(c) 2001 by W. H. Freeman and Company Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic...

(c) 2001 by W. H. Freeman and Company

Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

Robert E. RicklefsThe Economy of Nature, Fifth Edition


Acid Rain and Forest Growth

Decline in forests, noted in northeastern US and central Europe in the 1960s, appeared correlated with acid rain.

The Clean Air Act of 1970 reduced emissions of sulfur oxides and particulates in the US.

Forests did not show signs of recovery. Why?


Slow Recovery of Forests from Effects of Acid Rain

Studies at Hubbard Brook Experimental Forest in New Hampshire showed why forests did not recover after passage of Clean Air Act: acidity of rain declined slowly emissions of particulates declined, reducing an

important source of calcium at Hubbard Brook leaching of calcium and other nutrients by

acid rain left lasting effects on soil fertility


Lessons from Hubbard Brook

Acidity itself is not the cause of tree death: long-term leaching of nutrients kills trees

Natural recovery will be slow on nutrient-poor soils: restoration of nutrients will require

weathering weathering is a slow process


More Lessons from Hubbard Brook

Effects of acid rain on soils may remain for years, even if causes of the problem are addressed.

Understanding nutrient cycling and regeneration is crucial to understanding ecosystem function.


Nutrient regeneration occurs in soils.

Nutrients are added to the soil through weathering of bedrock or other parent material.

How fast does such weathering occur? estimates can be made for positive ions

such as Ca2+, K+, Na+, and Mg2+

at equilibrium, net losses must be balanced by replenishment from weathering


Weathering of Ca2+ at Hubbard Brook

Watershed budgets: precipitation inputs = 2 kg ha-1 yr-1

streamflow losses = 14 kg ha-1 yr-1

assimilation by vegetation = 9 kg ha-1 yr-1

net removal thus = 21 kg ha-1 yr-1Total weathering of bedrock to offset Ca+2

losses is 1,500 kg ha-1 yr-1 or 1 mm depth.Later analyses showed this to be an

overestimate; the system was not in equilibrium.


Quality of detritus influences the rate of nutrient regeneration.

Weathering is insufficient to supply plants with essential elements (Ca, Mg, K, Na, N, P, S, etc.) at the rates required.

Rapid regeneration of these elements from detritus is essential for ecosystem function.

In forests, detritus is abundant: includes plant debris, animal excreta, etc. >90% of plant biomass enters detritus pool


Breakdown of Leaf Litter

Breakdown is a complex process: leaching of soluble minerals:

10-30% of substances in leaves are water-soluble

consumption by large detritivores:assimilate 3-40% of energymacerate detritus, speeding microbial activity

breakdown of woody components by fungi decomposition of residue by bacteria


Quality of Plant Detritus

Litter of various species decays at different rates: weight loss in 1 yr for broadleaved species varied from

21% for beech to 64% for mulberry needles of pines and other conifers decompose slowly resistance to decay is largely a function of composition,

especially lignins, which resist decay

Fungi play special roles in degrading resistant materials: fungi especially capable of degrading cellulose, lignins


Mycorrhizae

Mycorrhizae are mutualistic associations of fungi and plant roots: endomycorrhizae - fungus penetrates

into root tissue ectomycorrhizae - fungus forms

sheath around rootMycorrhizae facilitate nutrient

extraction from nutrient-poor soils, enhancing plant production.


Function of Mycorrhizae

Mycorrhizae are effective at extracting nutrients: penetrate larger volume of soil than roots alone secrete enzymes and acids, which extract nutrients

Endomycorrhizae are associated with most plants: apparently an ancient association fungi are specialists at extracting phosphorus

Ectomycorrhizae are also widespread: sheath stores nutrients and carbon compounds fungi consume substantial amount of net production


Climate and Nutrient Regeneration

Nutrient cycling is affected by climate: temperate and tropical ecosystems differ

because of effects of climate on:weatheringsoil propertiesdecomposition of detritus

In temperate soils, organic matter provides a persistent supply of mineral elements released slowly by decomposition.


A Tropical Paradox

Tropical forests are highly productive in spite of infertile soils: tropical soils are typically:

deeply weatheredhave little claydo not retain nutrients well

high productivity is supported by:rapid regeneration of nutrients form detritusrapid uptake of nutrientsefficient retention of nutrients by plants/mycorrhizae


Slash-and-Burn Agriculture

Cutting and burning of vegetation initiates the cycle: nutrients are released from felled and burned

vegetation 2-3 years of crop growth possible fertility rapidly declines as nutrients are

leached upward movement of water draws iron and

aluminum oxides upward, resulting in laterite


Is Slash-and-burn sustainable?

Traditional agriculture is sustainable: 2-3 years of cropping depletes soil 50-100 years of forest regeneration

rebuilds soil qualityPopulation pressures lead to

acceleration of the cycle: soils are insufficiently replenished soils deteriorate rapidly, requiring

expensive fertilizer subsidies


Vegetation and Soil Fertility

Vegetation is critical to development and maintenance of soil fertility: clear-cutting of an experimental watershed

at Hubbard Brook, NH resulted in:several-fold increase in stream flow3- to 20-fold increase in cation lossesshift from nitrogen storage to massive nitrogen

loss:• uncut system gained 1-3 kg N ha-1 yr-1

• clear-cut system lost 54 kg N ha-1 yr-1


Soil versus Vegetation Stocks of Nutrients

Litter and other detritus do not form a large reserve of nutrients in the tropics: forest floor litter as percentage of

vegetation plus detritus:20% in temperate needle-leaved forests5% in temperate hardwood forests1-2% in tropical forests

soil to biomass ratio for phosphorus in forests is 23.1 in Belgium, 0.1 in Ghana


Eutrophic and Oligotrophic Soils

Tropics have both rich and poor soils: eutrophic (rich) soils develop in

geologically active areas with young soils where:erosion is highrapid weathering of bedrock adds nutrients

oligotrophic (poor) soils develop in old, geologically stable areas with old soils where:intense weathering of soils removes clay and

reduces storage capacity for nutrients


Nutrient Retention by Vegetation

Retention of nutrients by vegetation is crucial to sustained productivity in tropics.

