is a risk, given thecomplexity and scale of the

climate challenge, that another cycle of greatsignificance will be neglected.This is the nutrientcycle – made up of two great geochemical fluxes,those of nitrogen and phosphorus.The essays in thisbooklet explore the scale of the disruption thathuman development has unwittingly brought tothese cycles. As with the carbon cycle, we haveoverloaded natural systems in a short span of time aswe have industrialised, and in particular as we havedeveloped and applied synthetic fertilisers. Again, aswith the carbon cycle, our injection of vastquantities of nitrogen and phosphorus compounds

into the environment has brought immense short-term gains in prosperity and output. But the pricecould be long-term degradation of essential naturallife-support systems – adding to the greenhouseeffect, damaging water quality, reducing soil quality,and soaking up resources in end-of-pipe pollutioncontrol and damage limitation.

The nitrogen and phosphorus cycles are immenselycomplex and ‘leaky’, as made clear in the essays thatfollow.The policy challenges are also complex,involving many established commercial interests,dependence on manufactured fertilisers, and theneed for a radical shift in nutrient use so that we‘close the loop’ and recycle as many nutrients aspossible.

There is another important cycle to consider whenwe address these policy challenges.That is thepolitical attention cycle.The fabled ‘top of thepolitical agenda’ is always a crowded place, andenvironmental concerns cannot be guaranteed toremain there, with the exception (one hopes) ofclimate action.The risk is that climate policy comesto dominate debate and action at the expense ofother, related challenges. Nutrients policy isvulnerable.The nutrient cycle demands action on theleast glamorous and electorally appealing aspects ofenvironmental policy – soil management, farmingsystems, the treatment of sewage sludge and so on.The nutrient cycle is not getting the attention itdeserves.

forewordIan Christie, Green Alliance Associate

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“ the nutrient cycle demandsaction on the leastglamorous and electorallyappealing aspects ofenvironmental policy”

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3The answer may be to see the disruption of thenitrogen and phosphorus cycles for what they are:closely connected to the problem of the carbon cyclethat is now rightly commanding attention fromgovernments, civil society and business worldwide.For many of the current and projected problemsfrom climate disruption – over-heating, droughts,flash flooding and erosion – will interact with thethreats that nutrient overload poses for water qualityand soils. We need to see the nutrients issue as asubset of a much bigger general problem – ourunwitting overloading of fundamental natural cyclesand life-support systems – and as a cluster ofproblems that could be exacerbated severely byclimate change. Dealing with the climate threat andthe nutrients problem calls for a common approachrooted in closed loop management of energy, waterand nutrients, bringing emissions back into balance,and achieving the great gains in resource efficiencythat we know we can make.

So it will be important to make thenecessary connections betweenstrategies for climate action andmeasures to close the nutrients loop,many of which are discussed in theessays below. Another significantopportunity to raise the profile of thenutrients cycle is offered by emergingdebates on land use and the future of farming. Inthis respect two developments in 2007 are worthnoting, both involving the Environment Secretary,David Miliband. First, there is his call for theagriculture sector to take seriously the concept of‘One Planet Farming’, drawing on the growinginterest in the ‘One Planet Living’ concept recentlypromoted by WWF and BioRegional. Second, there

is a wide-ranging Defra review in hand on land use,and David Miliband has called for a national debateon a sustainable vision for land use over the longterm – in this case the next 80 years.

Finally, the emerging debate about the mix ofdemands on farming – what the balance should bebetween food production, management of landscapeand wildlife, and production of bio-fuels – providesanother opportunity to give the nutrient cycle theprominence it deserves. As with the carbon cycle, therealisation that we have managed to disrupt one ofthe great geochemical cycles should prompt urgentrethinking of policies, technologies and attitudes,and open up huge opportunities for innovation aswe try to restore the balance we have lost.

“ we need to see thenutrients issue as asubset of a muchbigger generalproblem”

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“We are fertilising the Earth on a global scale and ina largely uncontrolled experiment”, GlobalEnvironment Outlook, UNEP, 1999

Society owes a lot to the nutrient cycle. As studentsof biology and geography know, virtually everyaspect of the modern economy is dependent on thefact that nutrients such as nitrogen and phosphoruscirculate in different forms through water, air andthe soil.

Depicted neatly in the typical school textbookdiagram, the N-cycle and the P-cycle look almost toogood to be true.They are ‘true’ – even if, as theexpert contributions to this pamphlet remind us,they are more complex than the textbooks canconvey. And they are certainly ‘good’, in the sensethat life could not function without them and the

modern economy relies on them. Butnutrients are also a source of pollution andthe dangers of nutrient overload arerecognised. So the question has to be asked –is the operation of the nutrient cycle as goodas it could be?

The nutrient cycles of the modern world arevery different from those that existed before

the joys of burning fossil fuels were discovered,before (in 1912) Haber and Bosch found they couldsynthesise nitrogen fertiliser and before phosphatewas found to be beneficial to crop growth andimportant in the manufacture of detergents and

other household products. Nutrients still cyclethrough the economy but the amount in circulationis greatly increased. In the case of nitrogen, it isestimated that human activity has doubled theamount in circulation; in the case of phosphorus, wehave tripled the amount available since the industrialrevolution.

This might not of itself be a concern but it needs tobe considered alongside the indications thatnutrients are also, on some occasions, turning up inthe wrong place.

A water industry spends significant amounts ofmoney removing phosphates from wastewater.Theredoes not appear to be a definitive figure but someestimate it to be around £35 million a year.Furthermore, between 2005 and 2010, Englishwater companies will have to incur capitalexpenditure of about £300 million and annualoperating costs of £6 million to reduce nitrate levelsfor the public drinking water supply. Removing thephosphates is an energy-intensive process and alsorelies (in most cases) on chemical dosing with ferricsalts. The disposal of the nitrate-rich residues ofdrinking water treatment is a challenge that theindustry currently addresses by blending with lownitrate sources or removal via ultra filtration.

This indicates a problem with nutrients when wateris being treated for human consumption and use.What about nutrients at other stages in the cycle?

The nutrient cycle: closing the loopJiggy Lloyd, Green Alliance Associate

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“ human activityhas doubledthe amount ofnitrogen incirculation”

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The Environment Agency tells us that diffuse inputs ofnutrients are a major reason why our rivers are at riskof not meeting the Good Ecological Status (GES)required by the EU Water Framework Directive by2015. Specifically, nitrogen is cited as the problem forabout 40 per cent of rivers and phosphorus for nearlyhalf of them.

Nutrient pollution damages the countryside byaltering plant growth rates, changing plantcommunities and disrupting the food chain forwildlife. Nutrient inputs also trigger eutrophication ofshallow lakes; declines in fish diversity may be tracedback to this.

About 40 per cent of UK emissions of nitrous oxide(a potent greenhouse gas) come from agriculture;emissions are closely related to the form and themanner in which nutrients are applied.

Meanwhile, we continue to add to the amount ofnutrients in circulation and this has its ownenvironmental implications. Fertiliser manufacture forthe UK is believed to generate about six milliontonnes of carbon dioxide per year; some estimatessuggest it may be 20 million tonnes or more. Worldsupplies of phosphate rock are finite and the transportof phosphate fertiliser takes place over long distances.

Something seems not quite right here. On the onehand, nutrients circulate through the economy and theenvironment and the total amount in circulation issufficient to meet our needs.The nutrient content ofavailable organic materials such as biosolids, greenwaste, biodegradeable municipal wastes and farmyardmanures is sufficient to replace all use ofmanufactured fertilisers. On the other, we are

expending both money and energy on manufacturingfertiliser, and then further money and energy becausewe have to deal with nutrients in the wrong place.

It doesn’t sound logical does it?

But of course logic is not the only factor. Nature andhistory come into the story.

Nature dictates that nutrients do not cycle in perfect,leak-proof pathways.The miracle of nitrogen, inparticular, is its ability to exist in a range of differentforms. It is society’s bad luck that the form in which itis most available to the plants, nitrate, is also the formin which it most prone to move to other media –such as water. Phosphates are slightly less fickle – theyhave a happier tendency to remain bound to mineralsoil surfaces and incorporated into the molecularstructure of soil organic matter. But they move too –and end up in the wrong places as a result. From theperspective of the farming industry, the losses ofnitrogen and phosphorus that do occur (measured inenergy or in money) do not look bad in comparisonwith the value of the end product; they can becompared quite favourably with efficiency ratios inother industrial processes (which do not have theadded challenge of taking place outdoors, at the mercyof the weather!). And since the chief domestic use ofphosphates is in detergents, one might be tempted tosay that inevitably, the phosphates will end up inwater. Nevertheless, we can’t ignore the consequences.

History has got us to where we are today. Our abilityto add to the pre-1912 stock of nitrogen in circulationhas been the key factor underpinning the notablesuccesses of UK agriculture, and in feeding the world’sgrowing population.

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So, few would dare argue that we reduce the amountof nutrients in circulation to their pre-industriallevels; to do so would require an accompanyingproposition as to how world food production wouldbe maintained. And it would take a real optimist toconceive of a situation in which no fluxes withinand between media happened and supply alwaysexactly matched demand, year in year out, regardlessof the nuances of the weather, the soil and themarket. Even the most ardent advocates of precisionagriculture don’t claim we can be this clever.

But there are clearly significant benefits to berealised if we can reduce the economic andenvironmental costs associated with nutrients in thewrong place and reduce the need to manufactureand transport additional nutrients in favour of usingmore effectively those that are already in circulation.

The immediate benefits would be a reductionin greenhouse gas emissions – both carbonand nitrous oxide, improved water qualitywith benefits for biodiversity, and lessexpenditure of farmers’ money (onfertilisers) and water customers’ money (ondrinking water treatment).

But there is a longer-term objective here aswell. The elephant in the room (or hidingbetween the pages of this pamphlet) is theconcern about the global implications of

nutrients. Some argue that the global nitrogen cyclehas the potential to become as significant as theglobal carbon cycle. It is certainly justified to askourselves the following: if the developed world hasinflated the nutrient cycle this much so far, withthese sorts of side effects, how much more damage

is going to occur globally as food production andindustrialisation continue to increase?

So whether your perspective is domestic or global,the conclusion looks the same:

• We need to stop relying on policy measureswhich tackle nutrients as individual substancesconflicting with desired standards for soil, wateror air;

• And we need to start using a policy frameworkwhich aims, at its core, to cycle nutrientsthrough our economy with fewer unwantedeffects (‘leaks’) and an overall gain in resourceefficiency.

This would be the policy framework based onclosing the nutrient loop. It would have thefollowing principles:

1. The use of nutrients already in the economywould take priority over the manufacture andimport of additional nutrients.

2. Nutrients would be transferred between theplayers in the nutrient cycle – the producers andusers of nutrients – because of their value;regulations governing nutrient concentrations orprescribing nutrient management practices wouldbe secondary.

3. Soils (cultivated or otherwise) would beexplicitly recognised as part of the nationalnutrient resource bank.

4. Innovations which aided the recycling and re-useof nutrients and/or the maintenance of thenutrient resource bank would be encouraged andcould be supported using measures which

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“ some arguethat the globalnitrogen cyclehas thepotential tobecome assignificant asthe globalcarbon cycle”

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discouraged further additions to the nutrientcycle in excess of demand.

5. Information on nutrient stores and transfers(involving air, soil or water) would be used toestablish, at national level, the size of our nutrientsurplus and the options for limiting its growth.

This policy would be used to deliver the benefitsnoted above and also to ensure the UK is wellpositioned in a global debate about nutrients -which is well overdue.

Policies for closing the nutrient loopIf this framework were in place, what would need tobe done differently from, or additionally to, theplethora of nutrient-related measures already inplace?

First, there are some obvious enabling measures, asidentified by the expert pieces in this pamphlet andby the project partners/steering group.

The work already done by the water and foodsectors to devise a safe and workable system for theuse of biosolids on land (the Safe Sludge Matrix andrelated schemes) needs statutory underpinning.Thiswill ensure that necessary investments can be madeand, as importantly, financial commitments to less-sustainable options avoided.

