ATMOS 397G Biogeochemical Cycles and Global Change Lecture 18: Nitrogen Cycle

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ATMOS 397GBiogeochemical Cycles and Global
ChangeLecture 18 Nitrogen Cycle

• Don Wuebbles
• Department of Atmospheric Sciences
• University of Illinois, Urbana, IL
• March 20, 2003

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Biospheric Processes that Transport Nutrients
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Simplified Ecosystem Nutrient Cycle Model. In the
example below only the primary production trophic
level is shown. Harvest by humans or other
animals occurs if the material is removed from
the site otherwise the consumed portion of
vegetation is returned to the system as fallout.
The circles represent ecosystem nutrient
reservoirs (storage compartments). Two
independent inputs are involved (1) the
nutrients provided through soil development from
the soil parent material (weathered rock) and (2)
nutrients contributed from the atmosphere to the
surface with precipitation or as dry atmospheric
deposition (dustfall). Nitrogen, for example, can
volatilize and then flow to and from the
atmosphere. To simplify the model, we can
consider respiration as a positive net input. The
ecosystem would collapse if the balance was
sustained as negative. All other transfers in the
model are set as annual rates (percent)
multiplied by the storage in the contributing
nutrient reservoir. Harvest, erosion, and
leaching represent losses to the local ecosystem.
In many managed ecosystems, there is additional
human input of nutrients through fertilization.
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The model above can be expressed as a series of
equations for each annual time step and for each
nutrient storage compartment in the simulation
model we use in this activity. L1 (L0 (B0
f) n r) - ((L0 d) L0 e). S1 (S0
(L0 d) w) - ((S0 u) S0 l). B1 (B0) (S0
u) - ((B0 f) B0 h). Where d decay, e
erosion, f fallout, h harvest, l
leaching, n nutrients applied, r respiration,
u uptake, w weathering, and for time period
1, L1 litter, S1 soil, B1 biomass. Loss rates
from a compartment cannot total more than 100.
If all nutrient storage compartments started (at
time 0) with 33 units and the transfer rates were
as follows d 0.9, e 0.05, f 0.05, h 0.0,
l 0.2, n 0, r 9, u 0.7, and w 0.01,
then at time 1 we would get L1 (33 (33
0.05) 0 9) - ((33 0.9) 33 0.05)
43.65 -31.35 12.3 S1 (33 ((33 0.9)
0.01) - ((33 0.7) 33 0.02) 35.71 - 29.7
6.01 B1 (33 (33 0.7) - ((33 0.1) 33
0.0) 56.1 - 3.3 52.8 In this example, the
high transfer rates result in a rapid adjustment,
i.e., it would take only a brief period before
the nutrient storage in the different
compartments would stabilize.
From t0 to t1
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Integrative Biosphere Models Comparison of Net
Carbon Storage During the 1980s
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Simplified Model of the Biosphere
From Scott Denning
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Integrated Biospheric Simulator (IBIS)
ATMOSPHERE (prescribed atmospheric datasets)
Vegetation Dynamics Module
Biomass Production GPP, total respiration, NPP
Land Surface Module
GPP, foliage respiration, CN ratios
Canopy Physics energy water balance,
aerodynamics
Aboveground Carbon Cycling
Soil Physics energy and water balance
Vegetation structure biomass
Belowground Carbon Nitrogen Cycling Module
Plant Physiology photos. leaf respiration,
stomatal conductance
Canopy nitrogen allocation
temperature, photosynthesis
Daily LAI
Vegetation Phenology Module budburst senescence
t minutes to hours
t years
t days to weeks
Adapted from Kucharik et al. (2000)
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Discussion Questions

• How would acid rain affect nutrient availability
  to an ecosystem/
• Do human activities (other than the contribution
  to acid rain) affect nutrient availability in
  ecosystems? If so, how?

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In an aging Picea abias stand in Russia, the
canopy becomes more open after 70 years, and
understory vegetation increases in volume and
importance in nitrogen cycling
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Ecologist view of nitrogen cycle