Geoscience Reference
In-Depth Information
over long time periods, the amount of carbon in an
ecosystem is controlled not by the rate of input, i.e.
photosynthesis, but by the mean residence time of C (
).
The theory is that global warming will speed up soil
respiration, decrease
and release CO
2
in a positive
feedback loop. However, the assumption that respiration
and decomposition rates are temperature-dependent has
been challenged (Giardina and Ryan 2000), further
emphasizing our lack of understanding of key fluxes in
the carbon cycle. Soil carbon consists of several different
pools; 10 per cent is easily decomposable, but 50 per cent
is relatively inert with a
AND NITROGEN CYCLES
The carbon and nitrogen cycles have much in common.
Both elements are relatively enriched at the Earth's surface
due to microbiological activity. Heterotrophic micro-
organisms are responsible for the breakdown of organic
matter, and in their metabolism they require nitrogen
and other nutrients in addition to carbon. Thus it arises
that plant roots and microbes compete for mineral
nitrogen, with microbes generally being better competi-
tors. Fresh plant material has a typical carbon : nitrogen
ratio of 60 : 1 to 100 : 1, soil humus has a ratio of 10 : 1
whilst microbes have a ratio of 5 : 1 to 15 : 1.
Figure 21.8
illustrates how the fluxes of carbon and nitrogen are
related.
The symbol
value of thousands of years.
Fluxes in and out may be very small but can constitute a
large proportion of active soil carbon. These fluxes seem
to be insensitive to temperature.
The extent of nitrogen and phospherus limitations on
future responses to CO
2
is a complicated yet important
question. The availability of nitrogen to plants is
dependent not just on the total amount of nitrogen in the
soil but on the rate of mineralization, i.e. the rate of
nitrogen release from the organic matter, which in turn is
linked to the rate of decomposition of organic matter and
its C/N ratio. Both plants and microbes in colder
ecosystems are considered to show 'nitrogen starvation'
largely due to the slow rate of decomposition at low
temperatures.
(tau) is used to denote
mean residence
time
(or
mean turnover time
) for an element in an
ecosystem. This is a fundamental property of global
biogeochemical systems. It is calculated by measuring the
flux of an element out of a subsystem in the ecosystem,
e.g. soil, and also measuring the amount of the element
in that subsystem, e.g. in the soil. Thus for the mean
residence time of carbon in soil:
soil
= C
soil
/ R
soil
An example from a subarctic boreal forest soil in Siberia
gives:
The phosphorus (P) cycle is similar to the nitrogen cycle
in many respects, though it is less spectacular, as valency
changes do not occur during transformations by micro-
organisms. At 2 per cent content, phosphorus is the next
most abundant nutrient in microbial biomass and soil
organic matter. A useful concept is that the phosphorus
in soil can be partitioned into 'stores' based on the
availability of various organic and inorganic forms to
plants.
Figure 21.9
shows the phosphorus cycle in relation
to the availability to plants of the different stores, and
whether the stores are inorganic or organic forms of the
nutrient. Phosphorus is a major plant nutrient which is
absorbed as the anion orthophosphate (PO
3-
). As an
anion it is denied an exchangeable reservoir on soil
colloids; being a solid, it is denied a large reservoir in the
atmosphere like nitrogen and sulphur. Phosphorus thus
faces some unique problems. Most soils contain much
phosphorus , but it is often a limiting nutrient because
most is unavailable to plants. Phosphorus stores on Earth
are marine sediments (850,000
soil respiration
25 mol C m
-2
yr
-1
soil carbon content
400 mol C m
-2
so,
= 400/25 = 16 years, a very low figure for the subarctic
(Lloyd, personal communication). A conclusion is that,
To leaves and shoots
Litter (C/N 60)
N
2
O
Soil organic matter
(C/N 20)
Roots
NO
3
-
NH
4
+
Microbes (C/N 10)
Leaching
10
12
kg), terrestrial
soils (100
10
12
kg), dissolved inorganic phosphate in the
