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Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems. Robert E. Ricklefs The 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.

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Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

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  1. Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems Robert E. Ricklefs The Economy of Nature, Fifth Edition (c) 2001 by W. H. Freeman and Company

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  35. 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

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