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

The 13 C 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 13 C 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‰ increase in the soil δ 13 C values (from −18.97 to −18.25‰). Based on these values, the Rayleigh fractionation constant (ε) of relic SOC was −3.45‰. At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg −1 ) and δ 13 C increased 3.2% (0.548 ± 0.332‰) during a 5‐yr period. The Rayleigh fractionation constant for this experiment was −6.94‰ (±4.74‰). In a separate experiment, the δ 13 C value of corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] residue remained unchanged after 4 mo. The impact of 13 C 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 C 4 residues are returned to soil derived from C 3 and C 4 plants, not considering 13 C enrichment during relic SOC mineralization will result in underestimating relic SOC half‐lives and overestimating the contribution of fresh C 4 biomass in the SOC. The effect of 13 C 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 C 3 biomass. To minimize these errors, SOC maintenance rate experiments should measure 13 C enrichment during relic SOC and unharvested biomass mineralization.