The vast majority of carbon on Earth is located underground in inorganic forms such as the carbonates in sedimentary rock, mostly calcite (CaCO3), and in organic forms such as deposits of coal, oil, and natural gas and accumulations of soil organic matter. Each year, soil microorganisms, in a process known as decomposition, break down about 2.5% of soil organic matter, converting the organic carbon to carbon dioxide (CO2) which is released to the atmosphere. Roughly an equivalent amount of organic carbon returns to soils as animal and especially plant material, a process known as soil organic carbon sequestration.
The balance between carbon decomposition and sequestration has a direct effect on atmospheric CO2levels. Evidence for this includes the prominent seasonal cycles of atmospheric CO2 concentrations. Atmospheric CO2 concentrations decrease during the summer, when photosynthesis and carbon sequestration exceed respiratory decomposition, and increase during the winter, when the balance reverses.
An elevated atmospheric CO2 concentration stimulates photosynthesis and plant biomass production (particularly under heavy nitrogen fertilization), which enhances soil organic carbon sequestration. Soil microorganisms proliferate under these amendments and accelerate respiratory decomposition. At more natural levels of nitrogen fertilization (low N), faster soil carbon decomposition balances faster sequestration, and so soil carbon contents generally do not change significantly under CO2 enrichment.
Time course of CO2 released (10–6 moles per day per gram of soil) during microbial decomposition of soil at 25°C and 35°C. After Conant et al. 2007.
Global warming may disturb this balance. Net photosynthesis is relatively insensitive to temperature, but a longer growing season should enhance plant productivity and thereby accelerate soil carbon sequestration. Sequestration, however, may not keep pace with soil carbon decomposition  because the activity of soil microorganisms responsible for decomposition increases exponentially with temperature.
Predicting the response of soil decomposition to global warming has proved difficult. ,  Soils vary greatly from place to place, and even a single soil type from one place contains thousands of different organic compounds. About 5% of these compounds, like sugars and proteins, are labile, (decompose rapidly). Another 60% to 85%, like cellulose, are intermediate, (decompose in years to decades), and the final 10% to 40%, like lignin and tannins, are recalcitrant (may persist in soils for millennia). Differences among soil constituents are responsible for large variations in decomposition rates over time.
In regard to global warming, respiratory CO2 efflux from soils declines exponentially with time of exposure to elevated temperatures. Initially the labile compounds in a soil decompose and release a substantial amount of CO2. As labile compounds break down, intermediate and recalcitrant compounds constitute a larger portion of the remaining soil organic carbon, and CO2 efflux slows down.
Environmental factors other than temperature also influence decomposition. ,  These include soil particle mass size (organic carbon inside larger soil aggregates decompose more slowly, since they are better shielded), oxygen concentrations (which determine the type of biochemical pathway available), and water availability (which affects soil microorganism activity).
Despite these uncertainties about soil decomposition in a warmer world, release of additional CO2 from soils is anticipated. Wetlands and permafrost soils (soils frozen year round) at high latitudes contain large amounts of organic soil carbon. These high latitudes are likely to experience more extreme warming than the rest of the planet and, thus, the labile pool of organic carbon in their soils will be subject to faster decomposition. Consequently, global climate models predict that by the latter half of the twenty-first century, landmasses will release more CO2 than they absorb.
 Davidson, E. A. and I. A. Janssens (2006) Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440:165-173.
 Kirschbaum, M. U. F. (2006) The temperature dependence of organic-matter decomposition - still a topic of debate. Soil Biology&Biochemistry 38:2510-2518.
 Six, J. and J. D. Jastrow (2002) Organic matter turnover. In: Encyclopedia of Soil Science, Lal, R., ed., Marcel Dekker, New York. pp. 936-942.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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