Combining Traditional and Isotopic Measure to Develop C Budgets

Allmaras et al. (2004) developed a C-budgeting approach based on combining experimental and isotopic techniques. The data needed to use calculations derived by Allmaras et al. (2004) required that the experiment contain corn stover harvested and removed treatments. These calculations assume that the removal of above-ground biomass will not influence the sequestering of below-ground biomass. The SOC derived from corn stover (sSOC) is calculated with the equation:

where SOCr was the total corn-derived C in the residue returned treatment and SOCh was corn derived from unharvested material (exudates, roots, and crown) in the stover harvested treatment. The SOCr and SOCh values were calculated by multiplying the f value (Eq. 8.20) times the SOC values from the residue returned (SOCr) and harvested (SOCh) plots. This approach requires that 13C isotopic discrimination (A) and total SOC at the end of the experiment be measured. Based on this analysis, Allmaras et al. (2004) concluded that harvesting above-ground corn residue over a 13-year period reduced SOC carbon derived from corn by 35%, and that to manage the entire C cycle, roots and rhizodeposition must be considered.

An alternative approach is to combine models with field measurements to assist in developing carbon budgets (Huggins et al. 1998). Clay et al. (2005) used first-order models to calculate the amount of residue returned to the soil from C3 and C4 plants over an 8-year period. Based on the mineralization rates and when the C3 and C4 residues were returned, the S13C signature of non-harvested biomass was determined. Based on the rates, carbon turnover, the amount of SOC mineralized, and the amount of fresh biomass incorporated into the SOC over an 8-year period were determined.

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