We need to recognise the nutrient value of food andcomposting wastes and encourage the diversion ofthese wastes from landfill to beneficial use on land.A certification-use scheme, akin to the Safe SludgeMatrix, would enable the food industry and landmanagers to make the necessary commitments and

underpin the transactions involved. Similar measuresare needed to ensure that as we divert biodegradablemunicipal waste from landfills, we make it possibleto use the nutrients that it contains.

Such measures might seem to run counter to thesuggestion that available land might be insufficientto receive the nutrients that are potentially available.As the contributions to this pamphlet demonstrate,limitations do arise and they, coupled with the rate-response of crops to nitrogen fertilisers, have led tothe current situation. Enabling measures must tacklethese limitations.

For instance, a limitation on further use of biosolidson land is the mismatch between their availablenutrient content and crop demand. In part, this isdue to the effect of wastewater treatment in over-concentrating the phosphate load in relation tonitrates. A case might therefore be made for a levyon phosphates derived from non-dietary (detergentand industrial) sources, or a ban on some productscontaining phosphates. Discouraging their entry intowastewater and/or using levy proceeds to stimulatethe ‘mining’ of phosphorus from wastewater wouldbe a real example of closing the loop.

In some locations, a limitation on more effective useof animal wastes is the farm storage infrastructurethat enables the nitrates, as well as the organicnitrogen they contain, to be returned to the land atappropriate rates and timings. In others, diffuseinputs of agriculturally-derived phosphate may becausing a water quality problem and a levy onphosphate inputs to farming (operated at theregional or catchment-level) might be a means ofstimulating investment in nutrient recovery from

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farm waste. Nutrient storage and nutrient recoverycould be the starting point for a west-to-east transferof nutrients that would help close the loop withinthe UK farming sector.

The framework of a closed nutrient loop should alsoilluminate, from the perspective of a resourceefficient economy, two key debates. The first is thecase for and against a levy on nitrogen inputs to theeconomy.The contributions seem to suggest thatimproved agricultural knowledge and practice, withstrong cost-drivers in farming backed up by nitrateregulations and codes of good practice, are movingus in the direction we want to go; if there is a casefor using price to tip the balance further in favour oforganic sources of nitrogen, it may be argued thatenergy taxes affecting fertiliser manufacture arealready doing this. However, these lack the benefit ofrevenue-recycling which could be used to supportthe other measures already noted.

The second debate, related to the first, is how societyshould optimise the distribution of costs andbenefits. Given that a totally closed loop may not befeasible without a significant agricultural yieldreduction, some remaining costs will be internal toand paid for by, the production system; others willremain to be borne by society.

Improvements to feeding regimes in the intensivelivestock sector and access to precision farmingtechniques in intensive arable must feature in theclosing the loop framework. Both are central to theaim of reducing nutrient losses and improvingefficiency.

Policies based on closing the loop should alsounderpin our use of land management as a means ofachieving water quality objectives. If our aim is tomanage nutrients better for the long-term, then itmay well be the case that spending water customers’money on land management or land use adaptationschemes that will encourage nutrient storage in thesoil is at least as attractive an option as yet moreexpenditure on the energy-intensive treatment ofpoint sources. Such schemes also have the potentialto provide other valuable benefits, such as importantwildlife habitats. These arrangements must betransparent, however, and a careful distinctionmaintained between the most sustainable solution toan established problem, and arrangements thatmight imply a long-term acceptance of poorpractice.

But closing the nutrient loop is also about attitudesto nutrients and an appreciation of their place in asustainable economy.

There has been a commendable move to moreprecise use of nutrients in agriculture, especially inthe arable and horticultural sector.This has beenstimulated by the drive for efficiency andchallenging markets under the new farm supportregime. But it is not yet a universal approach and isassociated, in the main, with enterprises makingsubstantial use of manufactured fertilisers.

We need a more widespread and wholesale advocacyof the soil as a nutrient bank. Well-managed, withinputs not exceeding those that allow the system tofunction properly, it supports land-based ecosystemservices including food production and nutrient

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management. Abused or disregarded, it becomes partof the problem. Nutrient management needs tofeature more strongly in cross-compliance.There is astrong case for requiring nutrient management plans(i.e. the practice of ‘closing the loop’ at farm leveland if necessary planning for the management of thesurplus) on livestock units as part of cross-compliance.The enabling measures noted above areurgently needed to counter the caution about use oforganic wastes that has been found amongst farmers,buyers and consumers, but we also need strongermessages about the long-term merits of organic,locked-in storage of nutrients. Global warming hasstimulated an appreciation of the difference betweencarbon that is locked-up in soil, biomass and fossilfuels, and that which is causing the increased carbonconcentration in the atmosphere; it shouldn’t take asimilar crisis in nutrients to extend this appreciationto nitrogen and phosphorus.

Away from the landmanagement community,there is also scope to makenutrients a core concept insustainability, alongsidewater and energy. Can wecapture the current high

rate of ‘willingness to recycle’ with convincingmessages about the importance of food andmunicipal waste composting as a means ofconserving nutrients? Is this a means of counteringrecycling-fatigue (why do I bother? what is it for?)with strong messages that are not just about energybut nutrients as well?

A framework based on closing the nutrient loop willinevitably raise questions about risks. It is importantthat, for instance, the recycling of nutrients incomposting and municipal wastes is seen to be ascientific approach, not a throwback to ‘primitivepractices’. But the current tendency to be over-cautious about high profile and emotive risksobscures the equally significant, if different, risks ofnot acting sustainably. Closing the nutrient loopshould be presented with this in mind.

What is closing the loop?At its simplest level, the concept of closing the loop,as applied to waste and resources, expresses a desireto move away from a linear process of resourceextraction, manufacture, consumption and disposal,towards a cyclical system where resources remain inuse almost indefinitely. In global terms, andparticularly in ‘developed’ countries, a relativelysmall proportion of resources are cycled, or goround in loops, and a large proportion are inproductive use for a very short time.

This is unsustainable because it means: depletion ofnon-renewable resources (minerals and metals);degradation of habitats, landscapes and biodiversity;dangerous over-use of what should be renewableresources and over-use of the environmental ‘sinks’needed to absorb the pollution we generate as weproduce and consume. Circulating resources aroundthe economy, rather than just passing them throughonce, reduces the environmental impacts associatedwith extraction of resources and disposal of waste.

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“ abused ordisregarded,soil becomespart of theproblem”

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Nitrogen gas in air

Lightning

Denitrifyingbacteria or

fungi

Fixed by bacteriain root nodules of

leguminous plants or soil

AmmoniaNitrates

Nitrifyingbacteria Nitrites

Plants

Animals

Decay and animal waste

Nitrifyingbacteria

Nitrogen gas in air

NOx from burning fossil fuels

AmmoniaNitrates

Nitrites

Plants

Animals

Decay and animal waste

Run-off, leaching, eutrophication

Acid rain Aerosols, ground and ozone, smog

Globalwarming

N2O (nitrous oxide) & NOx

The natural nitrogen cycle

The nitrogen cycle and its human influences and effects

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Humancommunities

Humanwastes

Treatedwastewater

Biosolids

Fertiliser

Animalhusbandry

Decomposing microbes

Food chains

Animal wastesOrganic phosphate in

plant biomass

Phosphate in soil

Phosphate in water and sediments

millions of years

Sedimentation formation ofposphate rock

Erosion Mining ofphosphate rock

Food

Agriculture

P in chemicalse.g. detergents

The natural phosphorus cycle and its human influences (shaded)

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Industry: effluents Other diffuse sourcesHouseholds:human waste

and householdproducts

Public water supply

Sewage effluent Biosolids LandOther diffusesources

Further N&Premoval

Combustionprocesses

About 30% of N entering water is from STW effluent,about 70% N is from agricultureAbout 65% of P entering water is from STW effluent,about 25% P is from agriculture

Other diffuse sources also contribute

Water (surface and groundwater)About 75% of new N entering the atmosphere is from agriculture

About 25% of new N entering the atmosphere is in the form of NOx from combustion of fossil fuels

Air

About 90% of N entering STWs is from households, about 10% N is from industryAbout 85% of P entering STWs is from households, about 15% P is from industry

Other diffuse sources also contribute

Sewage Treatment Works (STWs)

Nitrogen and phosphorus through the environment in the UK

This diagram is provided to summarise and illustrate information inthis pamphlet. All figures are broad estimates and illustrative of theUK as a whole. In practice, there are significant variations dependingon time and location.

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The water cycle is perhaps something we have takenfor granted for too long in the UK.This is changingas the frequency of weather extremes seems toincrease which, combined with increasing demandsfrom our affluent and growing population, placesmore and more pressure on our water resources.

The water industry has a fundamental and invaluablerole in today’s society: abstracting, treating andsupplying water to customers, and recovering,treating and returning to the environmentwastewater and trade effluent. However, the watercompanies do not manage the water cycle, but tapinto it in order to provide an essential service to thepopulation. In carrying out these essential duties,there is potential to impact on virtually all aspects ofthe water cycle, which can affect both the qualityand the quantity of this valuable resource.

Threats to water qualityWater company activity interacts with water qualityin two separate ways. Firstly, surface water quality isinfluenced by the quality of point source dischargesfrom sewage treatment works (STWs). Watercompanies also take ‘raw’ water into their drinkingwater treatment plants: the cleaner the receivingwater, the less the need for expensive treatment.

Annual monitoring data from the EnvironmentAgency shows that the quality of our waters isimproving year on year. However, this doesn’t quite

square with a recent assessment, also by theEnvironment Agency, that most of the waters inEngland and Wales are at risk of contamination frompollution. Furthermore, companies are having toinvest increasing amounts of our money todecontaminate drinking water from pesticides andnitrates. In short, the UK and its water companieshave a challenge on their hands.

Pollution by nitrogen and phosphorus is known toimpact significantly on the ecology of water bodiesin the UK and elsewhere.The Environment Agencyhas identified that 38 per cent of rivers are at risk orprobably at risk from diffuse active compounds ofnitrogen (N)1 and 47 per cent from phosphorus(P).2 The risk maps do not distinguish between Nand P for point sources, but identify that 18 per centof rivers are at risk, or probably at risk, from pointnutrient sources. It is estimated that approximately60 per cent of river lengths contain concentrationsgreater than 0.1mg per litre P and 30 per centconcentrations greater than 30mg per litre N.3 Ingeneral, it is recognised that P is the limitingnutrient (and hence most ‘active’) inland with Nbeing the more important in estuaries and the sea.However, this is almost certainly anoversimplification and increasingly it is recognisedthat the N:P ratio has an impact on the biologicalresponse of an aquatic system.

The cost of increasing N removal from sewage worksbeyond the quantity already removed is potentially

the water industry’s role in today’s nutrient cycleDr Stephen Bolt

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high, particularly where low-energy percolatingfilters are currently used and would have to bereplaced with high-energy solutions. Similarly, Preduction is commonly carried out by chemicaldosing with ferric or aluminium salts which havetraces of hazardous substances.The less well-developed use of biological P removal hasoperational difficulties, and like N removal alsoincreases energy consumption.

Removal of N and P from sewage works musttherefore be carried out in conjunction witheffective diffuse pollution control. The worst-casescenario is that despite high dosing levels of ironand/or aluminium salts coupled with increasinglyenergy hungry processes, levels of phosphorus maystill not be low enough in the water to result in abiological response. In other words, all the dis-

benefits of an engineering solutionwith none of the ecological benefitsachieved. Clearly this unsustainableoutcome must be resisted.

Although not driven byenvironmental drivers, the DrinkingWater Inspectorate (DWI)requirement to achieve 50mg/l Nfor drinking water does haveincreasingly important financialimplications for the water

companies.The least costly (both in terms of energyand cost to customer) method is to blend highnitrate water with a low nitrate source so that theresultant drinking water passes the standard.However, in some areas low nitrate water isbecoming increasingly scarce and will inevitablyincrease the amount of expensive N removal from

the raw water source. In addition there is potentialfor disposal challenges with the resultant high Nwastewater.

