WK10: Communities & Ecosystems: Nutrient Cycling and Retention

Learning Objectives

  • Understand nutrient cycling: what it is, why it matters, and human impacts.
  • Describe the role of decomposition in nutrient cycling.
  • Describe factors driving changes in nutrient cycling (case studies).

Roadmap

  • Ecosystem part of the unit: nutrient dynamics.
  • Previous lecture: limits to productivity (bottom-up and top-down).
  • Today: movement of energy and materials through ecosystems.

Nutrient Flows and Productivity

  • Nutrients drive productivity.
  • Example: Wetland system.
    • Influx of nitrogen and phosphorus (fertilizers).
    • Algal blooms.
    • Changes in abiotic environment for macrophytes.
    • Changes in wetland structure and function (animals colonize or leave).
  • Nutrients at base of food web: large-scale, multi-level impacts.
  • Nutrient flow depends on the ecosystem type.
    • Pristine vs. agricultural forest.

Nutrients: Elements

  • Periodic table:
    • Red: 97% of body weight (H, C, N, O).
    • Dotted circle: Important (Na, K, Mg).
    • Blue: Trace elements (Cu, Zn).
    • Green: Possibly essential for some species.

Origin of Elements

  • Unlike energy from the Sun, elements are generated by stars.
  • Matter is neither created nor destroyed (thermodynamics).
  • Focus on pools (storage) and fluxes (movement).

Nutrient Definition

  • Elements required for development, maintenance, and reproduction.
  • Examples: phosphorus, carbon, nitrogen.
  • Used over and over.

Nutrient Cycling Example: Phosphorus

  • Phosphorus ion in a lake.
  • Diatom skeleton uptake.
  • Cladoserine eats diatom.
  • Minnow eats cladoserine.
  • Pike eats minnow.
  • Pike dies, phosphorus released.
  • Diatom reuptake.

Nutrient Cycling Defined

  • Use, transformation, movement, and reuse of nutrients in ecosystems.
  • Involves:
    • Storage in nutrient pools.
    • Nutrient flux (movement).
  • Ecological interest:
    • Factors controlling nutrient cycling rate (e.g., log decomposition).
  • Nutrient retention: How much stays vs. is lost.
  • Human impact: Radically changing nutrient cycling and productivity.

Why Care About Nutrient Cycling?

  • Monetary argument: Ecosystem services.
  • Ecologically mediated functional processes essential for biodiversity and humans.
  • Implications:
    • Public health.
    • Economy.
    • Human well-being.
  • Examples:
    • Clean air and drinking water (soil filtration, tree roots).
    • Greenhouse gas removal (plants).
    • Biomass (trees, fish, crops).
    • Pollination services (insects, mammals).
  • Nutrients underpin everything from monetary goods to biodiversity.

Nutrient-Specific Discussion

  • Phosphorus, nitrogen, and carbon.
  • Structure:
    • Diagram of pools and fluxes.
    • Description of pools and fluxes.
    • Importance of nutrient.
    • Examples of human influences.

Phosphorus Cycle

  • Pools (Teragrams 101210^{12} grams):
    • Marine sediments (massive).
    • Oceans (dissolved).
    • Mineable rock.
    • Land: organisms (moderate), soil (a lot).
  • Mostly a rock thing.
  • No substantial atmospheric pool.
  • Large quantities in mineral deposits and oceans/marine sediments.
  • A lot in soil, but much unavailable to plants (bound compounds); mycorrhizae help.
  • Importance:
    • ATP (energy molecule).
    • DNA, RNA.
  • Fluxes:
    • Inputs: weathering of rocks, decomposition, geological uplift.
    • Outputs: fixation (insoluble compounds), leaching.
    • Internal cycling: plant and microbial demand.
  • Human impact:
    • Experimental lakes in Canada (adding N vs. P).
    • Phosphorus is limiting in many systems.
    • Phosphate in laundry detergents (banned in 80s).
    • Phosphate-based fertilizers.
    • Soil disturbance during agriculture/forestry releases phosphorus.
    • Consequence: Eutrophication (often with nitrogen).

Nitrogen Cycle

  • Significant amount in oceans, soil, and atmosphere.
  • Fluxes: Denitrification (N to atmosphere), fixation (atmosphere N to usable form).
  • Nitrogen fixing bacteria do fixation.
  • Large atmospheric pool (80%), but inaccessible as a gas.
  • Oceans, plants, soil, organic matter are pools.
  • Amount available to organisms is very small.
  • Importance:
    • Amino acids and nucleic acids.
  • Fluxes:
    • Inputs: precipitation, nitrogen fixation.
    • Outputs: denitrification, leaching.
    • Internal: plant demand, denitrification, nitrogen fixation.
  • Nitrogen-fixing bacteria:
    • Free-living soil bacteria.
    • Rhizobia (legume roots).
  • Denitrification: Bacteria produce molecular nitrogen.
  • Nitrogen fixing bacteria: N2 to NH3 (usable form).
  • Rate-limiting step: Nitrogen fixation from the atmosphere.
  • Energetically demanding, anaerobic conditions.
  • Lightning can fix nitrogen.
  • All nitrogen in ecosystems is fixed to be used.
  • Human impacts:
    • Global nitrogen fixation over time.
    • Industrialization: Haber-Bosch process (N to NH3).
    • Requires high pressure/temperature.
    • Depletes soils, increases nitrous oxide.
    • Crop rotation (legumes).
    • Intensive agriculture, fossil fuel combustion, clear cutting.
    • Climate change, acid rain, eutrophication.
  • Hubbard Brook Experimental Forest: clear-cut basin example.
    • Clear cutting increases nitrate concentration in stream.
    • Nutrients not retained in deforested basin.
    • Changes ecosystem processes and leads to eutrophication.

