Soil organic carbon measure

Indirect methods of estimating sorption have been used when actual measurement of sorption isotherm is impossible (44). For instance, sorption coefficients have been estimated from soil organic carbon and a specific surface of soil, and from semiempidcal equations using pesticide properties. 

Skjemstad JO, Spouncer LR, Cowie B, Swift RS (2004) Calibration of the Rathamsted organic carbon turnover model using measurable soil organic carbon pools. Aus J Soil Res 42 79-88 Soon YK, Malhi SS (2005) Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer. Can J Soil Sci 85 579-587... 

Paul, E. A., Morris, S. J., Conant, R. T., and Plante, A. F. (2006). Does the acid hydrolysis-incubation method measure meaningful soil organic carbon pools Soil Sci. Soc. Am. J. 70(3), 1023-1035. 

Given that soils only contain a few percent OM by mass, direct measurement of SOM mass loss is much more difficult than for litter studies. The four most common techniques for measuring soil organic carbon (SOC) turnover are (1) laboratory incubations (2) in situ soil respiration measurements (3) stable isotope measurements and (4) radiocarbon methods. We will introduce the basic methodology and highlight some of the strengths and weaknesses of each of these four techniques. For much more detailed discussions of these methods, see the references cited within. 

The undecomposed litter (4% of the soil carbon) has a turnover time measured in tens of years, and the 22% of the soil carbon in the form of fulvic acids is intermediate with turnover times of hundreds of years. The largest part (74%) of the soil organic carbon (humic acids and humins) also has the longest turnover time (in thousands of years) (Holmen, 1992). 

Adsorption to soil organic carbon is measured as the organic carbon/water partition coefficient (Koc), calculated as the distribution among a soil and an aqueous phase, normalised to the organic carbon content of the soil [27]. 

At this stage, as noted, Kow may become very helpful. In fact, the adsorption to soil organic carbon is more often estimated rather than measured, starting from the Kow since in Kow measurement -octanol mimics soil organic carbon, a relation between Koc and Kow exists, and algorithms for the estimation of Koc as a function of Kow have been developed and largely applied. 

River transport of organic carbon, estimated earlier as 0.1 Pg C/yr, brings the sum of non-respiratory outputs to 7 Pg C/yr. Total respiration should therefore be around 50 Pg C/yr. This figure is in agreement with estimates of soil respiration rates determined from compilations of ecosystem types and their measured soil respiration rates (Ajtay et ai, 1979). 

Methyltins are less likely than the butyl- and octyl-tins to partition to sediments, soils, and organic carbon. Modelled data for K c suggest much lower capacity for binding to organic carbon than do measured values, often by several orders of magnitude. Measured data have been used in preference to model environmental fate of the compounds. The compounds also bind strongly to clay minerals, montmorillonite in particular. 

The quantification of gross root production, rhizodeposition, microbial assimilation, and the production of organic materials in soil has made increasing progress ever since stable ( C) and radioactive ( C) carbon isotopes have been used (see Chap. 12). Measurements of soil organic matter dynamics without these isotopes are difficult due to the large amount present as compared to the smaller rates of input. 

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