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SOIL & WATER MANAGEMENT & CONSERVATION

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

^a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
^b USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108
^c Former Research Associate at South Dakota State Univ.

^* Corresponding author (david.clay{at}sdstate.edu).

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 increase in the soil ¹³C values (from –18.97 to –18.25). Based on these values, the Rayleigh fractionation constant () of relic SOC was –3.45. At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg^–1) and ¹³C 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 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.

Abbreviations: NHC_a, the amount of non-harvested C applied • PCR, plant biomass C returned to soil • PCR_incorp, new biomass C incorporated into SOC • SOC, soil organic carbon • SOC_retained, the amount of soil organic carbon retained in soil after mineralization • SOC_final, soil organic carbon contained in soil at the end of the experiment • SOC_initial, soil organic carbon at the beginning of the experiment • SOC_lost, the amount of organic C lost • ¹³C_{soil final}, ¹³C value of soil at the end of the experiment • ¹³C_PCR, ¹³C value of plant material remaining in soil after mineralization • ¹³C_{SOC retained}, ¹³C of soil organic carbon at the beginning of the experiment that is retained in the soil after mineralization • , Rayleigh fractionation coefficient