Composition and Cycling of Organic Carbon in Soil

Soil organic carbon (SOC) represents a significant reservoir of carbon within the global carbon cycle that has been estimated to account for 1,200–1,550 Pg C to a depth of 1 m and for 2370–2450 Pg C to a depth of 2 m (Eswaran et al. 1995; Lal 2004a). Comparative estimates of organic C contained in living biomass (560 Pg) and atmospheric CO2-C (760 Pg) (Lal 2004a) indicate that variations in the size of the SOC store could significantly alter atmospheric CO2-C concentrations. A 5% shift in the amount of SOC stored in the 0–2 m soil profile has the potential to alter atmospheric CO2-C by up to 16%. Land-use change can induce emission or sequestration of carbon depending on a range of soil and environmental properties and land management practices. Carbon sequestration in soils is a slow process but may offer the most efficient natural strategy for offsetting increased atmospheric CO2-C concentrations induced by fossil fuel burning and conversion of natural terrestrial systems to agriculture (Lal 2004a; Metting et al. 1999; Post et al. 1999). It has been suggested that, over the next century, improved land management strategies could sequester up to 150 Pg CO2-C (Houghton 1995; Lal 2004b; Lal et al. 1998); however, considerable uncertainty exists in such estimates because of an inability to accurately predict the total carbon sequestration potential of soils. Improving our understanding of SOC cycling processes and how these are affected by land management practices will be important to defining future opportunities for carbon sequestration in soils. In addition to its importance in the global carbon cycle, SOC contributes positively to a range of biological, physical and chemical properties important to defining the potential productivity of a soil (Baldock and Skjemstad 1999; Part I Principles of Nutrient Cycling