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

Abstract

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%o increase in the soil δ 13 C values (from -18.97 to -18.25%o). Based on these values, the Rayleigh fractionation constant (e) of relic SOC was -3.45%o. 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%o) during a 5-yr period. The Rayleigh fractionation constant for this experiment was -6.94%o (±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.