The solution? An integrated approachNutrient pollution and associated problems insurface and groundwater are not new, nor is this aproblem restricted to the UK. Most of Europe hassimilar challenges, and there are many EU Directivesthat seek to tackle aspects of the problem. Forinstance, the Urban Waste Water Directive aims toidentify and protect sensitive waters suffering fromeutrophication, triggering significant investment bythe water industry in phosphorus removal at thelarger-scale sewage treatment works. OtherDirectives, such as the Habitats Directive, seek tolimit nutrients reaching the environment. Indeed, by2010, over half the inland population of East Anglia(an area particularly prone to eutrophication) will beserved by sewage works with active P removal.

The most recent EU Directive impacting on waterquality and water resources is the Water FrameworkDirective (WFD), which seeks to achieve goodecological and chemical status for all water bodiesby 2015. Clearly, key to this ambitious target will bethe successful control of nutrients reaching water.This will be achieved by setting Programmes ofMeasures (PoMs): actions that will need to beundertaken on a catchment scale to decreasepollution.

However, in order to meet this ambitious target, it isfirst important to understand sources of pollutantsand factors that affect their transport to water.Thefirst thing is to ‘apportion’ pollutants to sources, i.e.

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“ increasing control ofpoint source N and Pinputs will achievelittle unless diffusepollution issimultaneouslyaddressed”

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15be confident about from where they derive.Increasing control of point source N and P inputs,for example from sewage treatment works, willachieve little unless diffuse pollution issimultaneously addressed. It is therefore importantto first know the sources of N and P into acatchment or basin so that any programme ofmeasures has a realistic chance of achieving itstargets. Indeed previous standards, imposing fixedconcentrations on a sewage works depending on thesize of the discharge, were often criticised as failingto understand the problem.

Much work has been carried out in seeking toapportion N and P inputs to a catchment, and it isclear that there are large variations, both betweencatchments and at different times of the year. Forexample, sewage works are responsible for 62 percent of P to the Warwickshire Avon, with 30 per centfrom agriculture, whereas in the Ant catchment inthe Norfolk broads, only 18 per cent P is fromsewage works with 59 per cent from livestock and21 per cent from inorganic fertiliser.4 In addition,there are significant differences between summerand winter when river flows and agricultural run offare very different. Many rivers in the UK consist of ahigh proportion of treated sewage effluent duringdrought periods in summer.

To support important decisions that aim to protectour water resources, we therefore need a goodscientific understanding of catchment hydrology andpollution sources. It is clear that the situation isoften complex, which calls for an integratedapproach to a solution, tackling several pollutionsources and using a variety of approaches.Thisphilosophy moves away from what we have done to

date: clean up point sources at all costs. Whereas thisapproach has certainly brought improvementsrelatively quickly, tackling diffuse pollution is nowour major challenge.

Actions ‘on the ground’ -the catchment scaleIf the UK is to achieve the goals of the WFD, it willbe necessary to structure the programme ofmeasures to deal with nutrient transport at thecatchment level. Setting standards for individualsources is highly likely to fail to achieve anybiological response in the receiving watercourse.TheEnvironment Agency risk maps sought to identifycontrolled waters at risk or probably at risk of failingthe WFD standards.The maps distinguished betweenwaters at risk from point source and those at riskfrom diffuse source, however, critically, they did notseek to apportion the relative contribution betweensources.

There are, therefore, some crucial questions thatrequire answering if nutrient pollution control is tostand a chance of success and if the programme ofmeasures is to be based on sound science.

Firstly, a concentration limit for a nutrient in waterin order to achieve good ecological and chemicalstatus must be determined and, secondly, a nutrient‘budget’ for the catchment must then bedetermined.This then leads to a contentious issue -are these standards achievable without incurringexcessive cost? Sensibly, the WFD does allowconsideration of ‘disproportionate cost’, whichmeans where that the options to improve waterquality are very expensive, a derogation can be

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sought. Even more problematic is how excessive costshould be calculated - clearly it needs to include allthe three pillars of sustainable development -environmental, social and economic. If there areareas where derogations would need to be soughtfrom the European Union on grounds of excessivecost, then this will require robust evidence tosupport the case.This could be costly and timeconsuming and serious consideration needs to begiven now to how this could be achieved.

The alternative to basing the programme ofmeasures on sound science is to implement aniterative ‘best endeavours’ strategy.Thisapproach seeks to implement measuresthat are achievable with limitedresources. For instance, a catchmentofficer may seek to implement farmmanagement plans based only on whatcan be relatively easily andinexpensively achieved and not onwhat is needed to achieve WFDstandards. An example of this wouldbe to seek to implement a soilmanagement plan to reduce sedimentloading and associated P loss to the localwatercourse.This approach would probably result ina programme of measures with further reductions inP (and possibly) N standards on point sources,coupled with farm nutrient management plansseeking to reduce diffuse sources of nutrient. Thisintegrated approach is probably the least cost option,but could well be interpreted as planning to fail andcould be subject to third party challenge andultimately possible infraction proceedings by the EU.

Where next?From a water company perspective, it will be veryimportant that the issue of nutrient control is indeedbased on robust catchment based apportionment,since it may be attractive to the regulators to overcontrol point source N and P discharges tocompensate for lack of control of the diffuse inputs.Over control of point sources should be avoided onthree grounds: economic (poor value for money),social (water bills would need to increase withlimited or no value to the customer), andenvironmental (very stringent N and P limits wouldbe heavily energy dependent).

Given this risk, it is puzzling thatOfwat (now the Water ServicesRegulation Authority, the waterindustry economic regulator) tookthe view that the water industryshould not contribute to WFD-driven catchment managementschemes in the 2004 periodicreview since the water companieshad invested so much in point

source control. There is a strong argument that someinvolvement, including financial, in widespreadcatchment management to answer the questionsabove would be very much in water customers’ bestinterest and in the medium to long term wouldprovide high value for money. Although Ofwat didnot allow catchment management projects as part ofthe AMP4 programme (2005-2010), there is onenotable exception of United Utilities’ SCaMP(Sustainable Catchment Management Programme).SCaMP’s vision includes improvements in waterquality as well as a focus on bird populations and

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“ SCaMP’s visionincludesimprovements inwater quality as wellas a focus on birdpopulations andbiodiversity”

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17biodiversity.This was allowed partly since UnitedUtilities own some 57,000 hectares of land andtherefore have specific duties of care under theCountryside and Rights of Way Act (CROW). Inaddition, Ofwat recognised the strength of the RSPBlobbying between the draft and final determination.The programme is being developed in conjunctionwith the RSPB to develop an integrated approach tocatchment management.

Ofwat also recognised the advantages in allowingWessex Water to postpone AMP4 investment whilecatchment officers employed by the company seek toaddress specific nitrate and pesticide issues ingroundwater. Unfortunately, similar approaches putforward by other water companies were not allowedin Ofwat’s final determination. It is hoped thatOfwat will take a more favourable view of catchmentapproach projects in the next periodic review in2009. Several water companies are interested inexploring such approaches and are meeting todiscuss a possible united approach to the periodicreview negotiations.

There is some scope for reducing nutrient inputs tosewage works, for instance the introduction of low Pdetergents can significantly reduce P input to asewage works. However, the increase in dosing ratesof phosphorus compounds to reduce plumbo-solvency (lead solubility) to meet tighter leadstandards in drinking water may increase P loadingto sewage works by up to 10 per cent. Conversely,there are known technologies for ‘mining’ P fromsewage works and hence helping to recycle usable Pin the form of struvite (a white crystalline substanceconsisting of magnesium, ammonium andphosphorus). However, unless there were financial

incentives it is difficult to see what driver therewould be for the water industry to invest in thistechnology although it has clear environmentalattractions.

Once a nutrient budget for a catchment at risk iscalculated and targets set, then the programme ofmeasures needs to target sources of nutrient in anappropriate manner. If, as seems likely, diffusepollution control is required, there is the additionaldifficulty of who pays for the control and how it canbe effectively regulated. If the issue is agriculturalinput (although urban, transport and aerial must notbe underestimated), then the ‘polluter pays’ principlewould seek to increase the price of the produce inorder to fund changes in farming practice that maybe necessary. However, since produce procurementcompetes at a global level, it is unlikely that this verysimple link can be effective. If the matter of whoowns and ultimately pays for diffuse pollution is notaddressed effectively, this increases pressure oncontrol of point sources where full cost recovery ispossible from the water and other industry throughdischarge consents (and indirectly throughabstraction licences).

The importance of the nutrient cycle and its impacton our natural aquatic ecosystems and drinkingwater supplies cannot be over-emphasised. Aquantitative understanding as to sources within acatchment and whole catchment based solutions isneeded. Only then can we make decisions as towhether the targets of good ecological and chemicalstatus are achievable or whether derogations need tobe sought. It is becoming increasingly clear that inorder to control both diffuse and point sources in anappropriate and sustainable manner, there must be a

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fundamental increase in both the scale and pace ofprogress towards these goals. With the programmeof measures only three years away, there is a veryreal risk that a lack of overarching regulatoryownership of catchment control will lead to thefailure to implement the WFD in an appropriatemanner. More work is needed to look at ways toincentivise reductions in nutrient input to sewageworks and the potential for nutrient recovery beforedischarging to controlled waters. The continuedcontrol of point source nutrient inputs needs to beconsidered alongside diffuse pollution and acatchment approach to better controlling thenutrient cycle adopted.

Dr Stephen Bolt is head of integrated water and

environmental management, ADAS

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Recycling organic residuals on land to improve soilfor crop production was recognised by the earliestadvanced human societies as one of the keyprinciples of sustainable development. Closing thenutrient cycle by using sewage sludge on farmland iseffectively the modern equivalent of this basicprinciple and a cornerstone of sustainability.

Sewage sludge is an essential, and inevitable, residualby-product of urban wastewater treatment. It isproduced as a consequence of the removal of solidsfrom urban wastewater, discharged from homes andindustry and collected as drainage from pavedsurfaces, so that the treated effluent can be safelydischarged to the water environment.The mostimportant constituents requiring treatment arebiodegradable organic matter and reduced forms ofinorganic molecules naturally present in wastewatersuch as ammonia.These constituents originateprimarily from dietary sources and industrialprocesses, but also include a large range of organicchemicals, such as surfactants, used in the home orby industry.

Wastewater treatment – the basics Wastewater treatment typically has three principalstages that contribute to the production of sewagesludge.The first involves separating the solids thatare removed by a primary sedimentation process.This process produces an effluent, settled sewage,and a residue, primary sludge.The next stage

removes soluble and colloidal material from thesettled sewage by the ‘activated’ sludge process – themost common form of aerobic biological treatment.This step relies on the activities of a complexcommunity of microorganisms that oxidisebiodegradable constituents and nitrify inorganicammonia to nitrate. A third stage of treatment isbeing increasingly introduced to meet the tighteningdischarge consents to surface waters for thenutrients, nitrogen (which is mainly in the form ofnitrate with traces of ammonia) and phosphorus.These nutrient elements originate predominantlyfrom dietary sources, and in the case of phosphorus,from detergent residues (the relative phosphoruscontribution from dietary, detergent and industrialsources is equivalent to approximately: 60, 25 and15 per cent, respectively).

Nitrogen removal is a biological process, known asdenitrification, which reduces nitrates to nitrogengas that is released to the atmosphere. Phosphoruscan also be removed biologically, but is more widelyrecovered by using chemical dosing techniqueswith, for example, ferric chloride. Phosphorusremoved during wastewater treatment is transferredto the sewage sludge, thus significantly raising itsconcentration in sludge. Primary and secondarysludges are usually prethickened separately toincrease the dry solids content, and are thencombined together for subsequent treatment, by, forexample, anaerobic digestion or other processes, to

recycling biosolids to landDr Stephen R Smith

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generate a product that is suitable for recycling as afertiliser and soil conditioner on farmland.

The management optionsApproximately 1.4 million tonnes dry solids ofsewage sludge are produced annually in the UK.Themain outlets for managing the residual sludge fromwastewater treatment are shown in Figure 1.

The practice of dispersal at sea was ended by the EUUrban Wastewater Treatment Directive in 1998.Landfilling has never been regarded as a long-termsustainable option for sludge disposal by the UKwater industry, although it provides a short-term oremergency disposal route if necessary.The LandfillDirective also places restrictions on the disposal ofbiodegradable waste to landfill.

