Soil organic carbon definition

Sorption coefficients also have been expressed on an organic matter basis (Kom) by assuming that the organic matter content of a soil or sediment equals some factor, usually between 1.7 to 1.9, times its organic carbon content on a mass basis (Hamaker and Thompson, 1972 Lyman et al., 1982). Often 1.724 is used as this factor, implying that the carbon content of organic matter is 1 /1.724 or 60%. However, Koc is considered a more definite and less ambiguous measure than Kom (Hamaker and Thompson, 1972). 

Humin has been regarded as the most intractable component of SOM. It must be considered to be a very important component, however, because typically it represents more than 50% of the organic carbon in a soil (Kononova, 1966 Stevenson, 1982,1994) and more than 70% of the organic carbon in unlithified sediments (Durand and Nicaise, 1980 Rice, 2001). The definition of humin (Section 1.3.3) is similar to that of a protokerogen (Calvin and Philip, 1976 Rice, 2001), which is often used in petroleum geochemistry to describe insoluble organic matter in unlithified sediments. 

Case 3. The raindrop now runs off the calcite rock and flows into a calcium-carbonate-ffee soil, where carbon dioxide is added from the decomposition of organic matter until the total CO2 is five times its original value. For simplicity, it will be assumed that no other components are added and no other reactions take place between the soil and the raindrop. In this case calcium concentration and total alkalinity are constant, and the total CO2 and initial concentrations are known. From the definition of terms and equilibrium relations we can write ... 

The value of the distribution coefficient, K, varies with several soil properties, most notably the organic carbon content. This has led to the definition of a normalized sorption coefficient, Koc, which is calculated as (12) ... 

This definition of indicates a coefficient dependent only on the partition properties of a substance between two phases (organic carbon and water) and independent of other possible processes of sorption or other interactions with the inorganic matrix of the soil. 

Histidinol is oxidized to histidine by a dehydrogenase that appears to catalyze both steps required. - The most definitive studies on this dehydrogenase were carried out with preparations of Arlhrobacter hi i-dinolovorans, a soil organism isolated by an enrichment technique using histidinol as a carbon and nitrogen source. Similar enzymes occur in E. coli and yeast. Two equivalents of DPNH are produced in this reaction. The reduction of the first mole of DPN by these enzymes should result in the formation of the aldehyde, histidinal. This compound, however, has not been detected in incubation mixtures, even when aldehyde binders were added. Histidinal, an unstable compound in neutral solution, does serve as a substrate for the dehydrogenase, with both DPN... 

Although these studies utilizing Incorporated debris are valuable because they show the potential for allelochemlcals to be released from plant litter, they suffer from a disadvantage. The amount of debris added and Its carbon to nitrogen ratio might lead to alterations In nutrient contents In the soil as the result of proliferation or shifts In populations of micro-organisms. Thus, a control In which a material of similar C/N ratio but lacking allelochemlcals needs to be Included for such studies to be conclusive. The above studies did not Include such controls and thus are not definitive. 

Natural Abundance Stable Carbon Isotopes (I3C). There are trends in 13C of plant, litter, and organic constituents in soil that can be used to investigate carbon cycling, but to date the trends have proven too subtle, the variation too high, or the mechanisms too poorly understood to exploit these patterns definitively. The 813C of plant litter and particulate SOM is typically close to that of the plant source,... 

Aramentano, T. V. (1979). The role of organic soils in the world carbon cycle problem definition and research needs. The Institute of Ecology, 37 pp. 

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