Biogeochemical cycles

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Biogeochemical cycles global change
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• Despite uncertainties about details of human
  impact on global system, there are certainties!
• Changes in the N cycle
• Changes in atmospheric CO2
• Changes in land use

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Units appropriate for global-scale
studies teragram tg 1012g petagram pg
1015 g 109 t 1 gt
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Humans have long sought to alter biogeochemistry.
Sheep were used for this purpose in Medieval
Europe.
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Application of manure for over a period of
centuries provided fertile lands at the expense
of the rest of the landscape. Note the
grass-dominated foreground.
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Typical, nonfertilized Calluna heathland in The
Netherlands. Photographed in 1977. Note bump of
trees in background.
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Repeat photograph in 1983 (six years later) with
Calluna mostly gone. Note bump of trees in
background.
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Repeat photograph from 1987. Note bump of trees.
Calluna entirely replaced by same grass as
dominated on fertilized field.
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Units km3
The global hydrologic cycle is critical for
understanding biogeochemistry because many
elements move in water.
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Units teragrams
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Global nitrate levels have increased over the
last 40 years.
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• Changes in the global nitrogen cycle
  Implications
• - In atmosphere NO2 ? NO O O2 O ? O3
• Various oxidants harm plant growth
• Could be responsible for tree mortality
• Dysfunctional plant carbon allocation
• NO3 is a big component of acid rain, along with
  SO4
• Distribution NW Europe, E North America
• Leaching of calcium from soils
• Death of lakes
• Al toxicity
• Appears to have killed thousands of lakes on
  granite areas
• Could have leaching effects on soils
• Could be major component in tree death
• - Altered community composition loss of
  diversity

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Experiments illustrate impact of nitrogen
addition on community diversity.
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Global carbon cycle Humans are increasing
global CO2
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Human activity has increased atmospheric CO2 from
280 to 360 ppm since 1800.
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Current high CO2 levels are unique in the last
160,000 years
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The increase in atmospheric CO2 is derived from
fossil fuels (the primary source of carbon low
in 14C).
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FF(6) Veg (0.9) Atm(3.2) Ocean(2.0)
Unk(1.7) PgC/yr
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Global warming is real. The most recent years
have been higher than any shown on the graph.
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Consequences of increased CO2 Some extreme
projections show increase in temp by 5 C by 2100,
with some melting of polar ice, increase in sea
level by 6-8m, and increased droughts in central
US, Eastern Europe, and adjacent Asia. What is
certain is that we should be cautious. Do not
know what we are getting in for. - Change in
rain temp - Change in sea level - Change in
species distributions - Rapid migration in a
fragmented landscape
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Increases contribute to acid rain.
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Sodium pool sizes and transfers Na into sea
during 500 million yrs 119.4 x 1015
t Current dissolved Na 14.1 Deep sea
sediments 5.1 Sub ocean sediments
5.4 old sedimentary rocks 2.6 Rock salt
reserves 0.4 Total 27.6 75 of
119.4 is missing
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Land Use Changes Total Terrestrial
Photosynthesis 122 pg/yr NPP used by
humans Consumed by people 0.8 Consumed by
domestic animals 2.2 Wood used 2.4 Total
5.2 (4) NPP dominated by humans, but not
used Croplands, converted pastures Plantations
, land clearing Total 41 (31) NPP lost to
human activity Decreased NPP of cropland vs
natural 10 Desertification 4.5 Occupied
2.6 Total 17 (8)
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• Summary Points - 1
• - Humans have significantly altered global
  biogeochemical cycles.
• Nitrogen fixation has been more than doubled by
  human activities. Negative impacts of enhanced
  atmospheric nitrogen deposition include acid
  precipitation, global warming, lake
  acidification, alteration in carbon allocation in
  plants, change in community composition, and loss
  of biodiversity.
• Atmospheric CO2 has increased from 280 ppm to
  360 ppm over the last 200 years. Much of this
  increase is attributable to burning of fossil
  fuels. The increase in CO2 is causing an
  increase in global temperatures.

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• Summary Points - 2
• - Phosphorus mining has resulted in increases in
  phosphorus export in world rivers by a factor of
  two.
• Ratios of elements in the oceans do not match
  the ratios in stream water flowing into the
  oceans. To understand the differences it is
  necessary to consider, among other factors, the
  rates of deposition in sediments, movement as
  aerosols back to land, and movement and erosion
  of rock.
• Approximately 40 of the potential net primary
  photosynthesis of the earth is now within human
  dominated ecosystems systems.