Figure 1:The main outlets for sewage sludge management in theUK for 2004 (values denote t x 1000 and per cent totalproduction of dry solids)

IncinerationIncineration has played a sizable role in sludgemanagement in the past and is likely to do so infuture.There are currently nine sewage sludge

incinerators operating in the UK, but it is amongstthe most expensive of the treatment options availablefor sludge with high capital and operating costs.Incineration is a treatment process that maximisesdry solids destruction, however, a significant amountof residual non-combustible ash material, typicallyequivalent to 35-40 per cent of the initial sludge drysolids, remains requiring final disposal, usually bylandfilling. In some cases the ash may be classified ashazardous waste potentially increasing disposalproblems and costs.

Obtaining sufficient solids content is critical forautothermic combustion of the sludge to take place,otherwise additional fuel is necessary to supportcombustion.This is particularly challenging forsludge due to its high initial moisture content(sludge initially contains 95–97 per cent water).With the most advanced technology available, theenergy balance of an efficiently run incinerator plantmay be self-sustaining without the need for anexternal fuel source, but there is little capacity togenerate additional power from the process. Therecovery of surplus energy could improve in futurewith the development of more energy efficientprocesses and effective sludge dewateringtechnology. For the time being, and until suitableuses for the residual ash are found, incinerationshould be considered as a disposal method, withlittle opportunity for value recovery. Incineration isjustified, and is currently the only viable optionavailable, when the concentrations of contaminantsin the sludge prohibit its use in agriculture or whenthe sewage treatment works is located within a largeconurbation and agricultural land is inaccessible.

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Farmland

Landfill

Incineration

Land restoration

Other

878, 64.2%15, 1.1%

265, 19.4%

150, 11.0%

60, 4.4%

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21Recycling biosolids to farmlandApplying sewage sludge to farmland is a long andwell-established practice in the UK and representsthe only management option that providessignificant benefit from recycling of the nutrientsand organic matter contained in sludge. Currently,about 64 per cent of the annual sludge productionin the UK is recycled to farmland (Figure 1). Allsludge is treated before it can be used on land.Mesophilic anaerobic digestion is the most commontype of conventional sludge treatment process,which also has the advantage of producingrenewable energy in the form of methane gas that isused for combined heat and power (CHP)generation. Heat is used for digester heating; theelectrical power demand for wastewater treatmentplant operation can usually be met and typicallythere may be a surplus of approximately 20 per centof the electrical power generated which is suppliedto the national grid. Currently, more than 50 percent of sludge produced in the UK is treated bymesophilic anaerobic digestion.

Land application of sludge does not represent anygreater risk to the environment than many othertypes of livestock manure or organic residual spreadon land: in its 19th Report on the Sustainable Use of Soil,the Royal Commission on Environmental Pollutionnoted that there was no evidence linking thecontrolled use of sludge on farmland to disease inthe human population.The practice receivesstatutory control to ensure any potential risks to theenvironment and human health are minimised.Agricultural recycling is regulated by EuropeanDirective 86/278/EEC, which was transposed intoUK law by the Sludge (Use In Agriculture)

Regulations 1989 (as amended). A Code of Practice forAgricultural Use of Sewage Sludge also supports thestatutory regulation.

Two important features of the controls include limitvalues for potentially toxic elements in sewagesludge amended soil and restrictions on the use ofland after application as a precaution to enable thenatural attenuation of pathogenic microorganisms.The Code also includes a list of effective sludgetreatment processes designed to significantly reducethe numbers of pathogens that may be present,providing an effective multi-barrier approach to theprevention of disease transmission. As a result, theUK government considers recycling sewage sludgein agriculture as the Best Practicable EnvironmentalOption (BPEO) for sludge management under mostcircumstances.

Sludge treatment is designed to reduce odour anddisease vectors, as well as the pathogen content.Odour is probably the single most important factorinfluencing negative publicperceptions about recycling sludge,and effective measures to controlodour are critical to successfulrecycling operations.The mechanismsresponsible for producing malodourfrom sludge are poorly understood,however, so this is an area requiringfurther research. Contaminants insludge are the other potential concern.Concentrations of potentially toxic elements, such aszinc and copper, and of persistent organic pollutantshave declined markedly in sludge over the past 30years with improved industrial practices, restrictionson emissions, production and use of dangerous

“ applying sewagesludge to farmlandis a long and well-established practicein the UK”

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substances within European Union, as well aseffective trade effluent control measures taken by thewater industry.

The most recent important development influencingland spreading of sludge is the voluntary agreementbetween the water utilities and food retail sector,known as the Safe Sludge Matrix, which definedmore specifically the acceptable uses of sludge ondifferent categories of crops.TheMatrix introduced a two-tier systemfor use on cropped land, accordingto the degree of treatment of thesludge. It ended the practice ofapplying untreated sludge on landused for food production inDecember 1999, and use for non-food, industrial crops in December2005.The categories of sludge thatare acceptable for land spreading aredefined as conventionally orenhanced treated and specific microbiologicalstandards apply to each category. Furthermore,sludge treatment processes are managed followingthe Hazard Assessment Critical Control Point system,introduced by the food industry, to ensure that themicrobiological quality standards are met. Sludgemay be applied to farmland as a liquid product,typically with a dry solids content of 2-5 per cent,but increasingly it is dewatered mechanically toreduce transportation costs. The dewatering process,by centrifuge or belt press, produces a stackable cakecontaining approximately 25 per cent dry solids.Treated sludge that is suitable for beneficial use onland is increasingly referred to as ‘biosolids’ todistinguish this as a product from untreated sludge.

The Safe Sludge Matrix was adopted by the UK waterindustry voluntarily in 1999 and Defra agreed toincorporate these measures in an amendment of thecurrent Statutory Instrument and Code of Practice.So far this undertaking has not been fulfilled: thefirst consultation on the proposals to amend thestatutory controls for the agricultural use of sludgeclosed in January 2003.The apparent lack ofprogress on this important issue has been a serious

constraint to the agriculturalrecycling of biosolids in someareas, and has the potential toseverely undermine confidence inthe practice within some sectors ofthe food industry that control theavailability of the land bank forapplying sludge. Concerns have alsobeen expressed by a number ofimportant sectors within the foodindustry that international marketsand brands could be damaged by

the use of sludge in the crop production cycle. Giventhat recycling on farmland is recognised as the BPEOby government, a positive and proactive stance byDefra, the Scottish Executive, the Food StandardsAgency, the Environment Agency, and ScottishEnvironmental Protection Agency is essential tounderpin confidence in agricultural recycling withinthe food and agricultural industries as a whole.Thiscould include supporting the Sustainable OrganicResources Partnership (SORP), a stakeholderpartnership that aims to provide a national forum forsharing and disseminating information on allorganic materials applied to land to encourage theuse of organic residuals on land as a long-termsustainable activity and resource.

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“ the apparent lack ofprogress on thisimportant issue hasbeen a seriousconstraint to theagricultural recyclingof biosolids”

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23At a national level, nutrients are returned to less than1 per cent of the agricultural land in biosolids,which is a comparatively small input relative to thatof livestock manures, for example. As the totalvolume of organic resources recycled to landincreases (including composted materials andindustrial biowastes, due to their diversion fromlandfill), there will be greater competition for theavailable landbank for spreading all organic materialsin future.The question ariseswhether, given the increasedamounts of wastes going to landand the restrictions on applicationrates in Nitrate Vulnerable Zones(NVZs), and other futureconstraints that may be required bythe WFD, there is sufficient landavailable to receive the volume oforganic material generated.To answer this, Defra iscurrently funding the development of a tool toquantify the national capacity of agricultural land toaccept organic residuals: Agricultural Land andOrganic Waste – A National Capacity Estimator(ALOWANCE), which will be critical in developingan integrated land-based approach for recyclingorganic residuals.

The role of biosolids in closing the loopThe total concentrations of nutrients in dewateredbiosolids are typically in the range of 4-5 per centfor nitrogen and 2-3 per cent for phosphorus.However, phosphorus removal during wastewatertreatment can more than double the concentration ofthis nutrient in sludge.The contribution of biosolidsnitrogen to agricultural systems is equivalent toapproximately 3 per cent of the total N applied to all

crops in Great Britain in mineral fertilisers.Phosphorus supplied in sludge, on the other hand,supplies a much larger contribution, equivalent toapproximately 15 per cent of the total phosphorus inmineral fertilisers - a significant quantity relative toartificial fertiliser. Together, the notional fertiliserreplacement value of the nutrients supplied inbiosolids is in the region of £20 million. Otherbeneficial nutrient elements supplied in sludge

include useful quantities of sulphurand magnesium.

A major economic incentive tofarmers for accepting sludge is thenitrogen replacement and yieldresponse value. Recent fieldinvestigations by Imperial CollegeLondon on a range of dewatered

biosolids showed that the nitrogen fertiliserreplacement value of biosolids is equivalent toapproximately 35-40 per cent of the total nitrogencontent of artificial fertiliser. This compares wellwith solid farmyard manure, for example, whichmay have a nitrogen availability of up to 25 per cent.

Spreading rates used in the UK are determined bythe nitrogen content of the sludge. Within NVZs, anaverage maximum farm-based limit on the totalamount of nitrogen that can be applied in all organicmanures (including sludge) to arable land is 170kgper hectare, in compliance with the EU NitratesDirective. For grassland the UK limit is 250kg perhectare.There is also a maximum field-based limit of250kg N per hectare in any 12 month period andrestrictions on timing applications of liquid digestedsludge to prevent spreading of this high nitrogenavailability product (nitrogen availability in this case

“ the notional fertiliserreplacement value ofthe nutrients suppliedin biosolids is in theregion of £20 million”

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is approximately 60 per cent, due to the largecontent of mineral nitrogen, and is similar tolivestock slurry) in the autumn period, when thereis greatest risk of nitrate leaching.The Code of GoodAgricultural Practice for the Protection of Water (TheWater Code) recommends a limit of 500kg N perhectare in alternate years for dewatered sludge typesoutside the NVZs in England and Wales, and 250kgN per hectare is allowed per year for liquid forms.The rates of sludge application are adjusted inaccordance with these limits on maximum nitrogenadditions. Consequently, the water industry adheresto strict controls on the amounts of nitrogen thatcan be applied in sludge, and the timing andfrequency of sludge applications within, as well asoutside, NVZs, to reduce potential impacts on waterresources of nitrate leaching from agriculturalutilisation.

In terms of the sustainability of the practice,recycling phosphorus in the food chain is arguablymore important than for nitrogen. Sludge provides along-term maintenance dressing for this nutrientand can replace phosphorus inputs from inorganicfertilisers in a crop rotation. Recycling phosphorusto soil in biosolids has a much wider global benefitthan simply reducing farmers’ costs, however, bycontributing to the conservation of geologicalmineral phosphorus reserves and reducing externalinputs of geogenic cadmium into the food chain.World reserves of currently exploitable phosphaterock are estimated to be 40 billion tonnes and, at thepresent rate of consumption (150 million tonnes peryear), this will be exhausted within 250 years.Cadmium inputs to soil from rock phosphatefertiliser production has also been a concern withlong-term implications for soil fertility and human

health. Recycling phosphorus in biosolids istherefore a key prerogative for long-termsustainability. Phosphorus recovery duringwastewater treatment is highly efficient and thesludge is an effective phosphorus fertiliser sourcethat closes the nutrient loop through the food chain,provided it is carefully managed. Under thesecircumstances, it may not be necessary to controlinputs of phosphorus to the wastewater collectionsystem. In any case, the largest input of phosphorusoriginates from dietary sources, which emphasisesthe important link apparent between recycling insludge and the food chain.

