Monday, November 13, 2006 - 9:15 AM
102-1
13C Fractionation During Soil Organic C Mineralization Impacts on Calculated Half-Lives.
David Clay, South Dakota State University, South Dakota State University, Plant Science Department, Brookings, SD 57007, C. