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Biogeochemical cycles
& global change
Despite uncertainties about
details of human impact on
global system, there are
-Changes in the N cycle
-Changes in atmospheric CO2
-Changes in land use
Units appropriate for global-scale studies
teragram = tg = 1012g
petagram = pg = 1015 g = 109 t = 1 gt
Humans have long sought to alter biogeochemistry.
Sheep were used for this purpose in Medieval Europe.
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.
Typical, nonfertilized Calluna heathland in The Netherlands.
Photographed in 1977. Note bump of trees in background.
Repeat photograph in 1983 (six years later) with Calluna
mostly gone. Note bump of trees in background.
Repeat photograph from 1987. Note bump of trees. Calluna
entirely replaced by same grass as dominated on fertilized field.
Units = km3
The global hydrologic cycle is critical for understanding
biogeochemistry because many elements move in water.
Units =
Global nitrate levels have increased over the last 40 years.
Changes in the global nitrogen cycle:
- 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
Experiments illustrate impact
of nitrogen addition on
community diversity.
Global carbon cycle – Humans are increasing global CO2
Human activity has increased atmospheric CO2 from ~280 to
~360 ppm since 1800.
Current high CO2 levels are unique in the last 160,000 years
The increase in
atmospheric CO2 is
derived from fossil
fuels (the primary
source of carbon low
in 14C).
FF(6) + Veg (0.9) = Atm(3.2) + Ocean(2.0) + Unk(1.7) [PgC/yr]
Global warming is real. The most recent years have
been higher than any shown on the graph.
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
Increases contribute
to acid rain.
Sodium pool sizes and transfers
Na into sea during 500 million yrs =
119.4 x 1015 t
Current dissolved Na =
Deep sea sediments =
Sub ocean sediments =
old sedimentary rocks =
Rock salt reserves =
Total =
75% of 119.4 is missing
Land Use Changes
Total Terrestrial Photosynthesis
NPP used by humans
Consumed by people
Consumed by domestic animals
Wood used
NPP dominated by humans, but not used
Croplands, converted pastures
Plantations, land clearing
NPP lost to human activity
Decreased NPP of cropland vs natural
122 pg/yr
5.2 (4%)
41 (31%)
17 (8%)
Summary Points - 1
- Humans have significantly altered global biogeochemical
- 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
- 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.
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.