Nutrient cycling in ecosystems: Lecture Content • Introduction to nutrient cycles • Driving forces for nutrient cycles in ecosystems • Water (hydrological) cycle as a physical model of nutrient cycling • Case study of N, Ca limitation: Hubbard Brook Experimental Forest, NH • Major nutrient cycles & their pool sizes, transfer rates, control mechanisms, human impacts • Nitrogen • Phosphorus
What dets comm structure, comp, distribution? Big scale, big picture things….. E in system, nutrients, solar radiation, rain…….=productivity. Thermodynamics review: Means that Sun+nutrients + water = plants Herbivores eat them, preds eat them……. E always lost w. each transformation from one trophic level to next….. (What is missing? (Omnis).
Raymond Lindeman: ecosystems are systems that transform energy. • This transformation and transfer of energy from one trophic level to the next (feeding level) is inefficient so some energy is lost at each level.
How it is lost, Primary production –Unabsorbed energy given off as heat. Photosynthesis, Respiration Secondary production – wastage (bones, stems, uneaten material, ie. Production and Consuption efficiencies), heat Trophic level transfer efficiency is around 10%. What OTHER very important trophic level receives lots of available energy due to inefficiency of primary producers and secondary producers (consumers)?
What are the limits, determinants of primary production (see biome lecture!) Secondary productivity- well…. Primary productivity • Nutrients, unlike energy, are not constantly renewed and used up
Introduction to nutrient cycling • They are cycled, between organic (living) and inorganic pools (and among organic and inorganic ones) • Movement, or cycling, of nutrients requires (ultimately) energy input into ecosystems, e.g., to initiate chemical reactions • We will focus on particular nutrients in this lecture, to try and understand those that are most critical to ecosystem function • One way to understand nutrient dynamics is to use compartmental model to identify both the pools (organic and inorganic) and the fluxes between pools
Sedimentary cycles (e.g., P) Generalized compartmental model of nutrient cycles Atmospheric cycles (e.g., N)
To see the coupling of nutrient cycling and energy, consider a simple redox chemical reaction: Energy releasing reaction is paired with energy requiring one; oxidation side must release more energy than reduction side requires; rest lost as heat Assimilatory reactions (e.g., photosynthesis) incorporate inorganic forms of nutrients (e.g., carbon) into organic forms (e.g., carbohydrates); dissimilatory rxns. the reverse
Represents difference between evaporation and precipitation over sea, i.e., 425 - 385 Global hydrological cycle drives other cycles (units g18 = teratons (tt) = 1012 metric tons for pools (dark blue). Fluxes in light blue, units of tt/yr. Represents difference between precipitation, & evaporation, i.e., 111 - 71 25% of total solar radiation on Earth used to drive hydrological cycle! 97% of global H2O pool in oceans
Which nutrient cycles to study? Those that are most limiting to plants (& thus ecosystems), i.e., N, P, S, sometimes Ca because demands high relative to supply (soils, lakes, oceans)
Case study: N, Ca limitation in Hubbard Brook Experimental Forest, NH • Simplified nitrogen budget for Hubbard Brook (temperate deciduous forest--northern hardwoods) • Inputs via bulk precipitation (hydrological cycle) & net nitrogen fixation by soil bacteria • Outputs via stream water, & by denitrification • Internal transfers of N are small relative to pool sizes, which is typical of limiting nutrients • Mineralization (chemical, dissimilatory reactions that convert nutrient from organic to inorganic form) is slow in Hubbard Brook soils • Low movement of N (low turnover time=pool size/flux) is due to how tightly N is held & cycled by organisms there
Nitrogen budget for forested watershed, Hubbard Brook Experimental Forest (values in boxes are pool sizes, kg/ha; arrows give fluxes in kg/ha/yr)
The hypothesis that nutrients like N tightly held, tightly cycled was tested experimentally at Hubbard Brook • Methods: Entire watershed (water-catchment basin, defined by topography) was cut, harvested (to disrupt biological activity • Tree re-growth suppressed; Nutrient inputs, outputs measured (precipitation gauges, stream gauges) • Bedrock “tight” at Hubbard Brook (no groundwater losses), allowing all outputs to be measured in streams • Results? • N, P, Ca increased dramatically in streamwater because of inhibited biological uptake • Why? Increased streamflow (40%), less plant uptake • Implications? Forest clearcutting destructive
Clear-cut watershed used to test hypotheses about nutrient cycling by vegetation uptake