Much research has been completed to determine thefertiliser replacement value of nitrogen, and to alesser extent phosphorus, in sludge: the phosphorusavailability in sludge is typically given as 50 per centrelative to mineral fertiliser. This information isreadily available to operators and farmers in the formof advisory fertiliser recommendations, for examplethe Fertiliser Recommendations for Agricultural andHorticultural Crops (RB 209). Phosphorus inputs insludge exceed the demand of the first receivingcrop, however, recommendations are given on thebasis that a single dressing of dewatered biosolidswill supply adequate phosphate for most three tofour year crop rotations.This is justified on the basisthat phosphorus is retained in sludge-amended soil;recent research shows that phosphorus applied inthis form is bioavailable to crops, yet it is lesssusceptible to leaching and run-off compared toapplications in livestock manures. However,improved understanding of the release of residualphosphorus applied to soil in biosolids in thegrowing seasons after application is an arearequiring further attention to more precisely define

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25the long-term phosphorus balance in sludge-amended soil. As the UK moves towardsimplementing the EU Water Framework Directive,action is likely to be taken to reduce diffusepollution from agricultural sources, focussingparticularly on nitrogen and phosphorus. It isunclear at this stage what form these measures arelikely to take and what their potential impact onrecycling biosolids would be. Recognising that landvaries in terms of its potential risk of contributingnutrients to receiving waters may provide apragmatic approach to managing nutrient inputs tosoil in biosolids and avoid unacceptable losses to thewater environment.

Ensuring that full account is taken of the nutrientvalue provided by biosolids, as well as in otherorganic manures, by adjusting supplementarymineral fertiliser applications to amended soil iscritical to reduce wastage, impacts on crop qualityand losses of nutrients to the environment. Furtherprogress could be made in this area and would havedirect benefits in terms of improved nutrient use andreduced pollution as well as providing economicsavings in fertiliser costs.

Dr Stephen R. Smith is director of the centre for

environmental control and waste management,

department of civil and environmental engineering,

Imperial College London

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Farm businesses produce food to make profits.Nutrients are valuable resources that should bemanaged effectively to support profitableproduction; they are imported onto farms in animalfeed, organic manures, composts and waste materialsspread to land, as well as in chemical fertilisers.Unfortunately, agricultural systems use nutrientsinefficiently, particularly nitrogen and phosphorus,and ‘leakage’ to the wider environment iswidespread, substantial and, at some level, inevitable.

About half the nitrogen added to soil as fertiliser forarable crops and grass is either emitted to theatmosphere as ammonia, nitrous oxide or othergaseous forms, or finds its way into surface andground waters as nitrate.The conversion efficiencyof nitrogen in feedstuffs to animal protein is lessthan 10 per cent, and a significant proportion of thenitrogen in manures and slurries is emitted to theenvironment, rather than being incorporated intothe soil, taken up by plants and so eitherincorporated into food products or recycled in theagricultural system. Agriculture accounts for about60 per cent of UK emissions of nitrous oxide, a gaswhich is 296 times more potent as a greenhouse gasthan carbon dioxide. Some 90 per cent of UKammonia emissions to air are from agriculture.Nitrogen emitted to the atmosphere as ammoniareturns to land as wet and dry deposition,

sometimes causing damage via nutrient enrichmentto sensitive ecosystems, such as heath land, orcontributing to acid rain.The amount of nitrogendeposited to soil surfaces in England and Wales hasnot reduced materially over the past two decades.Large reduction in non-agricultural industrialemissions have lowered long-range nitrogen exportsto Scandinavia but agriculture and other sourcessuch as transport, have ensured that local depositionhas persisted.The quantities being deposited varygeographically but often exceed 20kg per hectareper year, which is biologically significant. Themajority of nitrate in UK surface waters is fromagriculture. Excess nitrate entering ground andsurface waters compromises natural ecosystems anddegrades water supplies. Nitrate nitrogen persists inmany aquifers for decades so their pollution isirreversible in the medium-term.The environmentaldamage from agriculture to the wider UK economyarising from these different nitrogen emissions to airand water – nutrient removal by the water industry,greenhouse gas emissions, and remediation ofdamaged aquatic and terrestrial habitats – isconsiderable.

About a quarter of the phosphate in UK surfacewaters is thought to be from agricultural sources,(although this can vary widely from less than 10 percent in the Thames to over 60 per cent in western

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26 implications of farm management on thenutrient cycle Professor Mark Kibblewhite

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27Wales).5 This is a consequence of inefficient uptakeby crops and grass of the phosphate present infertilisers, manures and slurries. The naturalavailability of phosphate limits the productivity ofaquatic ecosystems and their characteristicbiodiversities reflect this constraint; phosphatepollution from agriculture, as well as other sources,damages these ecosystems and in more severe casesleads to eutrophication - a condition where highnutrient availability causes excessive algal growth.

There is great variety and variability in the types andscales of agricultural production. A useful distinctionis between intensive and extensive systems. Intensivesystems maximise production and profits per hectarevia larger inputs, including fertiliser and animalfeed. Examples are cereal, oil seed and potatoproduction, dairy and cattle enterprises with higherstocking densities and large-scale pig and poultryproduction. In these systems, high nutrient inputsare used to maximise yields and so greater emissionsto water and air are likely. Extensive systemsoptimise profits where land is not suited to intensiveproduction by controlling inputs at maintenancelevels and relying more on natural nutrient cycles.Examples are sheep and beef cattle production in theuplands. In extensive systems, inputs are lower andthe risk to the environment from nutrient leakage isreduced. Nonetheless the environmental impact offarming remains significant in extensive systems,particularly because of nitrogen emissions fromanimal wastes and where soil erosion releasesphosphate-containing sediments into surface waters.In all cases the impact of farming systems is stronglyinfluenced by the effectiveness of management; apoorly managed extensive system can be as, or evenmore, polluting than a well managed intensive one.

Not surprisingly, a tension exists betweenagricultural businesses, who perceive that continueduse of higher nutrient inputs is a pre-condition forprofitable farming, and wider society, which ispaying for the environmental damage.The challengefor society is to encourage the adoption ofagricultural systems and management practices thatcan sustain food production while releasing fewernutrients.

The mid-term reform of the Common AgriculturalPolicy (CAP) is likely to encourage a permanent shiftto extensive land management. For example, withthe decoupling of production and support payments,it makes sense to put arable land down to grass,where cereal and other commodity crop yields arenot sufficient to fully recover production costs atmarket prices. However, the market prices can go upas well as down, influencing land managers’decision-making differently. At present, in the UK,marginal livestock enterprises appear to bewithdrawing from production or adopting moreextensive approaches. Such trends will tend toreduce the environmental burden of nutrients fromagriculture. But conversely land that continues to beused for commodity production can be expected tobe managed in larger and more intensive units tomaximise profits under unsupported marketconditions, which may present a greater risk to theenvironment if protective measures are insufficient.

Agricultural systems are open ones - they absorb andrelease energy and materials from and in to theenvironment. Agriculture requires natural resourcessuch as soil and water to support a profitable level offood production, but to be sustainable it must alsoonly return nutrients at levels which can be safely

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absorbed by these same natural resources. Overall,current emissions of nutrients exceed the capacity ofthe wider environment to absorbthem. Better information is needed onthe natural capacity of localenvironments to assimilate nutrientsand on the emissions from differentagricultural systems. And wherecapacity is exceeded, agriculture hasto change, either by reducingemissions through improvedefficiency, reducing production, orshifting to a different type of production.Thischange is unlikely to happen if left to market forcesbecause many environmental damage costs are notincluded in on-farm costs. The benefits of nutrientadditions accrue to the farmer while many damagecosts present beyond the farm gate and are metelsewhere, either directly as remediation costs orthrough loss of resource quality. And it is irrationalfor a farmer seeking to maximise individual farmprofit to forego profit for the benefit of the widercommunity. Nutrients such as nitrogen are relativelycheap and are a highly cost-effective means toimprove yields of arable crops and grass. In theabsence of regulatory controls, it pays to apply alittle more fertiliser than is strictly necessary, as aninsurance against reduced yields, even if this causesenvironmental damage. A legal framework thataddresses this market failure is essential, which mayinclude a mix of environmental regulations,incentives and voluntary measures.The current legalframework is incomplete. For example, NitrateVulnerable Zones (NVZs) may protect the waterenvironment, but at present there are no controls onnitrogen emissions from agriculture to theatmosphere, except from those larger pig and

poultry units controlled under the PollutionPrevention and Control (PPC) regulations.

The introduction of Single FarmPayments, accompanied by crosscompliance measures, is a highlyimportant step towards encouragingbetter land and soil management, andthis should reduce nutrient emissions tothe environment. For example, therequirement for implementation of asoil management plan to reduce erosion

risk will lower the risk of transfer to surface watersof phosphate bound to soil particles. Looking ahead,further CAP reform may offer opportunities tointroduce cross-compliance conditions or agri-environment measures that require specific actionsto reduce nitrogen and other nutrient emissions, inreturn for continued support payments. A good starthas been made with agri-environment schemes,which include measures such as buffer zones tolimit phosphate pollution of surface waters.

Better management of nutrients within intensivesystems is both needed and possible. Greater policyemphasis is needed on supporting the developmentof improved technology and encouraging itswidespread adoption.The role of the agriculturalengineer is critical. There is existing commercialtechnology which can reduce nutrient inputs whilesustaining or even slightly increasing yields, basedon precision nutrient application rates derived fromanalysis of information about soil conditions,previous cropping and weather patterns.Theseprecision agriculture techniques have the potential tomaterially reduce polluting emissions to air andwater but have not been targeted by UK policy,

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“ in the absence ofregulatorycontrols, it pays toapply a little morefertiliser than isstrictly necessary”

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29although they are mature and have been widelyadopted elsewhere in the world. UK farmers needincentives to adopt (precision agriculture) thistechnology. More positively, the animal sector isintroducing altered nutritional regimes, whichincrease protein conversion efficiency, reducingwaste nitrogen emissions. And the leaders in the pigindustry are shifting to energy recovery from slurriesvia anaerobic digestion in place of direct slurryapplication to land, which offers better controls onnutrient management as well as making animportant contribution to reducing greenhouse gasemissions.

A particular challenge is to make extensiveagricultural systems and also organic ones moreefficient in their use of nutrients. While thesesystems import less nutrients and rely more onnutrient cycling and on natural inputs and biologicalfixation of atmospheric nitrogen, they are also lesseasily controlled, which increases the risk of fugitivelosses to the wider environment. For example,animal wastes arising directly during grazing orspread from winter housing may be washed intosurface waters during wet periods and this mayneither be predictable or avoided entirely. In truth,there are different but significant risks associatedwith both intensive animal production systems,which have constructed facilities for wastecontainment, and extensive systems with little wastemanagement infrastructure.Those farms thatcombine some intensive practices with extensiveones, such as many dairy units, present manyopportunities for nutrient emissions to both waterand air, which argues for their closer regulation;including by an extension of the PPC regulations tocover larger dairy units. For example: to minimise

losses to the environment, the timing of fertiliserapplications to permanent grass is critical, but canoften be constrained by grazing schedules, and localstocking densities may exceed the carrying capacityof the land in winter months where there isinadequate investment in housing and wastemanagement. Current good practice requires thatanimal wastes are collected, stored, treated andreturned to land at rates and with timings thatmaximise recovery of nutrients back in to theproduction cycle, so minimising emissions.There isclear scope to improve performance, via acombination of regulation and capital support forbetter nutrient management infrastructure, such asslurry stores and additional winter-housing.

Agriculture delivers services other than food andfibre production, including waste management.Organic materials that would otherwise go to landfillor be incinerated can and are spread on land wheresome of the nutrients they contain arerecovered in to production.This serviceis an important means of recyclingnutrients in food wastes and sewagesludge. Nonetheless land spreading suchmaterials may harm the environment ifnutrient additions are not estimated andmatched to the requirements of theagricultural system.

There is a relatively good understanding of theprocesses that release nutrients to water fromnitrogen and phosphate fertilisers. There is scope forimproved management, such as by introducingprecision techniques, but the immediate highpriority is to lower nitrogen emissions to theatmosphere. For this, a better understanding is

“ UK farmers needincentives toadopt precisionagriculturetechnology”

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required of the processes andpatterns of nitrogen releases tothe atmosphere, especially fromgrazed land, as a basis foreffective controls and innovativetechnology.

Continued CAP reform shouldencourage a shift to moreextensive systems, especially onpoorer land, as the link betweensubsidy and yields is broken.Thisshould lower overall agriculturalemissions of nutrients to theenvironment. But CAP reformwill also drive the emergence of larger intensive andmixed intensive-extensive operations that maypresent higher environmental risks. There is anurgent need to ensure that more intensiveagricultural systems are not sited where there isinadequate environmental capacity to absorb theiremissions. Alongside this there is a requirement for acombination of public investment in research andincentives and advisory services that are targeted atimproving the efficiency of nutrient use, via newand better adoption of existing technology andhigher standards of management.

