Nutrient cycling in ecosystems: Lecture Content

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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

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What dets comm structure, comp, distribution?
Big scale, big picture things.. E in system,
nutrients, solar radiation, rain.productivity.
Thermodynamics review Means that Sunnutrients
water plants Herbivores eat them, preds eat
them. E always lost w. each transformation from
one trophic level to next.. (What is missing?
(Omnis).
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• 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.

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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)?
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Pyramids of Energy tend to reflect pyramids of
numbers
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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

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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

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Generalized compartmental model of nutrient cycles
Sedimentary cycles (e.g., P)
Atmospheric cycles (e.g., N)
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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
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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 evaporation and
precipitation over sea, i.e., 425 - 385
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
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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)
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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 timepool
  size/flux) is due to how tightly N is held
  cycled by organisms there

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Nitrogen budget for forested watershed, Hubbard
Brook Experimental Forest (values in boxes are
pool sizes, kg/ha arrows give fluxes in kg/ha/yr)
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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

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Clear-cut watershed used to test hypotheses about
nutrient cycling by vegetation uptake
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Weir, or stream gauge for quantifying water flow,
stream chemistry in watershed experiments such as
Hubbard Brook
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Nutrient increases after clear-cutting in Hubbard
Brook streams
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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

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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)

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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)

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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?

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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)

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Chemical transformations in the nitrogen cycle
Note control by microbes, soil oxygen level
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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)--gteutrophication aquatic
  systems

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Major pools fluxes of phosphorus globally
(units in billions of metric tons g15)
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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

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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

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Acknowledgements Some illustrations for this
lecture from R.E. Ricklefs. 2001. The Economy
of Nature, 5th Edition. W.H. Freeman and
Company, New York.