ATMOS 397G Biogeochemical Cycles and Global Change Lecture 15: Biosphere and Nutrients

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ATMOS 397GBiogeochemical Cycles and Global
ChangeLecture 15 Biosphere and Nutrients

• Don Wuebbles
• Department of Atmospheric Sciences
• University of Illinois, Urbana, IL
• March 18, 2003

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The Biosphere (AVHRR measurements)
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The Biosphere over Time
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Leafing Dates of Oak (1746present) - This graph
shows how the leafing dates of oaks in
southeastern England have changed over the past
256 years.
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Terrestrial Ecosystems

• C/N in leaf tissue ? 50
• NPP globally 60 x 1015 gC/yr ? 1.2 x 1015 gN
  needed each year
• N and P are often limited ? supply of these
  elements may control NPP
• Nutrients in greater quantities, e.g., Ca and S,
  have NPP determine their rate of cycling in
  ecosystems and losses to streamwaters
• The atmosphere is the major source of C, N, and S
  in terrestrial ecosystems
• Rock weathering is the major source for most
  remaining biochemical elements, e.g., Ca, Mg, K,
  Fe, P

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detailed overlay of N deposition patterns with
ecosystem types was key to predicting possible C
storage. Where high N deposition occurs on
forested systems, there is a potential for
significant C storage because forest vegetation
has large CN ratios and long tissue lifetimes in
wood. Thus, a map of modeled C uptake using
spatially defined estimates of fossil fuel N
deposition suggested potential carbon sink
hotspots in the mid-latitude forests of the
northern hemispere, while at the same time showed
that high deposition regions over grassland or
agricultural areas, such as the Great Plains,
were not likely to produce much carbon storage
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Nutrient Intrasystem Cycling

• Cycling of N within an ecosystem is often 10 to
  20X greater than the amount received from
  outside.
• Soil chemical reactions (ion exchange, mineral
  solubility) set constraints for plant uptake of
  essential elements
• Plants can release organic compounds that enhance
  solubility
• Uptake of N and P is so rapid, and soil
  concentration so low, that there is often none of
  these in the vicinity of roots.
• Diffusion of P is slow and limits supply
• Plants respond by increasing root/shoot ratio
• Some plants respond by putting out enzymes to
  extract nutrients

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Nutrients Needed for Growing Corn
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Nutrient Uptake by Wheat
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Nutrient Balance

• Plant growth is affected by the balance of
  nutrients in soil
• Some trees 100 N15 P 50 K 5 Ca 5 Mg 10 S
• More nutrients occur as positively charged ions
  in the soil solution
• Plants will often release H to maintain balance
  of charge
• Plants using NH4 as N source tend to acidify the
  immediate zone around their roots (NO3- uptake
  has opposite effect)

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Nutrient Availability Depends on pH of the Soil
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Nitrogen Assimilation in Roots of Plants
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Nitrogen Assimilation and Nitrogen Fixation

• Availability of NH4 or NO3- depends on
  environmental conditions
• Waterlogged soils NH4
• Desert conditions NO3-
• Most species show preference for NO3- even though
  NH4 is assimilated easier
• NH4 reacts easier in the soil
• Rate of delivery of NO3- to roots is higher
• Nitrification vs. denitrification

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Nitrogen in the Plant
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Nitrogen Fixation in Trees

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• Inorganic nitrogen cycle
• no nitrogen is found in native rock
• the ultimate source of nitrogen for ecosystems is
  molecular nitrogen (N2) in the atmosphere (78.1
  by volume)
• N2 may dissolve in water
• virtually all nitrogen would occur as N2 if not
  for biological processes occurring in the
  presence of oxygen
• Molecular nitrogen enters biological pathways
  through nitrogen fixation by certain
  microorganisms

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