Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Ecosystems