Plants retain nutrients by: retaining leaves withdrawing nutrients before leaves are

dropped developing dense root mats near soil

surface


Nutrients are regenerated from aquatic sediments.

Soils and aquatic sediments share similar regenerative processes (both processes occur in aqueous medium).

Soils and aquatic sediments differ in two profound ways: release of nutrients in soils occurs near

plant roots in soils, far from roots in sediments

release of nutrients is aerobic in soils, anaerobic in aquatic sediments


Nutrients and Aquatic Productivity

Productivity in aquatic systems is stimulated when nutrients are in the photic zone, resulting from proximity to bottom sediments upwelling of nutrient-rich water

Regeneration of nutrients by excretion and decomposition may take place within the water column.

Sedimentation represents a continual drain on nutrients within the water column.


Thermal stratification hinders vertical mixing.

Vertical mixing is critical to replenishment of surface waters with nutrients from below: results from turbulent mixing driven by wind impeded by vertical density stratification:

may be caused by thermal stratificationalso occurs when fresh water floats over denser salt water

Vertical mixing has positive and negative effects on productivity: nutrients brought from depths stimulate productivity phytoplankton may be carried below photic zone


Stratification inhibits production.

Thermal stratification in temperate lakes: nutrients regenerated in deeper waters cannot

reach the surface vertical mixing in fall brings nutrient-rich water

to the surfaceStratification in other aquatic systems:

arctic/subarctic and tropical lakes are not thermally stratified and mix freely

in marine systems, stratified and non-stratified water bodies may meet, stimulating production


Nutrients limit production in the oceans.

Primary production of marine ecosystems is tied closely to nutrient supplies: nitrogen is especially limiting shallow seas and areas of upwelling are

especially productive some areas of open ocean are unproductive,

despite adequate nitrogen and phosphorus:iron may be limiting in some areas of open oceansilicon may also be limiting, especially for diatoms


Oxygen depletion facilitates nutrient regeneration.

Nutrient regeneration is facilitated as anoxic conditions develop in hypolimnion and sediments of stratified temperate lakes: nitrification ceases, leading to accumulation of

ammonia iron is reduced from Fe3+ to Fe2+

insoluble iron-phosphorus complexes are solubilized, releasing iron and phosphorus

These processes reverse when oxidizing conditions return during fall overturn.


Phosphorus and Trophic Status in Lakes

Phosphorus typically limits productivity in freshwater systems: P is especially scarce in well-oxygenated

surface waters

Natural lakes exhibit a wide range of fertilities: productivity depends on:

external nutrient inputsinternal regeneration of nutrients


Temperate lakes exhibit varied degrees of mixing.

Productivity depends in part on degree of mixing of surface and deeper waters: shallow lakes may lack hypolimnion and

circulate continuously somewhat deeper lakes stratify sporadically,

with periods of mixing caused by:strong windsoccasional cold weather in summer

deepest lakes rarely mix completely, so productivity depends on external nutrient sources


Productivity varies in temperate lakes.

Lakes may be classified on a continuum from oligotrophic to eutrophic. oligotrophic lakes are nutrient-limited and

unproductive naturally eutrophic lakes exist in a well-

nourished and productive dynamic steady-state human activities can lead to inappropriate

nutrient loading resulting from:inputs of sewagedrainage from fertilized agricultural lands


Cultural eutrophication of lakes is harmful.

Nutrients stimulate primary production.Production is not inherently harmful, but:

biomass accumulates, overwhelming natural regenerative processes

untreated sewage also increases the amount of organic material in water

increased biological oxygen demand depletes oxygen, killing fish and other obligate aerobes


Estuaries and marshes are highly productive.

Shallow estuaries and salt marshes are among the most productive ecosystems on earth.

High production in these systems results from: rapid and local regeneration of nutrients external loading of nutrients


Marshes and estuaries export their production.

Adjacent marine ecosystems benefit from export of production from marshes and estuaries. For example, a Georgia salt marsh exported nearly 50%

of its net primary production to surrounding marine systems in the form of:organismsparticulate detritusdissolved organic material


Marshes and estuaries are critical to functioning of marine ecosystems.

Marshes and estuaries are important feeding areas for larval and immature stages of fish and invertebrates, providing: hiding places high productivity

These organisms later complete their life cycles in the sea.


Summary

Chemical and biochemical transformations are modified by physical and chemical conditions in each type of ecosystem.

Pathways of elements in ecosystems reflect patterns of nutrient cycling.


Summary: Terrestrial and Aquatic Systems

In terrestrial systems: ecosystem metabolism is mostly aerobic production is limited by regeneration of nutrients

from soils

In aquatic systems: anaerobic respiration and regeneration of nutrients

occurs in sediments, far from producers local regeneration of nutrients occurs in water

column productivity is ultimately limited by regeneration of

nutrients from deeper waters

(c) 2001 by W. H. Freeman and Company Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic...

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Transcript of (c) 2001 by W. H. Freeman and Company Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic...