Part of the reason it
accumulates in the lower parts of the soil is that there tends to be
less oxygen in that environment, and the lack of oxygen makes it even
more difficult for microbes to work on this humus and decompose it
further. Another pathway for carbon to move from the sedimentary rock reservoir to the … Carbon dioxide can be dissolved in
seawater, just as it can be dissolved in a can of soda; it can also
be released from seawater as the CO2 from a soda can also
be released. It is worth noting that the process
of organic carbon consumption on the seafloor is another microbial
process and is very similar to the soil respiration flow described
above. The CO2
concentration of seawater, relative to the atmospheric
CO2, determines whether the oceans can absorb or
release CO2 . This dense
water then sinks and flows through the deep oceans, effectively
mixing them on a timescale of about 1000 years or so (the Atlantic
Ocean mixes somewhat faster, which helps explain the smaller
SCO2
and alkalinity gradients seen in Figure 7.08). For our purposes, we will
assume that the global collection of plants has a ratio of around 2
to 1. Respiration also occurs within the
soil, as microorganisms consume the dead plant material. This
will be provide us with a very nice way of assessing the significance
of our modeling results. So, we can define
this flow in the form of a standard draining flow: where
Flf
is the flow in Gt C/yr and
land_biota
is the amount of carbon stored in the land biota (plants) at any
given time. Fossil Fuel Burning. The data used in our global carbon cycle model lead to a
residence time of about 26 years for the soil carbon
reservoir. The answer is, yes, it would, except for the
fact that there are other flows of carbon out of the this cold water
reservoir, most importantly, downwelling. Dead plant material enters the soil
in two ways -- it falls on the surface as litter, and it is
contributed below the surface from roots. The surface waters of the worlds
oceans are home to a great number of organisms that have as the basis
for their food web photosynthesizing phytoplankton. The carbon cycle is tied to the availability of other elements and compounds. To understand this process, we
need to have some sense of what happens to CO2 once it
gets dissolved in seawater. This distribution is shown in Figure 7.09
below. However,
some of the organic remains and the inorganic calcium carbonate
shells will sink down into the deep oceans, thus transferring carbon
from the shallow surface waters into the huge reservoir of the deep
oceans. In the
oceans, warm surface waters move up to the polar regions and are
added to the cold surface waters there, while some of the cold
surface water returns back to the equatorial regions, making a
large-scale cycle that is important for the transfer of heat energy,
as we discussed in our earlier work with energy balance climate
models. In the atmosphere, carbon is attached to oxygen in a gas called carbon dioxide (CO 2). What this amounts to is an increased efficiency of water use
by the plants -- they grow more while using less water. This makes this flow a first-order
kinetic process, like many of the other flows in this model. More precisely, this ratio is 100 to 50; these are the current
best estimates for the total amount of carbon, in gigatons, used and
released in photosynthesis and respiration each year on a global
basis. This difference is essentially made up by the carbonate and
bicarbonate ions. But how can this be the case? Some of the carbon, both organic and
inorganic (i.e., calcium carbonate shells) produced by marine biota
and transferred to the deep oceans settles out onto the sea floor and
accumulates there, eventually forming sedimentary rocks. When these planktonic organisms die, their soft parts are
mainly consumed and decomposed very quickly, before they can settle
out into the deeper waters of the oceans. They also release energy,
in the form of heat, or infrared radiation. The alkalinity of sea
water represents the positively-charged ions that need to be
countered by negatively-charged carbonate ions. At the same
time, many planktonic organisms extract dissolved carbonate ions from
seawater and turn them into CaCO3
shells. The flow units are grams of
carbon per year, which can then be converted to Gigatons of carbon
per year, using the relationship that 1Gt = 1 x 1015g. ... Take a bite of dinner, breathe in air, or a drive in a car — you are part of the carbon cycle. we can say that the equilibrium
constants, k1
and k2,
are given by a ratio formed by the concentrations of the various
compounds involved: We can do some algebra with these
equations, just like normal equations. (relevant for timescales of a
few hundred years), V. Modeling the Long-Term
Carbon Cycle. The equation for this flow is thus: where Fsed
is the transfer of carbon from the deep ocean reservoir to the
sedimentary rock reservoir, and Fswob
and Fscob
are the flows of carbon from the warm and cold surface ocean biota. In other
words, the circulation controls how many cubic meters of water are
transferred from the warm to the cold region, and the concentration
of carbon controls how many grams or moles of carbon are contained
within each cubic meter. This decomposition thus
returns carbon, in the form of CO2, to seawater. Downwelling will be
discussed in more detail below, but it does indeed balance this
addition from advection. The abundance of 14C in
seawater, and its distribution with depth are observations that
provide enough information to determine the coefficient of gas
transfer. There are several ways to construct
an equation that incorporates this information, but here, we use the
following: Fp is the global rate
of photosynthetic uptake of CO2 in GtC/yr. In reality, it is the litterfall that is actually measured in
studies of carbon flow through ecosystems; that, combined with a
measure of the gross primary productivity (the total amount of carbon
used in photosynthesis) gives an estimate of the plant respiration
flow according to the following equation: Having already chosen the initial
rates for photosynthesis and plant respiration, at 110 and 50, this
leaves us with a value of 60 Gt C/yr for the rate of carbon added to
the soil reservoir through the process of litter fall.
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