Containing ~2000 Pg C at any one time, soils play a key role in the global carbon cycle. About 300 Pg C can be found as detritus in the topsoil, with this carbon-rich material decomposing at varying rates depending on factors such as 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 CO2. The rest of the detritus carbon can be converted into 'modified soil carbon', which decomposes at a slower pace and so keeps the carbon from the atmosphere for a longer time. A small amount of this carbon is further decomposed to 'inert' carbon and this can remain locked away from the atmosphere for over a thousand years. Biomass burning may be particularly important here, as a small percentage of the material burnt can be left behind in the soil as inert 'black carbon'. These recalcitrant burning residues can constitute small

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