Decomposition

All vegetation is to some extent biodegradable and as a consequence some of the carbon sequestered in the vegetation will return to the atmosphere where it came from as this material is decomposed. As shown in Table 3 the annual carbon flux into the atmosphere due to soil organic matter oxidation/erosion is on the order of 61-62 gigatons/year.

As shown above the world’s hot deserts have the potential to take up 15.6 gigatons of carbon annually, in the form of CO2 121, of which, based on the ratio between soil organic matter oxidation/erosion and photosynthesis incorporation shown in Table 3 – 62/110 x 15.6 gigatons, 8.8 gigatons would be returned to the atmosphere 21 for a net sequestration of 6.8 gigatons of carbon annually.

Methane (CH4) is a greenhouse gas that remains in the atmosphere for approximately 9-15 years. Methane is over 20 times more effective in trapping heat in the atmosphere than CO2 over a 100-year period. A problem would arise therefore if the carbon taken up the desert in the form of CO2 was return to the atmosphere in the form of Methane. In this unlikely circumstance the deserts would become net contributors to the problem of global warming.

Methanogenic bacteria in soil produce methane when decomposition occurs under anaerobic, reducing conditions. Wetlands represent the most important natural source of methane emissions to the environment. As the rate of methane emission is often reported to increase with temperature, there is potential for a positive feedback due to climate change.

As it is the intention of one aspect of this invention to sequester atmospheric CO2 to reduce global warming, it would be counterproductive to have this carbon returned to the atmosphere in the form of methane, which is a 20 times more effective greenhouse gas.

As the reducing conditions that produce methane are most often associated with wetlands they would not be found in the desert. It is an objective therefore of the current invention to ensure the amount of greenhouse gas produced by an embodiment of the current invention never exceeds the amount of carbon sequestered.

When organic materials decompose in the presence of oxygen, the process is called "aerobic." The aerobic process is most common in nature. For example, it takes place on ground surfaces such as the forest floor, where droppings from trees and animals are converted into relatively stable humus.

In aerobic decomposition, living organisms, which use oxygen, feed upon the organic matter. They use the nitrogen, phosphorus, some of the carbon, and other required nutrients. Much of the carbon serves as a source of energy for the organisms and is burned up and respired as CO2. Since carbon serves both as a source of energy and as an element in the cell protoplasm, much more carbon than nitrogen is needed. Generally about two-thirds of carbon is respired as CO2, while the other third is combined with nitrogen in the living cells. However, if the excess of carbon over nitrogen (C:N ratio) in organic materials being decomposed is too great, biological activity diminishes. Several cycles of organisms are then required to burn most of the carbon.

Aerobic decomposition results in a net uptake of CO2, which is the goal of global warming mitigation.

Hot, dry and windy deserts are oxygen rich environments, which favour aerobic decomposition. The non-marketable by-products of the crops grown in an irrigated environment can therefore be composted to further enrich the desert soils without undercutting the objective of sequestering excess atmospheric carbon.