Weir, or stream gauge for quantifying water flow, stream chemistry in watershed experiments such as Hubbard Brook
Hubbard Brook Study also important to understand effects of acid precipitation on forest dynamics, health • Acid precipitation (low pH of rain, snow) caused by human activities • Combustion of fossil fuels, other industrial processes put nitrous oxides, sulfur oxides in atmosphere, which react with water to form nitric, sulfuric acids • Acidity could affect plants, animals both directly (acid burns) or indirectly (altered soil nutrient availability) • Which was important at Hubbard Brook? • Long-term studies show importance of indirect effects
Long-term recovery from acid precipitation, Hubbard Brook, slow Clean-air Act, 1970 • Factors preventing recovery of ecosystem after Clean-air Act? • Sulfur emissions remained high (fossil fuels not controlled enough) • Particulate emissions dropped, but this reduced Ca inputs in rain! • Long-term leaching of Ca from soils via hydronium ions (attaching to clay particles in soil) • Ca in tree tissues has dropped, causing widespread forest die-back (spruce, sugar maple)
Lessons from Hubbard Brook studies • Nutrient limitation, dynamics illustrated by descriptive (compartmental models) and experimental methods • Trees died because of indirect effects, which are difficult to quantify and demonstrate • Natural recovery of acid-damaged ecosystem does take place, but estimated to be slow (centuries) for nutrient restoration (depends on flux rates) • Nutrient dynamics, regeneration processes important to understand ecosystem processes, effects of human impacts • Regeneration in terrestrial ecosystem via soil processes: microbial activity in detritus food chains (e.g., N), bedrock weathering (Ca, P)
Nutrient cycles & their controls • Things to notice: • What are major inorganic sources? • How many chemical forms of nutrients? • What aspects of physical, biological environment determine the transformations (fluxes)? • What limits the availability of these nutrients in terrestrial and aquatic ecosystems?
Summary of the nitrogen cycle • Ultimate source is atmosphere (huge gas pool) • Proximate sources are nitrogen-fixation and lightning • Nitrogen fixation is important in variety of ecosystems, but barely offsets N-losses due to denitrification • Oxygen (oxidation potential) determines which reactions in cycle are important (via microbes) • N occurs in many forms because of many oxidation states (it can act as oxidizing agent or reducing agent) • Regeneration in soils via decomposition organic matter; in H2O via mixing of nutrient-rich sediments • Humans add as much N to global ecosystem (fertilizer) as combined natural causes, leading to eutrophication (increased 1º production)
Chemical transformations in the nitrogen cycle: Note control by microbes, & soil oxygen level
P cycle also of great biological importance • Phosphorus cycle relatively simple chemically, due to fewer oxidation states (plants uptake primarily PO4 3-) • Large inorganic pools in soils, bedrock, ocean sediments • Control of availability to organisms complex • At low pH, P unavailable by binding to clay, Fe, Al in soil • Also unavailable at high pH • Mycorrhizae important scavenging P from soils • In high-O2 systems, P precipitates out of water, constituting constant rate of loss from ecosystems • Rock weathering, soil decomposition make P available • Humans contribute some P to global ecosystems via fertilizers (& runoff)-->eutrophication aquatic systems
Major pools & fluxes of phosphorus globally (units in billions of metric tons = g15)
Carbon cycle is of great importance to humans • Three classes of processes cause C cycling: • Assimilatory, dissimilatory reactions involve living things • Exchange of CO2 between atmosphere, oceans • Sedimentation, precipitation of carbonates in water (limestone, dolomite) • CaCO3 (insoluble) + H2O + CO2 Ca2+ + 2HCO3- (insoluble) • Uptake of CO2 by plants, corals pushes reaction to left, causes Calcium Carbonate sedimentation (or in case of corals, deposition into reef-building structures • Human impacts on carbon cycle (see next lecture): • Consumption of fossil fuels increases atmospheric CO2 • Global warming causes increased plant uptake, but even greater release of C (decomposition) from tundras
Conclusions: • Energy (sunlight) ultimately required for chemical circulation (e.g., water movement), transformations • Hubbard Brook Experimental Forest studies show some factors controlling cycling, availability of N, Ca • Different nutrient cycles are very different in terms of the pools, fluxes, interaction with biological organisms, and impacts of humans • Humans are causing global changes in N, C, P cycles, among others, that are altering the biosphere
Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.