Submitted to: Encyclopedia of Soils in the Environment
Publication Type: Book / Chapter
Publication Acceptance Date: May 15, 2004
Publication Date: October 15, 2004
Citation: Cambardella, C.A. 2004. Carbon cycle in soils: formation and decomposition. In: Hillel, D., editor. Encyclopedia of Soils in the Environment. Vol. I. Amsterdam: Elsevier Academic Press. p. 170-175.
Plants use the energy of sunlight to transform carbon dioxide (CO2) to reduced forms of organic carbon, where the energy is stored in chemical bonds. Nearly all of the carbon entering the soil ultimately comes from plants. Decomposition is the physical and chemical breakdown of organic material. In aerobic environments, decomposition is mediated by heterotrophic organisms, which derive their energy and carbon from organic matter produced by plants. Aerobic decomposition consumes oxygen and returns carbon to the atmosphere as CO2. The rate of decomposition under anaerobic conditions is much slower than when oxygen is plentiful. Decomposition is controlled by soil microclimate variables, such as moisture, temperature, and oxygen content, by intrinsic soil properties, such as soil texture and pH, and by the quality of the organic substrate available to the decomposers. Rapidly decomposing materials generally have higher concentrations of labile substrates and lower concentrations of recalcitrant compounds than do slowly decomposing materials. Soil organic matter is a diverse mix of plant, animal, and microbial residues that vary in their susceptibility to microbial decomposition. The cycling of carbon in soils is the summation of processes leading to rapid turnover of plant litter in the surface soils, and processes leading to slower production, accumulation and turnover of humus deeper in the soil profile. Soil disturbance increases soil organic carbon decomposition by exposing new soil surfaces to microbial attack and by increasing the oxygen content of the soil. Soil organic matter has been conceptualized as a series of pools that comprise a continuum based on decomposition rate. This concept has been extended further through the development of simulation models that mathematically describe the processes of organic matter formation and turnover. Generally, these pools are conceptualized as one small pool with a rapid turnover rate (labile pool) and one to several pools of greater size and slower turnover rate (recalcitrant or stabilized pools). These concepts are useful in the assessment of the impact of climate and land use change on soil ecosystem function.