Mark Kibblewhite is head of the natural resources

department and Professor of applied soil sciences at the

National Soil Resources Institute, Cranfield University

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30 “ continued CAPreform shouldencourage a shiftto more extensivesystems,especially onpoorer land, as thelink betweensubsidy and yieldsis broken”

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The composting industry is a relatively new arrivalin the UK. In little more than ten years it has grownfrom back garden compost heaps to a multimillionpound industry which is making a positivecontribution to increasing recycling, reducinglandfill and returning significant amounts of carbonand nitrogen to the soil.

While the industry might still be young,composting, that is to say, the process of producingcompost through aerobic decomposition ofbiodegradable organic matter, has taken place sincethe dawn of time. For the vast majority of that time,composting took place with little input from peopleand no technology.The composting of human faecalmatter, night soil, together with other vegetable andanimal manure waste has been practised in China forcenturies. It has been considered the key tosupporting high population densities andmaintaining soil fertility and structure over some4,000 years. Even without the benefit of scientificstudies, farmers throughout history recognised theimportance of returning nutrients to the soil.

However, since the Industrial Revolution this cruciallink between food production and returning surplusorganic matter to soils has become severelydiminished. In the rush towards industrialisation,this historical knowledge was discarded and it has

only been in relatively recent years that mainstreamscientists and farmers have once again recognised theimportance of recycling organic waste back to thesoil. Classifying these materials as ‘wastes’ andrequiring sanitary disposal methods, such as landfillor incineration, in order to safeguard public healthin our growing urban areas, inadvertently resulted ina loss of valuable nutrients and organic matter whichwe are only now rectifying.The words of Dr F.H.King ring even truer today than when they were firstpublished nearly a century ago:

“Man is the most extravagant accelerator of wastethe world has ever endured. His withering blight hasfallen upon every living thing within his reach,himself not excepted; and his besom ofdestruction in the uncontrolled hands ofa generation has swept into the sea soilfertility which only centuries of lifecould accumulate, and yet this fertilityis the substratum of all that is living.”6

It is only in recent years that the environmentalimpact of these waste disposal techniques has beenquestioned and that European governments havebegun to act upon them.The principal driver behindthis development has been the EU Landfill Directive(EC/31/99), which aims to reduce emissions of thepotent greenhouse gas methane into the atmosphere.

the role of composting in closingthe nutrients loop

Dr Jane Gilbert

“ composting hastaken place sincethe dawn of time”

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The Directive sets binding targets for member statesto divert biodegradable municipal waste fromlandfill; the knock-on effect of this is that localauthorities in the UK are now collecting garden andfood wastes separately for biological treatment,either through composting or anaerobic digestion.The UK’s composting industry currently processes inexcess of three million tonnes of waste a year,producing more than a million tonnes of compost.Impressive though this may sound, especially whencompared to the few thousand tonnes composted inthe early nineties, this increased need for recyclingmeans that the composting industry is setto witness somewhere between a three andfive-fold increase in production within thenext 10-15 years.

The benefits of compost use are wide andvaried.The ‘black gold’ loved by gardenersis now the subject of serious scientificstudy by academics across the world, whohave begun to translate qualitative effects intoquantitative benefits. The complexity of soils and theflora and fauna they support suggest that benefits arediverse, with synergistic effects observed in long-term trials.

A range of research projects and trials have beencarried out in the last five years to prove the benefitof compost use in a range of applications.Theevidence from these trials has shown that compostcan play a pivotal role in the adoption of sustainablepractises in certain markets. Compost’s moistureretention capabilities and its slow release nutrientshave made it an attractive option to landscapers andhorticulturists. Recent trials in agriculture haveshown that the application of compost to soil can

increase yields over three to five years of repeatedapplication. Furthermore, compost also has apotentially large market in restoring and ‘repairing’soil on brownfield sites such as former factories andindustrial areas, as it can be an ideal component intopsoil manufacture where existing soil is scarce orof poor quality.

Most notably though, composts have a direct impacton the soil’s physical structure, helping to increaseorganic matter content, improve structure andworkability, reducing erosion and compaction as

well as acting as a sponge to help hold-in moisture.This water-retentioncapacity has direct agronomic benefitsespecially as parts of the UK are nowstarting to experience greater periods oflow rainfall followed by intenseprecipitation. An added benefit is thatthe organic matter acts as a buffer,helping to mop up and retain inorganic

fertilisers that would otherwise run-off and pollutewater courses.

Compost, being a living substrate, introducesbeneficial microorganisms into the soil, and helpsprovide food for indigenous soil fauna.Thedevelopment of ever more sophisticated molecularbiology techniques has moved the understanding ofthe microbiology of composting to new horizons.Scientists are now able to track the growth andsuccession of microbial communities duringcomposting and assess how these interact with soilmicrobes. One area that is receiving considerableinterest is the ability of composts to suppress plantpathogens. Research into the disease suppressiveeffects of compost began in the United States over

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“ compost canplay a pivotalrole in theadoption ofsustainablepractises”

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3330 years ago. Since then, there has been a significantamount of research into the effects of compostsmade from a wide range of feedstock wastes onplant diseases. One material that has real potentialfor disease suppression is green waste compost.Where these pathogens are of agronomic concern,the application of compost can help reduce afarmer’s reliance on artificial pesticides, which haveover the years been linked to numerousenvironmental problems.

Notably, composts contain a range of plant nutrients.The macronutrients N, P and K are available to plantsin different ways. In temperate regions between 10-15 per cent of the total nitrogen content in compostis typically available for plant growth during the firstyear of application.This means that applyingcomposts helps build up a ‘bank’ of nutrients thatare released into the soil over time so that benefitsare observed over a period of years. Compost usecan, therefore, help improve soil fertility and cropyields not only in its year of application, but also insubsequent years. The benefits of compostapplication to crops have been demonstrated innumerous long-term field-scale trials globally. Recentstudies in the UK have suggested that after threeyears yield increases of 7 per cent compared withconventional fertiliser applications were observed.7

Research in Germany has suggested that not all ofthe nitrogen in compost is used as a fertiliser. Rather,a proportion is retained in the soil and used by soilmicrobes to help generate more organic matter insitu. As the UK, in particular, has experiencedsignificant organic matter loss during the lastcentury, this benefit cannot be overstated. Figuresfrom England and Wales show that percentage of

soils with less than 3.6 per cent organic matter rosefrom 35 per cent to 42 per cent in the period 1980-1995.8 Recent research published in Nature suggeststhat loss of soil carbon is linked to climate change.9

By inference, the controlled application of compostcan help counter some of this loss, thus mitigatingcarbon dioxide emissions.The CompostingAssociation has provisionally calculated thatcomposting an estimated 15 million tonnes ofbiowaste could have the potential to offset carbondioxide emissions equivalent to those of over amillion cars a year.

The UK now produces a ready supply of qualityassured compost. Compost produced to meet the BSIPAS 100 standard and the Quality Protocol forCompost means that it need no longer be classified asa waste in England and Wales. Compost is the firstrecovered waste type to be subject to a qualityprotocol. It was chosen because of the growingdemand from landscapers, horticulturists, developersand farmers who know the benefits of applyingcompost to their soil and because there is aprofessional industry in place to meet this demand.

The Quality Protocol for Compost sets out criteria forthe production of compost from specific waste types,and compliance with these criteria is consideredsufficient to ensure that the recovered product can beclassified as fully recycled andused without risk to theenvironment or harm to humanhealth.

In the UK, the Single PaymentScheme has replaced previousCommon Agricultural Policy

“ composting 15 milliontonnes of biowastecould have the potentialto offset carbon dioxideemissions equivalent toa million cars a year”

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payment schemes. Organic matter decline inEuropean soils is one the main emerging issues in EUpolicy terms, because it is threatening the capacity ofsoils within EU Member States to remain fertile andto keep performing their environmental functions. Inorder to receive their subsidy payment, farmers mustcomply with Statutory Management Requirementsand Good Agricultural Environmental Condition(GAEC) standards. Better productivity is linked withgood soil management that prevents soil erosionfrom fields.The effective maintenance of soil organicmatter and good soil structure are central to meetingthe GAEC standards. In addition, agri-environmentschemes within the UK provide additional funding tofarmers and other land managers who delivereffective environmental management on their land.Although the schemes differ in some details, soil andmanure management planning is a common elementto all.

The Composting Association believes thatthe relevance of certified quality assuredcomposts should be made more apparentin government GAEC standards, agri-environment scheme guidance and in anyassociated guidance on the reformedCommon Agricultural Policy, and thatgovernment considers policy drivers thatwould provide a clear link betweenbiowaste and soil management practices.

Soil comprises a delicate balance of minerals andliving organisms but our recent emphasis onapplying inorganic fertilisers has rapidly destroyedthis ecosystem. Compost can play a vital role inreturning nutrients to the soil and thereby arrest thedecline in the quality of soil in this country.The

nutrients present in quality compost will provide asource of food for microbes in the soil and helpfarmers to move as close as possible to closing theorganic loop.

With our rush towards the use of fertilisers theorganic loop has been broken; it’s time that wereconnected that link between organic waste and thesoils on which they arose in the first place. Farmersthroughout history have known that it was importantto return waste to the soil, they might not haveknown why it was important or what they weredoing but they knew it was important and that itworked.The use of inorganic fertilisers may have hadshort-term benefits but it has had massiverepercussions in the longer run.This will change.Recycling and composting are high on thegovernment’s agenda, the Quality Protocol forCompost has been published and there is a real desireamongst the public to recycle and take more care ofthe environment. With potentially in excess of 7million tonnes of compost produced every year, thiscould represent just under a million tonnes oforganic carbon and over 50,000 tonnes of nitrogenwhich would otherwise be lost. We now have theknowledge and the opportunity to improve thequality of soil and to move in the right direction; ouragri-environment schemes need to reflect this.

Applying compost to the soil will have major benefitsto the environment. After all, there’s a reason whyfarmers have been doing it for 40 centuries.

Dr Jane Gilbert is chief executive

of the Composting Association

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“ it’s time that wereconnected thelink betweenorganic waste andthe soils on whichthey arose in thefirst place”

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Human activity is altering the fertility of thecountryside. Industrialisation and agriculturalintensification have been accompanied by dramaticchanges to the natural cycling of plant nutrients -specificially nitrogen and phosphorus. On a globalscale, human activity has more than doubled thefixation of nitrogen gas from the atmosphere intoforms that can affect the environment. Humans havealso tripled the amount of biologically availablephosphorus available since the industrial revolution.

Both these nutrients directly affect plant growth innatural and semi-natural habitats with knock-onconsequences for wildlife. It is difficult todisentangle the effects of nutrients from the otherpressures contributing to the decline in individualspecies but the effects can be viewed by looking indetail at the effects on habitats. This chaptersummarises a detailed review of the evidence forindirect (including food-chain) effects ofphosphorus and nitrogen on UK bird populations.10

It focusses on the impacts of nutrients on farmland,aquatic habitats and upland moor and lowland heathhabitats.

FarmlandFarmland is a deliberately fertilised habitat. The plantgrowth acceleration properties of nutrients are key toincreasing yields and have helped to change the waycrops, including grassland, are grown and managed.Inorganic fertiliser use has been one factor that hashelped to drive a massive intensification ofagriculture since the 1950s. Problems arise whenintensification has negative environmentalconsequences; when nutrients affect non-crop plantson farmland; or when unused nutrients are dispersedfrom the agricultural system.

