Soil organic matter dynamics: Controls and management for sustainable ecosystem functioning

The composition of soil, particularly its organic matter content, reflects its role as a major controller of ecosystem functioning and soil fertility (Paul and Collins 1998, Basso et al. 2011, Bhardwaj et al. 2011). Soil organic matter (SOM), the largest global reservoir of terrestrial organic carbon (C), contains three to four times as much stored C as either the atmosphere or plant biomass. The soil biota, consisting of microorganisms as well as fauna, account for 1–3% of total soil C and complete the terrestrial C cycle by mineralizing SOM to carbon dioxide (CO 2 ) (Paul and Collins 1998, Robertson and Paul 2000). Soil organic matter is a complex, multistructured, multicomponent pool of organic materials including decomposing plant residues, associated microorganisms and their products, and a biochemically transformed fraction, sometimes called humic material, that is complex in structure and often associated with soil minerals. Microbially derived C, because of its unique chemical structure and intimate association with minerals, is selectively protected and represents a particularly important component of total SOM. This pool serves as a dynamic source of labile nutrients and contributes to soil aggregate formation and erosion resistance (Robertson and Paul 1998, Grandy and Neff 2008). The investigation of SOM dynamics provides a wealth of information on how organisms, including vegetation and soil biota, interact with climate, parent material, landscape, and management over time to influence ecosystem functioning (Collins et al. 1997). One of the most promising approaches for understanding SOM dynamics uses long-term incubations, together with tracers and density fractionation, to interpret SOM pools and their turnover. The multiple components that make up SOM can be divided into three pools based on their turnover times (Paul et al. 2001a, b). The most labile, active pool is Paul, E. A., A. Kravchenko, A. S. Grandy, and S. Morris. 2015. Soil organic matter dynamics: Controls and management for sustainable ecosystem functioning. Pages 104-134 in S. K. Hamilton, J. E. Doll, and G. P. Robertson, editors. The Ecology of Agricultural Landscapes: Long-Term Research on the Path to Sustainability. Oxford University Press, New York, New York, USA.