Carbon Cycle

  • Major pools: oceans, soils, atmosphere.
  • Fluxes: plant respiration, soil respiration, photosynthesis.
  • Importance: essential part of all organic molecules.
  • Inputs: photosynthesis (CO2 to O2).
  • Outputs: respiration and decomposition.
  • Carbon recycled via the detritol decomposition loop.
  • Human impacts:
    • Fossil fuel burning, deforestation (climate change).
    • Need to reduce CO2 production.
    • Policy changes.
    • Alternative fuels.
    • Increase CO2 uptake (forest cover, reduce forest clearance).
    • Ocean fertilization experiments (add iron).
    • Carbon injection into depleted gas/oil reservoirs or geological features.

Take Homes

  • Humans have changed all nutrient cycles (P, N, C), typically increasing availability.
  • Eutrophication in freshwater systems.
  • Impacts on ecosystems and food webs.
  • Implications are poorly understood.
  • Hard to reverse radical changes.

Decomposition in Nutrient Cycling

  • Mineralization: conversion of nutrients from organic to inorganic forms (e.g., animal carcass to NH3NH_3).
  • Decomposition: breakdown of organic matter, releases CO2CO_2. Mineralization occurs through decomposition.
  • Detritus: mix of dead animal/plant remains and excretory products (previously alive stuff).
  • Traditionally, detritus viewed as unimportant in food web studies due to difficulty isolating it and lack of techniques to measure energetic importance.

Decomposition: Biological Sources

  • Bacteria and fungi: primary decomposers (terrestrial).
  • Impact energy and nutrient cycling.
  • Convert energy into more accessible forms.
  • Symbiotic bacteria (cow guts) break down cellulose.
  • Decomposers recycle essential chemical elements.
  • Without decomposition, life on earth would stop (too many corpses, too few nutrients).

Decomposition: Biological, Chemical, and Physical Processes

  • Biological: fungi, bacteria, invertebrates (fragment and digest).
  • Chemical/physical: leaching, fragmentation (abrasion, streamflow).
  • Plant detritus is a major food source, but plants are poor food source (high C:N, cellulose, toxins).
  • Most plant detritus consumed after decomposition.
  • River red gums example:
    • Physical/chemical: leaching (25% mass loss/day of soluble material).
    • Microbial colonization and decomposition: fungi and bacteria degrade cellulose.
    • Biological/physical: invertebrates eat detritus, abrasion, crushing by flow.

Trophic Subsidy

  • Wetland food web.

  • Sun -> Autotrophs (paraphyton) -> Herbivores -> Higher Predators.

  • Parallel pathway with terrestrial input.

  • Leaves -> Detritus -> Decomposers -> Detritivores -> Higher Predators.

Detritus Decomposition Rate

  • Influences availability and persistence of nutrients.
  • Typically follows negative exponential decay.
  • T50T_{50} (half-life): time at which 50% of mass is gone.
  • Rate depends on temperature, moisture, chemical composition, decomposer species.
  • Example: precipitation (Spain) - higher rainfall leads to higher decomposition rates.
  • Example: Lignin content (difficult to decompose) - higher lignin leads to lower decomposition rate.

Case Studies on Factors Influencing Nutrient Cycles

  • Biomanipulation: using organisms to manipulate ecosystems.

    • Example: Lake Food web: Phytoplankton > Zooplankton > Fish.

  • Excess nutrients -> phytoplankton overgrowth.

  • Goal: to reduce phytoplankton.

  • Reduce fish -> zooplankton increase -> phytoplankton consumption -> macrophytes grow back -> nutrients taken up by macrophytes -> water clarity.

  • Success rates varied (8 definite, 8 partial out of 18)

  • Removing planktivorous fish increase zooplankton and decrease algal biomass, improving water quality and allowing macrophytes to grow.

  • Promotes a stable clear water state.

  • Does not always work because their is no redundancy in the system. Also strong overriding bottom up effects can reverse process.

  • Invasion of a plant, Mirakafea in Hawaii.

    • Soils in Hawaii are volcanic in origin and low nitrogen.

  • Miracafea has a symbiosis with Frankia bacteria that allows it to fix nitrogen.

  • The amount of nitrogen that can be fixed is much greater with Miracafea.

    • Miracafea invades Hawaii, fixes nitrogen, increases nitrogen and available to the organisms in the ecosystem.
    • Alters trophic interactions and habitat structure.

  • Allows other plants to grow with the new increase nutrients that native plants, invasive plants, native earthworms, alien bird species feed on Miracafea.
    *So a bit of mess, a bit of a massive disturbance is created.

  • Meta Analysis, humans change nutrient cycles and pools by making more amount available. This increase carbon and nitrogen.
    Far reaching impacts on nutrient cycles of plant invasions.

Take Homes

  • Primary productivity drives food webs influenced by cycling and recycling of nutrients and of energy.
  • Carbon, nitrogen, phosphorus and all nutrients described in terms of the flows through biotic and abiotic pools.
  • Changes in these flows are evident due to human activities.
  • Decomposers crucial component of nutrient cycling in all ecosystems.
  • Changes in trophic structure and nutrient availability change nutrient cycles within ecosystems having impacts on ecosystem structure.