ABSTRACT

The limitation in determining soil C sequestration under elevated C 02 conditions is the accuracy of the methods used to assess changes in total soil C. Methodologies to determine changes in soil C range from mass balance approaches to C isotope dilution studies. An accurate measure of soil bulk density is required by these methods to assess changes in soil C. Both approaches are direct but yield different types of information. The mass balance approach yields information on changes in the total soil C pool. However, small changes in total soil C are difficult to detect because C entering the soil through decomposition processes is small compared to the background soil C (Hungate et al., 1996). The 13C isotope dilution approach can overcome this problem by yielding additional kinetic information that describes the rate of C accumulation, turnover of recently added C, and pools of C within the total soil C pool (Balesdent and Mariotti, 1996). In combination, the mass balance and 13C isotope dilution methods provide information that can accurately determine the size and kinetics of standing soil C pools that are influenced by such factors as cultural practices or elevated C 02. However, both methods are limited by factors that affect soil variability, such as soil heterogeneity.