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Nutrient Cycles

1. Nutrient requirements

2. Biogeochemical cycles

3. Rates of decomposition

4. Plant adaptations in low nutrient conditions

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Nutrient Requirements for Plant Growth

• Taken up in gaseous form, Oxygen (O2), Carbon CO2, and from roots - Water (H2O).– Derived from water and carbon dioxide

• Rest are taken up from soil solutions– Macro-nutrients –Nitrogen (N), Phosphorous

(P), Potassium (K), – Calcium (Ca), Magnesium (Mg), Sulfur (S)– Micro-nutrients – Boron (B), Copper (Cu),

Iron (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn)

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Nutrient Cycles

1. Nutrient requirements

2. Biogeochemical cycles

3. Rates of decomposition

4. Plant adaptations in low nutrient conditions

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Biogeochemical Cycling

The cycling of nutrients through ecosystems via food chains and food webs, including the exchange of nutrients between the biosphere and the hydrosphere, atmosphere and geosphere (e.g., soils and sediments)

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•Ecosystems produce and process energy primarily through the production and exchange of carbohydrates which depends on the carbon cycle.

•Once energy is used, it is lost to the ecosystem through generation of heat

•Carbon is passed through the food chain through herbivory, predation, and decomposition, it is eventually lost to the atmosphere through decomposition in the form of CO2 and CH4 . It is then re-introduced into the ecosystem via photosynthesis.

•However, the amount of carbon present in a system is not only related to the amount of primary production, as well herbivory and predation (e.g., secondary production), it is also driven by the rates of decomposition by micro-organisms

•Atmospheric carbon is rarely limiting to plant growth

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•When we look at other nutrients, a somewhat different picture emerges than with the energy cycle – e.g., phosphorous in a food chain within a small pond.

•Algae remove dissolved phosphorous from the water

•The phosphorous is then passed through different trophic levels through herbivory and predation.

•At each level there is some mortality, and then the phosphorous is passed to decomposers

•These organisms release phosphorous into the water where it is again taken up by primary producers and the whole cycle starts up again

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Key Elements of Biogeochemical Cycles

a. Where do the nutrients that ecosystems use come from?

b. What happens to the nutrients within the ecosystem itself?

c. What happens to the nutrients once they leave the ecosystem?

d. Once nutrients are cycled through an ecosystem, how do they get back?

e. What are the rates of exchange of nutrients between the different pools?

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Nutrient Pools and Nutrient Flux

• Nutrient pool – a specific component or compartment where a nutrient resides– Can be a single organism, a population, a

community, a trophic level, and an abiotic feature (e.g., lake, soil, atmosphere, etc.)

• Nutrient flux – the rate of exchange (e.g., unit of material per unit time) of nutrients between pools

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•Example of changes in the amounts of tracer phosphorous being exchanged within an aquatic food web

•The values themselves represent changes in the pool levels, where each one of the lines represents a different pool

•Understanding the feeding relationship allows us to build a nutrient cycle model for this ecosystem

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Model of phosphorous cycle for an aquatic ecosystem – flux rates per day shown.1. This system is not closed – inputs, probably from run-off from land.2. Exports include herbivores moving outside of system and dead plant/animal material

moving out of system, probably through sedimentation.3. Rate of uptake by plants is directly proportional to net primary production.4. Exchange of nutrients by higher trophic levels is controlled by processes regulating

secondary production.5. Rates of inputs and outputs of nutrients from an ecosystem are driven by both biotic and

abiotic factors.

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Types of Biogeochemical Cycles

Three major categories of biogeochemical cycles based on slowest-changing pool(=reservoir):

1. Gaseous cycles of C, O, H20

2. Gaseous cycle of N, (S)

3. Sedimentary cycles of the remaining nutrients

Global scale

Local scale

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Sedimentary Cycles

Gaseous Cycles

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Major Components of Nitrogen Cycle

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Biological Nitrogen Fixers

• Cyanobacteria – blue-green algae

• Free living soil bacteria

• Mycorrhizae

– Symbiotic bacteria living in root nodules

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Root nodules on ? Cassia fasciculata

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NO from lightning

Lightning + N2 + O2 NO + O2 Nitrate (NO3)

