Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

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Chapter 8 Nutritional Regeneration in
Terrestrial and Aquatic Ecosystems

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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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?

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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

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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

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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.

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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 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-1
• Total weathering of bedrock to offset Ca2 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.
• gt90 of plant biomass enters detritus pool

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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 energy
• macerate detritus, speeding microbial activity
• breakdown of woody components by fungi
• decomposition of residue by bacteria

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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

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Mycorrhizae

• Mycorrhizae are mutualistic associations of fungi
  and plant roots
• endomycorrhizae - fungus penetrates into root
  tissue
• ectomycorrhizae - fungus forms sheath around root
• Mycorrhizae facilitate nutrient extraction from
  nutrient-poor soils, enhancing plant production.

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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

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Climate and Nutrient Regeneration

• Nutrient cycling is affected by climate
• temperate and tropical ecosystems differ because
  of effects of climate on
• weathering
• soil properties
• decomposition of detritus
• In temperate soils, organic matter provides a
  persistent supply of mineral elements released
  slowly by decomposition.

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A Tropical Paradox

• Tropical forests are highly productive in spite
  of infertile soils
• tropical soils are typically
• deeply weathered
• have little clay
• do not retain nutrients well
• high productivity is supported by
• rapid regeneration of nutrients form detritus
• rapid uptake of nutrients
• efficient retention of nutrients by
  plants/mycorrhizae

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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

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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
  quality
• Population 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 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 flow
• 3- to 20-fold increase in cation losses
• shift 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 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 forests
• 5 in temperate hardwood forests
• 1-2 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 Soils

• Tropics have both rich and poor soils
• eutrophic (rich) soils develop in geologically
  active areas with young soils where
• erosion is high
• rapid 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 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

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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 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.

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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 stratification
• also 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 ocean
• silicon 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 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 inputs
• internal 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 winds
• occasional 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 sewage
• drainage 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
• organisms
• particulate detritus
• dissolved 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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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.

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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 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