Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems
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Transcript of Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems
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Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems
Robert E. RicklefsThe Economy of Nature, Fifth Edition
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Acid Rain and Forest GrowthDecline 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?
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Slow Recovery of Forests from Effects of Acid RainStudies 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
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Lessons from Hubbard BrookAcidity 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
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More Lessons from Hubbard BrookEffects 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.
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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
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Weathering of Ca2+ at Hubbard BrookWatershed 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.
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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
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Breakdown of Leaf LitterBreakdown 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
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Quality of Plant DetritusLitter 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 decayFungi play special roles in degrading resistant
materials: fungi especially capable of degrading cellulose, lignins
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MycorrhizaeMycorrhizae 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.
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Function of MycorrhizaeMycorrhizae 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
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Climate and Nutrient RegenerationNutrient 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.
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A Tropical ParadoxTropical 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
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Slash-and-Burn AgricultureCutting 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
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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
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Vegetation and Soil FertilityVegetation 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
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Soil versus Vegetation Stocks of NutrientsLitter 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
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Eutrophic and Oligotrophic SoilsTropics 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
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Nutrient Retention by VegetationRetention 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
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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
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Nutrients and Aquatic ProductivityProductivity 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.
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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
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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 surface
Stratification 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
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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
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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.
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Phosphorus and Trophic Status in LakesPhosphorus typically limits productivity in
freshwater systems: P is especially scarce in well-oxygenated
surface watersNatural lakes exhibit a wide range of
fertilities: productivity depends on:
external nutrient inputsinternal regeneration of nutrients
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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
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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
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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
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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
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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
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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.
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SummaryChemical 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.
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Summary: Terrestrial and Aquatic Systems
In terrestrial systems: ecosystem metabolism is mostly aerobic production is limited by regeneration of nutrients
from soilsIn 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