Vegetation structureSome organisms are directly affected by the toxicityof nutrients applied to the land, butthe majority of wildlife impacts areindirect. Fertilisers promote faster andearlier spring growth and lead to taller,denser and more uniform crops andgrass swards. Fertilisers applied ontofield margins in the application processalso alter these habitats. This has anumber of consequences for non-cropplants and animals in farmland.

too much of a good thing? the impact of nutrients on birds and other biodiversity

M A MacDonald, J M Densham, R Davis and S Armstrong-Brown

“ human activity hasmore than doubledthe fixation ofnitrogen gas fromthe atmosphereinto forms that canaffect theenvironment”

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Species richnessIncreased fertiliser applications generally reduce thevariety of plant species found in fields.The first todisappear are the most sensitive plants which cannottolerate increased nutrient levels or compete withnutrient hungry crops, for example, lambs succoryand annual knowle.This can have knock-on effectsfor invertebrate species such as butterflies andcarabid beetles11 which require these plants as hosts,and has the potential to decrease invertebratebiodiversity on farmland.

Whilst some plant eating invertebrates may benefitfrom the increased nutritive content of fertilisedcrops and grass, some, such as grasshoppers andcrickets, are adapted to a specific sward structurewithin fields. When fertilisers cause a change to thestructure of plants these insects are affected12 but inaddition, they are disturbed by intensive grazing andcutting regimes because of their longer life cyclesand relatively low recolonisation ability.This meansthat they are more likely to disappear fromintensively managed farmland.The loss of these largeinvertebrates may also be a major cause of impactson birds, such as cirl bunting and red-backed shrike,and breeding waders in grassland.13 Dense grass andcrops also dry the soil forcing earthworms deeperinto the ground making them unavailable to birdssuch as snipe, starling and chough, that probe thesoil for food.

Land managementSilage production, with its intensive use of fertilisersfor rapid grass growth and repeated cutting,probably has the most radical effect on vegetationand the food-chain of wildlife, of any farmland

management. It has probably also had the greatesteffect on farmland bird populations because inaddition to reducing food resources, nest destructionduring silage cutting has a major impact on groundnesting birds such as whinchat and corncrake.14

Furthermore, intensive grazing on fertilisedgrassland can lead to increased nest trampling inground nesting species such as skylark15 andlapwing.16

Farming systemsAt the landscape scale the use of inorganic fertilisershas led to changes in patterns of land-use byreleasing farmers from the need to graze livestockfor manure production and/or to rotate leguminouscrops in order to provide plant nutrients. As a resultthere has been a loss of mixed farming landscapesand a polarisation of agriculture in the UnitedKingdom towards arable farming in the south andeast, and pastoral farming in the north and west.Birds such as starling and lapwing prefer thejuxtaposition of arable fields and pasture and arelikely to have been affected by the loss of mixedfarming from the landscape.17

Aquatic habitatsIn some forms nitrogen and phosphorus can betransported away from their intended targets, forexample in silage fields, and can disrupt habitatselsewhere. Nutrients also enter the aquaticenvironment as pollution from some sewagetreatment works.This pollution can cause aquaticsystems to become eutrophic with complex andhighly localised effects on the food-chain.There arewinners and losers when aquatic habitats becomeeutrophic: increases in nutrient concentrations may

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37be beneficial to wildlife by increasing food supplybut too great a change can radically impact thehabitats on which species rely for food and nestingsites.

Upland freshwater lakesUpland lakes in the UK are mostly naturally low innutrients and plant life making them sensitive tominor increases in nutrient loadings. Shallowfreshwater lakes are especially susceptible toeutrophication with the resultant shifts in waterquality and impacts on biodiversity being noticablerelatively quickly. Eutrophication of shallow lakesleads to a reduction of fish diversity, with roach andbream becoming dominant at the expense of perch,tench and rudd.18 The rare bittern may be affectedby this reduction in the numbers of rudd, one of itsmajor food items in Britain and itsadaptation to life in reedbeds mayalso mean it is affected by the actionof nutrients in changing the structureand composition of plantcommunities. An example of thischange has been seen in the NorfolkBroads where increases in nitrates in water havecaused the decline of a particular floating form ofreed, used by bitterns, known as hover.19

Invertebrate communities in freshwater are alsosensitive to nutrients, for example eutrophicconditions can radically affect the invertebrates livingat the bottom of water bodies, leading to a loss ofsensitive groups of species such as molluscs. Divingbirds, such as pochard, which dive from the surfaceto feed on them suffer from the resultant reducedfood supply.20 Phosphates are commonly transported

to the aquatic environment by being bound tosediments.This mechanism of pollution can increasephosphate levels in water but the sedimentsthemselves can also cloud water, prevent the growthof submerged plants, clog gravel needed forsalmonid eggs to survive and hatch, and affectfreshwater pearl mussel and native white-clawedcrayfish.

Estuaries and coastal watersNutrients flowing downstream reach esturaries andcoastal waters where their effects are less persistentbecause they are generally more quickly flushedthrough with water. However, where eutrophicationoccurs it typically leads to a decline in plantcommunities, such as seagrass and eelgrass beds, andtheir replacement by algal mats and phytoplankton

blooms.21 This growth can starve thewater of oxygen and force invertebratesto the surface providing a short-termflush of food for organisms higher upthe foodchain.This phenomenon canoccur in freshwater too, killinginvertebrates and fish in an affected

stretch of water. Small aquatic crustaceans such asthe mud shrimp and polychaetes like ragworm arerelatively tolerant of eutrophic conditions and canprovide abundant food resources to shorebirds ineutrophic tidal areas.These birds generally benefitfrom human influenced nutrient inputs, forexample, from sewage outflows, although birds withspecific prey requirements or foraging habits, suchas shelduck, may not.22

“ shallow freshwaterlakes are especiallysusceptible toeutrophication”

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Upland moor and lowland heathUpland moor and lowland heath, dominated byheather, are naturally nutrient poor habitats that aresensitive to increases in nutrient inputs. Fertilisersare not generally applied to moorland or lowlandheathland as a land management tool. Instead,increases in nutrient loading are the result ofatmospheric nitrogen deposition.This source ofnitrogen has increased enormously over the pastcentury, despite some reductions over the past twodecades.

VegetationHeather cover in UK upland moorland and lowlandheath declined by 40 per cent between 1947 and198023 due to overgrazing, afforestation, conversionto farmland, invasion of grasses, and succession toscrub. Nitrogen deposition in the form of nitrousoxides may have accelerated these processes, such asby increasing the nitrogen content of heather leaves,affecting the plant’s structure leaving it sensitive todrying and attack by heather beetle,24 which feedsexclusively on it. Exacerbated by grazingdisturbance, dieback can occur which allows grassesto invade, themselves fertilised by the atmosphericnitrogen. Plant-eating invertebrates may be helpedby the enhanced nutritive value of heather orthrough a preference for grasses but ultimatelychanges to the mosaic of grass and heather andintensive grazing of moorland will remove foliage tothe detriment of these invertebrate communities.

BirdsIn upland moorland, the shift from heather to grassmoorland over a large scale is likely to be the mostimportant indirect effect on birds of nitrogen

deposition. A number of bird species have beenidentified as sensitive to heather loss, especially redgrouse and stonechat. It is, therefore, possible thatthese birds are adversely affected by the impacts ofatmospheric nutrient deposition.

ConclusionsIncreased levels of nutrients are damaging habitatsacross the UK. Nutrient pollution force-feeds thecountryside, altering plant growth rates, changingplant communities and disrupting the food chain forwildlife. Red backed shrike is now extinct in the UK;its decline was largely driven by managementchange linked to fertiliser application to grassland.This process also helped drive corncrake and cirlbunting to the brink of extinction in the UK.Thesetwo species are slowly recovering as a result ofintensive conservation effort. Tougher action must betaken to ensure the efficient use and recycling ofnitrogen and phosphorus, and limit losses to theenvironment.The knowledge and technologies existto achieve this, through capturing and re-using thenutrients present in organic wastes. Now, policies arerequired to put this knowledge into practice,through a mixture of better regulation, incentivesand education.

This article is a summary of a recent evidence

review by RSPB. The full report is available at

http://www.rspb.org.uk/ourwork/policy/

water/issues/diffuse.asp

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Underpinning all agricultural production, the twomost important nutrient cycles in the UK are thoseof nitrogen and phosphorus. However they arenatural cycles and are prone to leakages andinefficiencies.

All food crops with the exception of legumesrespond positively to added nutrients. The yield of acrop of well-fertilised winter wheat will be doublethat of a crop which received no additionalnutrients. In vegetable crops, additional nutrients canbe the difference between a marketable crop and nocrop at all – cauliflowers grown with limitednutrients may fail to form a curd whereas those withadequate nutrients will form big marketable heads.To ensure an economic return on agriculturalproduction, farmers add to the naturally availablenutrients with synthesised fertilisers.

Nitrogen is an essential element, perhaps the essentialelement. It is a component of the DNA of all animalsand plants and is critical for plant growth. Globally,there is no shortage since our atmosphere is 80 percent nitrogen.The problem is that atmosphericnitrogen is inert and cannot be used by plants andanimals in that form. Plants need the ‘reactive’nitrogen compounds ammonium and nitrate forgrowth that are normally provided from thebreakdown of plant and animal remains in the soil.

Synthesised fertilisers, manufactured using nitrogenfrom the atmosphere by the Haber-Bosch processcan supplement this ‘natural’ supply. In fact, the largeincrease in world population since 1910 can belinked to this process that revolutionized modernagriculture and food production.

Humans have, of course, been supplementing thebackground or natural nitrogen cycle for hundredsof years but industrialisation and the manufacture ofindustrial inorganic fertilisers have since resulted in adoubling of the reactive nitrogen in the biosphere.The Global Nitrogen Enrichment ThematicProgramme (GANE) suggests, “While the globalcarbon cycle is being perturbed by less than 10 percent, the global reactive N cycle is being perturbedby over 90 per cent”.25 The increase can beattributed to the increased demand for food and tothe fact that food production isnitrogen inefficient. Cereal crops, themost efficient users of nitrogen, areonly able to take up 50 per cent of thenitrogen in the soil. Other agriculturalsystems are even more inefficient: theproduction of beef uses only 9 percent of the nitrogen supplied to it.

Within the UK, two industries,agriculture and power generation add

energy and cost implications oftoday’s nutrient cycle

Rob Lillywhite

“ while the globalcarbon cycle isbeing perturbed byless than 10 percent, the globalreactive N cycle isbeing perturbed byover 90 per cent”

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between them 2 million tonnes of new reactivenitrogen to the environment every year: 1.1 milliontonnes of nitrogen fertiliser, 0.4 million tonnes ofagricultural nitrous oxides and ammonia and 0.5million tonnes of nitrogen oxides from thecombustion of fossil fuels. Although only thenitrogen fertiliser is deliberately applied toagricultural soils, the other compounds are alsoreturned to the soils as wet and dry deposition.26

The costs and implications of adding this muchnitrogen to the UK environment on an annual basisare significant, but defining and quantifying them isnot easy. Some are financial; some environmental -and there is no easy way to make them comparable.

The nitrogen cycle, due to the mobility of nitrate, isinherently leaky and the subsequent environmentaleffects of excess nitrogen are well documented:acidified soils, loss of plant diversity and high nitrate

levels and eutrophication in fresh and marinewaters. Whilst most people would agree thatthese effects have a negative value it is farmore difficult to ascertain their cost. Let’s startwith the basics. Every year half a milliontonnes of nitrogen is leached from agriculturesoils. Although it’s only partially true, for thepurposes of this exercise we will assume thatthis is all commercial ammonium nitratefertiliser costing £160 per tonne (£464 pertonne of nitrogen). In financial terms, thismeans that £232 million is wasted.27

Costs can also be measured in terms of theenergy used.To manufacture one tonne ofnitrogen for ammonium-nitrate fertiliser takes40 gigajoules (GJ) and releases the equivalent

of 5.29 tonnes of CO2,28 which means that the 0.5

million tonnes of nitrogen lost as leachate every yearrequired 20 million GJ to manufacture and emittedthe equivalent of 2.65 million tonnes CO2. In total,the 1.1 million tonnes of nitrogen fertiliser used inthe UK required 44 million GJ of energy tomanufacture and emitted the equivalent of 5.8million tonnes CO2.

The cost of transporting and applying fertiliserrequires some assumptions but we estimate thatanother 1.3 million GJ are required to transport andapply the unused nitrogen, equivalent to another0.01 million tonnes of CO2.

