Modeling of Environmental Systems
• The next portion of this course will examine the
balance / flows / cycling of three quantities that are
present in ecosystems:
– Energy
– Water
– Nutrients
• We will look at each of these at two scales:
– Global
– Ecosystem
• Before we can build models of these phenomena, we
need to have some background on the functioning
of these systems with respect to these quantities
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Global Cycling of Nutrients
• In addition to the movement of water, there are other
forms of matter that are critical to sustaining
ecosystem function that move through the
environment in a cyclical manner
• A few elements of critical importance, which we
refer to as nutrients, include:
–
–
–
–
Nitrogen
Phosphorus
(Potassium, Sulfur, Magnesium, and Calcium)
Additionally, the movement of carbon is also quite
important, but because of carbon’s relative abundance, we
usually do not refer to it as a nutrient, but rather as
substrate or some similar term
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Nitrogen Cycle
• Organisms require nitrogen to live: While carbon is
a much more common element in tissues, nitrogen is
required as well
• Nitrogen is required by the enzymes which mediate
the biochemical reactions in which carbon is fixed in
photosynthesis and oxidized in respiration
• In plant tissues, the ratio of carbon:nitrogen is
around 50:1, and if nitrogen is not available in this
relative abundance to carbon, it can be the limiting
factor in ecosystem productivity
• On the global scale, nitrogen limits the rate of net
primary production on land and in the sea
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Forms of Nitrogen in the Environment
• In the gross sense, nitrogen is very common on
Earth: About 78% of the atmosphere is nitrogen in
its inorganic molecular form as N2
• However, most organisms are incapable of making
use of nitrogen in this form, so instead most
organisms need to gain access to nitrogen that are
part of several other chemical species, such as NH4+,
NO, N2O, NO2- , NO3• An important part of the global cycling of nitrogen is
its transformation between these various forms
– Certain organisms, particular microbes, subsist on the
energy gained through various transformations of
nitrogen
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Microbial Transformations of Nitrogen
Wollast, R. 1981. Interactions between major biogeochemical cycles in marine
Ecosystems, pp. 125-142 in G.E. Likens (Ed.), Some Perspectives of the Major
Biogeochemical Cycles. Wiley, New York.
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Nitrogen Transformations
• There are two sorts of transformations that are of
particular importance:
• Nitrogen fixation converts nitrogen from its
inorganic molecular form (an ‘even’ form) to one of
the organic (‘odd’) forms, e.g.
– A certain type of bacteria might fix molecular nitrogen to
ammonia: N2 Æ NH3
• The opposite sort of process also occurs, where
organic (‘odd’) forms of nitrogen are converted back
to inorganic (‘even’) molecular nitrogen, and this is
called denitrification, e.g.
– Another sort of bacteria might denitrify nitrate or nitrite:
NO2- or NO3- Æ N2
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Nitrogen Cycle
Atmosphere (3.9x109)
Lightning
Fixation
<= 5
100
Biological
Fixation
140
Denitrification
110
Denitrification
<= 200
Human
Activity
Biological
Fixation
15
River Flow
36
30
1200
Land
(3.5x103 in terrestrial biomass)
(95 to140x103in soil organic matter)
8000
Ocean (5.7x105)
• Flow units are 1012 g N/year, stores are (1012 g N)
• Transformation processes: Fixation Æ
Denitrification Æ
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Nitrogen Cycle
• Plants can only make use of nitrogen as NH4+ & NO3• Because of the strength of the triple bond between the
pair of nitrogen atoms in inorganic molecular
nitrogen (N2), this chemical species is practical inert
• All of the nitrogen that is available to biota was
made available through nitrogen fixation -- either by
lightning fixation ( <= 5 Tg/yr) or by microbes (140
Tg/yr, of which asymbiotic microbes accounts for
44 Tg/yr and symbiotic fixation in higher plants
accounts for the remaining amount)
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Nitrogen Cycle
• If we assume that terrestrial net primary production is
about 60 Pg/yr (1Pg = 1000Tg = 1015 g), and we
assume the average C/N ratio is 50:1, then the
nitrogen requirement of land plants is about 1200 Tg
• Thus, lightning and microbial fixation of nitrogen
only accounts for 12% of the demand. The
remaining of the nitrogen is derived from recycling
and decomposition of dead materials in the soil
• Rivers carry 36 Tg/yr into the ocean, half of which is
now from human additions of fixed nitrogen. This
has a great impact on the coastal sea and estuaries,
and is of both environmental and economic concern
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Human Impacts on the Nitrogen Cycle
• Humans have been modifying the nitrogen cycle
when we began planting crops that result in
