Globally, soils contain ~2000 billion tonnes of organic carbon with ~1500 billion tonnes of carbon in the top metre of mineral soil (Batjes, 1996). This constitutes about three times the amount of carbon in vegetation and twice the amount in the atmosphere (IPCC, 2000a). About 300 billion tonnes can be found as detritus in the top soil, with this carbon rich material decomposing at varying rates depending on temperature and soil conditions. During this decomposition some of the carbon in soil detritus is respired by the decomposing organisms (often fungi and bacteria), and the carbon returned to the atmosphere as carbon dioxide (CO2). The rest can be converted to soil organic matter, which decomposes more slowly and hence keeps the carbon away from the atmosphere for a longer period. A small amount of this carbon is further decomposed to forms that persist in the soil, sometimes for decades or centuries.
The conversion of soils from natural to agricultural use by humans has led to substantial loss in soil carbon stocks. Greater soil disturbance, such as that caused by ploughing, can result in rapid respiration and loss of large amount of soil carbon, which would otherwise decompose slowly.
Sensitive land use practice is key for balancing the soil carbon sink, and perhaps reversing recent trends of loss of carbon from soils. Farming practices such as 'no-till', whereby agricultural land is used without the soil disturbance and carbon loss that comes with ploughing, are becoming more widespread, and land use remains a key area of research in studies of human-made greenhouse gas (GHG) emissions and strategies to reduce them.
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