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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