heightened nitrogen fixation
• We now produce fertilizer (80 Tg N/yr) through the
burning of natural gas and molecular nitrogen at high
temperatures and pressures, a form of fixation that
produces ammonia
• Fossil fuel combustion releases about 20 Tg N/yr:
This is not technically fixation since much of this N
was already in organic form, but it takes N that was
stored and releases it to the atmosphere (as NOx),
which eventually precipitates on land, having
considerable impact on forested ecosystems
downwind (non-point source pollution)
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Phosphorus Cycle
• Phosphorus is also necessary for organic life: It is an
essential component in DNA, cell membranes, and the
two related organic compounds which provide a key
mechanism for the storage and release of energy
• Adenosine diphosphate (ADP) and adenosine
triphosphate (ATP) are made up of adenosine bonded
to either two or three phosphate groups
• When a phosphate group is removed from ATP to
produce ATP, energy is released:
– ATP Æ ADP + energy + phosphate
• When a phosphate group is added to ADP to produce
ATP, energy must be added:
“Rechargeable Batteries”
– ADP + energy + phosphate Æ ATP
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Phosphorus Cycle
• The ATP/ADP cycle provides energy for cellular
activity and is a key part of plant productivity
• Photosynthesis, respiration and ATP/ADP are
related: Photosynthesis stores energy, respiration
releases it, and ATP is the central molecule in this
process
• Thus, plants require phosphorus to live, and much
like nitrogen, if it is not available in sufficient
quantity, it can be the limiting factor in productivity
• However, the global phosphorus cycle differs from
that from nitrogen in several ways: In particular, the
global phosphorus cycle has no significant gaseous
component
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Phosphorus Cycle
A small amount of P in dust
Significant flow from land to
ocean via rivers (21 Tg/yr)
Terrestrial P derived from weathering of
Calcium phosphate minerals (apatite)
Schlesinger, W.H. 1997. Biogeochemistry: An Analysis of Global Change. Harcourt, Brace and Co., USA: p.397.
• Most phosphorus compounds are not very water
soluble, thus few chemical transformations
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Phosphorus Cycle
• The largest flow of phosphorus in the global cycle is
from rivers to the oceans (21 Tg/yr), and about 10%
of this is in reactive form which can be used by
marine organisms
• The remainder is strongly bound to soil particles
that deposit on the continental shelf. On a time scale
of hundreds of millions of years, these sediments
mineralize and become rock, and are uplifted and
subject to rock weathering on land
• So while there are significant stores of P on land and
in the sea, very little is accessible to organisms.
Thus, there is significant internal cycling where the
available P is reused quite efficiently in ecosystems
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Carbon Cycle
• Living tissue is primarily composed of carbon, so
estimates of the disposition of carbon globally (like
NPP) give us a good sense of the extent to which
ecosystems are thriving or struggling
• Carbon is abundantly available in the atmosphere
as two gaseous species, CO2 and CH4
• Carbon is withdrawn from the atmosphere and added
to organic biomass through photosynthesis, and
vice-versa occurs through the process of respiration
• Over the past billions of years, the concentration of
atmospheric CO2 has diminished as its removal
from the atmosphere has exceeded its addition,
demonstrating organisms’ ability to change the planet
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Carbon Cycle
Units are Pg
(1015) rather
than Tg (1012)
Fossil fuel
burning
Compare
Net increase
LULC change
as Veg. is cleared
~15% of the
atmospheric
pool is taken
by terrestrial
organisms
Another large pool in soils,
increasingly available as
permafrost melts?
Schlesinger, W.H. 1997. Biogeochemistry: An Analysis of Global Change. Harcourt, Brace
and Co., USA: p.359.
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
The Global Carbon Cycle
Units are Pg
(1015) rather
than Tg (1012)
Human
Activity
6.9
Potential
feedbacks:
Change in
the Atm.
5.2
•As [CO2]
increases,
plant uptake
increases
•As global
climate
changes,
vegetation
distribution
changes, and
[CO2] …
changes?
6.9 – 5.2 = 1.7 … Where are
the other 1.7 Petagrams of C?
•Uncertainties in figures?
•Boreal (and temperate) forest
growth?
Schlesinger, W.H. 1997. Biogeochemistry: An Analysis of Global Change. Harcourt, Brace
and Co., USA: p.359.
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Global Flux of Carbon
Botkin, DB, Keller EA. 2003. Environmental Science: Earth as a Living Planet. Wiley, USA: p.66.
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
NDVI from AVHRR
Feb 27-Mar 12
Jun 19-Jul 2
Aug 14-Aug 27
Apr 24-May 7
Jul 17-Jul 30
Nov 6- Nov19
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005
Remote Sensing of Vegetation in the News
David Tenenbaum – GEOG 110 – UNC-CH Fall 2005