Soil organic matter (SOM) is a generic term for all organic compounds in the soil that are not living roots or animals. SOM has been characterized in various ways: by origin, transformation stage, function, solubility, chemical constituents, elemental carbon:nitrogen (C/N) ratio, exchange capacity, functional activity level, or dynamics and stability (Parton et al., 1987; Anderson and Ingram, 1993; Feller and Beare, 1997; Paustian et al., 1997; Smith et al., 1997a; Baldock and Nelson, 1999). In well-drained, non-acid soils-which occupy most agricultural lands-there is a balanced and dynamic composition of chemical compounds with a high degree of humification, resulting in medium to low C/N ratios (10-12). Characterization of SOM based on C/N ratio may be a convenient proxy for the composition and stability of SOM in topsoils. In deeper subsoils, where the amount of humus is normally small, the ratios are less reliable.
When grouping specifically on stability of the total pool of SOM, its dynamics and residence, or turnover time, one distinguishes inert or passive, stable or slow release, and labile functional subpools (Schlesinger, 1986; Smith et al., 1997a; Batjes, 1999). Labile or active SOM largely consists of soil microorganisms and their immediate products, with a cycling or residence time of 1-10 years. Stable or slow-release SOM consists of neo-formations of polymeric substances, which can be extremely diverse in composition, depending on the litter source and other soil-forming conditions; it has a residence time of between 10 and 50 years (sometimes more). Inert or "recalcitrant" SOM, which is also diverse in composition, would not be destroyed for up to 500 years. The extent to which humic substances are stabilized against microbial mineralization largely depends on the type of bonding with clay minerals (Tate and Theng, 1980). In recent years, nuclear magnetic resonance (NMR) techniques have been used to establish the precise nature of these bondings (Kögel, 1997; Bayer et al., 2000). The results of the application of this technique over the full range of the world's ecosystems and agricultural and anthropogenic soils are still awaited.
Parties may wish to agree on standardization of residence times, in view of the time horizons of IPCC and those mentioned in the Kyoto Protocol: 0-5 years, 5-50 years, 50-100 years, and >100 years (see Chapter 4 for details).
None of the current definitions and subdivisions of SOM provide fully quantified and universally accepted parameters to define SOM quality and quantity that are relevant to measure carbon sequestration in soils as a Kyoto Protocol activity. For accounting purposes, a simplification to SOC-which is easily measured through fine-earth samples and possibly C/N ratios-may be sufficient. Carbon isotope ratio measurements are very useful for research and modeling work on the dynamics of SOM, especially at the conversion of forest to grassland and vice versa. These techniques are not yet available for routine monitoring of the fate of soil carbon.
SIC (soft lime, calcium carbonate concretions or pans, primary carbonate of rock fragments or sediments) is much less mobile than SOC, except in irrigated agriculture (Schlesinger, 1999). Measurement of SIC, as well as of charcoal, will be required for monitoring and modeling of soil carbon changes, but the intensity of sampling can be much lower than with SOC.
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