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Phosphorous Cycle

Phosphate – PO4-3

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Potassium

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Sources of Nutrients

Parent Material

Atmosphere

Run-off, Ground water

Floods

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Nutrient Cycles

1. Nutrient requirements

2. Biogeochemical cycles

3. Rates of decomposition

4. Plant adaptations in low nutrient conditions

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Simple Model of Soil Decomposition/ microbial respiration

MicrobialPopulation

Litter

Organic Soil

H2O, O2

Energy

Nutrients

CO2 or CH4

DissolvedNutrients

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Factors Controlling Microbial Respiration

1. Availability of oxygenCO2 versus CH4 production

2. Temperature

3. Moisture

4. Quality of material comprising dead organic matter

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Soil Moisture

Re

sp

ira

tio

n

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Simple Model of Simple Model of Soil Decomposition/ microbial respiration

MicrobialPopulation

Litter

Organic Soil

H2O, O2

Energy

Nutrients

CO2 or CH4

DissolvedNutrients

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k is the fraction of a material that decomposes in a given year

Decomposition as a Function of Lignin Content

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Residence Time

• Residence time is the length of time it takes for biomass or a nutrient to be completely decomposed or recycled from the forest floor

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Residence times

Coniferous forests have longer residence times than deciduous C/N controlBoreal forests have longer residence times than temperate forests temperature control

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Nutrient Cycles

1. Nutrient requirements

2. Biogeochemical cycles

3. Rates of decomposition

4. Plant adaptations in low nutrient conditions

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Tree Nutrient Content

% N % P % K

Temperate

Conifers

0.147 0.043 0.100

Temperate

Deciduous

0.289 0.025 0.178

Eucalyptus 0.194 0.008 0.127

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Translocation of Nutrients

• Prior to shedding leaves in the fall, translocation of nutrients often takes place in trees

• This allows tree to retain essential nutrients that are hard to come by

• Spruce trees remove more nutrients than other coniferous trees

• An adaptation to poor nutrient sites

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Question – do plants growing on sites with low soil nutrients have low

nutrient contents as well?

The answer is no –

• Plants on sites with low nutrients tend to have higher nutrient contents

• They have a higher nutrient use efficiency

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Nutrient Use Efficiency (NUE)

• Some plants are more efficient at using nutrients because it gives them selective advantages in low nutrient conditions

NUE = A / LA – the nutrient productivity (dry matter

production per unit nutrient in the plant)L – nutrient requirements per unit of plant

biomass

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A common pattern found in ecosystem productivity is saturation curve.

Productivity increases linearly with N availability, up to a certain point, when other resources become limiting (e.g., light, water, temperature, other nutrients)

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Three types of relationships with respect to limitations of nutrients:

A. Production is independent of resource availabilityB. Production is a linear function of resource availabilityC. At some point, another resource becomes limiting

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Factors Influencing Nutrient Availability

• Presence of nitrogen fixers

• Microbial activity

• Fire

• Precipitation patterns

• Soil drainage

• Soil temperature, moisture

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FireH2O - Precipitation CO2

Organic soil

Upper mineral soil

Lower mineral soil Leaching, run off

Through-fall nutrients

Litterfall

Internal translocation

Dissolvednutrients

nutrients

GHG

N fixers

Energy,Nutrients

Microbes

Nutrients

CH4, CO2

N2, O2

PhotosynthesisAeolian,AtmosphericDeposition

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 Forest Type

LivingBiomass

Pool

PrimaryProduction

Rates

Soil Carbon/Nutrient

Pool

DecompositionRates

Tropical Highest Highest Lowest Highest

Temperate Middle Middle Middle Middle

Boreal Lowest Lowest Highest Lowest 

Boreal forest has the largest available nutrient pool in soil, but lowest rates of production, where as tropical forest has lowest soil pool, and highest production.

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Role of Disturbances in Nutrient Cycling

• Type of disturbance important– Fire versus logging versus large-scale mortality

• Disturbances directly alter biotic and abiotic controls on nutrient cycling– Rates of primary production

– Controls on evapotranspiration

– Influences on surface runoff

– Soil temperature/moisture decomposition rates

• Linkages between terrestrial/aquatic systems

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Hubbard Brook watershed, upstate New Hampshire.

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Nutrient Cycles

1. Nutrient requirements

2. Biogeochemical cycles

3. Rates of decomposition

4. Plant adaptations in low nutrient conditions

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Upland White Spruce Succession

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Nutrient Cycling in Upland White Spruce