Carbon-13 Fractionation of Relic Soil Organic Carbon during Mineralization Effects Calculated Half-Lives

The 13C natural abundance approach for determining soil organic C (SOC) stability and turnover has been used to determine SOC mineralization kinetics. These calculations generally assume that 13C fractionation during relic SOC and unharvested biomass mineralization is insignificant. The objective of this study was to determine the impact of this assumption on calculated relic SOC half-lives. Study sites were located in Minnesota and South Dakota. At the Minnesota site, SOC contained in the surface 30 cm of soil in a fallowed area decreased from 90.8 to 73.2 Mg ha-1 during a 22-yr period. Associated with this decrease was a 0.72 ppt increase in the soil delta 13C values (from -18.97 to -18.25 ppt). Based on these values, the Rayleigh fractionation constant (epsilon) of relic SOC was -3.45 ppt. At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg-1) and delta 13C increased 3.2% (0.548 ± 0.332 ppt) during a 5-yr period. The Rayleigh fractionation constant for this experiment was -6.94 ppt (±4.74 ppt). In a separate experiment, the delta 13C value of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] residue remained unchanged after 4 mo. The impact of 13C enrichment during relic C mineralization on calculated C budgets depends on the type of residue returned to the soil. A simulation study showed that for systems where C4 residues are returned to soil derived from C3 and C4 plants, not considering 13C enrichment during relic SOC mineralization will result in underestimating relic SOC half-lives and overestimating the contribution of fresh C4 biomass in the SOC. The effect of 13C enrichment during relic SOC and unharvested biomass mineralization had cumulative impacts on C budgets and did not cancel each other out. The reverse was true for C3 biomass. To minimize these errors, SOC maintenance rate experiments should measure 13C enrichment during relic SOC and unharvested biomass mineralization.