Unused reactive nitrogen has been accumulatingwithin our environment for years. The task ofassessing and if necessary making safe or removingthat nitrogen is the responsibility of theEnvironment Agency and water companies. InEngland approximately 29 per cent of all rivers areclassified as having nitrate concentrations greaterthan 30 mg/l with some areas such as the midlands,Anglian and Thames recording concentrations of upto 80 mg/l. Wales, Scotland and Northern Irelandhave much lower levels due to less intensiveagriculture. In order to meet the EU drinking waterlimit of 50 mg/l, water companies must eitherblend high concentration water with lowerconcentration water or must remove nitrate by ionexchange or reverse osmosis. Both of these processesare expensive to construct and operate: Ofwat hassuggested that between 2005 and 2010, Englishwater companies will have to incur capitalexpenditure of about £300m and annual operatingcosts of £6m to reduce nitrate levels for the publicdrinking water supply.

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“ agricultureand powergenerationadd betweenthem twomillion tonnesof newreactivenitrogen to theenvironmentevery year”

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41In summary, unused agricultural nitrogen fertiliserthat cost £232 million to manufacture also has asignificant environmental cost - although the onlydirect costs are those operating costs borne by thewater companies of £6 million per annum.

The fertiliser form of phosphorus is phosphate, ofwhich the UK used 278,000 tonnes in 2004.Phosphates like nitrates are implicated ineutrophication although they are less mobile in soiland therefore less likely to leach.The current bestestimate is that 10 per cent of applied phosphates arelost from the soil on an annual basis whichassuming an average cost of £150 per tonne fortriple super phosphate (or £319 per tonne ofphosphate) is £8.9 million pounds wasted.

Phosphate fertilisers are manufactured from minedrock phosphate with an average energy input of 5 GJper tonne, although this energy input could be offsetby subsequent processing into triple superphosphate that can release 3.8 GJ per tonne.However for the purposes of assessing the energylost within the nutrient cycle we will use 5 GJ pertonne which for 27,800 tonnes is 139,000 GJ or3320 tonnes of oil equivalent and 10,000 tonnes ofcarbon dioxide.

However, losses of nutrients and their impact on theenvironment, be they economic or physical, must becontrasted against the gains: food production.Economically the £241 million cost of unusedfertiliser and £55 million that is spent annually inremoving it from drinking water is substantial. But isit significant? The farm gate value of UK producedfood is £16,000 million so the £296 million bill forwaste represents only 1.85 per cent. In many

commercial industries that level of waste would beacceptable.

Summing up the leakage in terms of energy isslightly more difficult. The embodied energycontained within UK produced food and animalfeedstock’s production is estimated to be 548million GJ, which is over ten times the amountcontained within unused fertiliser. Of course thereare other energy inputs into agricultural productionsystems but these figures still illustrate the large gapbetween input and output.

So there are certainly costs involved in leakages fromthe UK’s nutrient cycles which if considered inisolation appear substantial. However, they shouldnot be considered in isolation but in context ofnatural biological efficiency and overall foodproduction.The use of nutrients in agriculturalproduction systems and therefore the amounts ofthese nutrients lost to the environment have beenreducing year on year for the last ten years. In thattime plant physiology hasn’t changed but the totalamount of nutrients applied in the UK has droppedfrom 2.1 million tonnes per annum to 1.8 milliontonnes, without any reduction in overall agriculturaloutput.

It is possible that the use of nutrients in agricultureis nearing the point of diminishing returns and thatfurther reductions will only come at the expense ofreduced food production and taking vast swathes ofland out of agriculture. If we have reached thatposition and accept that leakages and topping up area necessary but evil part of food production thenperhaps we need to explore different ways ofreducing the impact of applied nutrients.

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If UK agriculture cannot do without added nutrientsthen perhaps we need to rethink the form that thosenutrients take. What would happen if we didn’t usesynthesised fertiliser and grew all our food usingsome of the strategies lifted from organic farming?Synthesised ammonium nitrate, as remarked onearlier, is very energy expensive to produce. Soreplacing it with low energy alternatives wouldreduce the energy bill and perhaps diffuse pollutionas well. Alternatives do exist, for example, bulky‘natural’ materials such as farm yard manures,slurries, composts, sewage sludges and digestatescontain plant nutrients and are readily available buthave been largely replaced in modern conventionalagriculture by more easily handled synthesisedgranular fertilisers. Although concerns regardinghazards to both health and the environment havebeen expressed for some of these materials, these are

overcome by adhering to existing strict Codes ofPractice such as the Safe Sludge Matrix and thenew Quality Protocol for Compost.

If the concerns of the Environment Agency, themajor retailers and their customers can beovercome there is no shortage of these naturalmaterials since meeting the demands of theLandfill Directive will ensure that biodegradableorganic material which in the past would havebeen land filled is now available for spreading toland. In terms of overall nutrient supply, acombination of source segregated biodegradable

municipal waste, green waste, sewage sludge andfarmyard manures could replace all use ofmanufactured fertilisers. However, a note of cautionhere.To implement such a complete sea change inagricultural practice would involve not only engagingUK farmers but also government and the generalpublic. Farmers would initially have to bear the

increased financial costs of implementing and usingnew systems but far more importantly, the generalpublic would need to accept the concept of reusingbiodegradable organic material as alternatives tosynthesised fertiliser and would have to accept thatthe true cost of food is higher than it is now.However, in the long term, the benefits in terms ofimprovements to the environment and energy savedcould be substantial.

In these days of increasing environmental awareness,ecological footprints, climate change and air miles,the task of reconnecting the general public to foodand farming is already underway. Recycling andreusing materials is now part of our everyday lives soextending that concept to the reuse of manures andsewage sludges would seem logical. In the last fewyears, public opinion has shifted to a point where thegreen agenda is no longer a minority position, soselling the new fertiliser has already begun althoughit would take determined government action and thesupport of the green and environmental groups tomake the change possible. However, if we are reduceor replace synthesised fertilisers, reduce our energydemand and close the loop we need to start byovercoming our own prejudices.

Whether food is produced domestically or imported,‘leakages’ and ‘topping up’ are unavoidablecomponents of agricultural systems. Differentmanagement strategies in agriculture and UK andEuropean legislation have reduced the impacts andcosts over the years, however there is still room forimprovement.

Rob Lillywhite is a researcher at Warwick HRI, University

of Warwick. He co-authoured the Biffaward

mass balance study, Nitrogen UK

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“ the generalpublic wouldhave toaccept thatthe true costof food ishigher thanit is now”

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1 Environment Agency, 2004, Map: Rivers at risk from diffuse source pressures (nutrient nitrogen), available athttp://www.environment-agency.gov.uk/commondata/acrobat/r_diff_nitrogen_v1_1008185.pdf

2 Environment Agency, 2004, Map: Rivers at risk from diffuse source pressures (nutrient phosphorus), available athttp://www.environment-agency.gov.uk/commondata/acrobat/r_diff_phos_v1_1008201.pdf

3 Defra, 2002, Directing the flow: priorities for future water policy, available athttp://www.defra.gov.uk/environment/water/strategy/pdf/directing_the_flow.pdf

4 Ibid.

5 Defra, 2006, Updating the estimate of the sources of phosphorus in UK waters, available athttp://www2.defra.gov.uk/research/project_data/More.asp?I=WT0701CSF&M=KWS&V=csf&SCOPE=0

6 F.H King, 1911, Farmers of Forty Centuries or Permanent Agriculture In China, Korea And Japan, Chapter 9 TheUtilisation of Waste.

7 Wallace, P., 2006, Compost Use in Agriculture, Enviros Consulting Ltd.

8 Favoino, E. and Hogg, D., 2006, The potential contribution of Biowaste to tackle climate change: shortcomings of lifecycle analysis concerning Biowaste and relevance to policy-making, Orbit 2006 Proceedings.

9 Bellamy, P., Loveland, P.J., Bradley, I., Murray Lark, R. &. Kirk G.J.D., 2005, Carbon losses from all soils acrossEngland and Wales, 1978–2003 Nature Vol 437.

10 MacDonald, M A., 2006, The indirect effects of increased nutrient inputs on birds in the United Kingdom: a review,RSPB, available at http://www.rspb.org.uk/ourwork/library/reports.asp

11 Kirkham, F.W., Sherwood, A.J., Oakley, J.N. and Fielder, A.G, 1999, Botanical composition and invertebratepopulations in sown grass and wildflower margins, Aspects of Applied Biology 54 pp291-298.

12 van Wingerden, W.K.R.E., van Kreveld, A.R. and Bongers, W., 1992, Analysis of species composition ofgrasshoppers (Orth., Acrididae) in natural and fertilised grasslands, Journal of Applied Entomology 113 pp138-152.

13 Beintema, A.J. 1991, Insect fauna and grassland birds. In: Curtis, D.J., Bignal, E.M. and Curtis, M.A. (eds),Birds and pastoral agriculture in Europe. Proceedings of the second European Forum on birds andpastoralism. Port Erin, Isle of Man, 26-30 October 1990. Scottish Chough Study Group: pp 97-101.

14 Green, R.E. and Stowe,T.J., 1993, The decline of the corncrake Crex crex in Britain and Ireland in relation to habitatchange, Journal of Applied Ecology 30, pp689-695.

15 Donald, P.F., Evans, A.D., Muirhead, L.B., Buckingham, D.L., Kirby, W.B. and Schmitt, S.I.A. 2002.Survival rates, causes of failure and productivity of skylark Alauda arvensis nests on lowland farmland.Ibis 144, pp652-664.

Notes and references

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16 Shrubb, M., 1990, Effects of agricultural change on nesting lapwings Vanellus vanellus in England and Wales. Bird Study37, pp115-127.

17 Wilson, A.M.,Vickery, J.A. and Browne, S.J. 2001, Numbers and distribution of northern lapwings Vanellusvanellus breeding in England and Wales in 1998, Bird Study 48, pp2-17.

18 Leach, J.M., Johnson, M.G., Kelso, J.R.M., Hartmann, J., Numann, W. and Entz, B. 1977, Responses ofpercid fishes and their habitat to eutrophication, Journal of the Fisheries Research Board of Canada 34, 1964-1971.

19 Crook, C.F., Boar, R.R. and Moss, B., 1983, The decline of the reedswamp in the Norfolk Broadland: causes,consequences and solutions, Report to the Broads Authority, Norwich, UK.

20 Allen, D., Mellon, C., Enlander, I. and Watson, G., 2004, Lough Neagh diving ducks: recent changes in winteringpopulations, Irish Birds 7, pp327-336.

21 Herbert, R.A. 1999, Nitrogen cycling in coastal marine ecosystems, FEMS Microbiology Reviews 23, pp563-590.

22 Burton, N.H.K., Jones,T.E., Austin, G.E., Watt, G.A., Rehfisch, M.M. and Hutchins, C.J., 2004, Effects ofreductions in organic and nutrient loading on bird populations in estuaries and coastal waters of England Wales. Phase 2 report,Report by the BTO for English Nature, English Nature Research Report No. 586, English Nature,Peterborough.

23 Thompson, D.B.A., MacDonald, A.J., Marsden, J.H. and Galbraith, C.A., 1995, Upland heather moorland inGreat Britain: a review of international importance, vegetation change and some objectives for nature conservation, BiologicalConservation 71, pp163-178.

24 Brunsting, A.M.H. and Heil, G.W., 1985, The role of nutrients in the interactions between a herbivorous beetle andsome competing plant species in heathlands, Oikos 44, pp23-26.

25 Lillywhite, R. and Rahn, C., 2005, Nitrogen UK, p6, Warwick HRI

26 Dry deposition occurs when a nitrogen compound is removed from the atmosphere by contact withthe surface of the land or an aerial-borne particle that subsequently falls to land. Wet depositionoccurs when a nitrogen compound is dissolved into water vapour and returned to land as rain.

27 Commercial ammonium nitrate fertiliser contains 34.5 per cent nitrogen.To covert weight ofnitrogen to weight of fertiliser multiply by 2.9 or to convert weight of fertiliser to weight ofnitrogen, divide by 2.9.

28 Jenssen,T.K. and Kongshaug, G., 2003, Energy consumption and greenhouse gas emissions in fertiliser production,The International Fertiliser Society, Proceedings No. 509, 1-28.

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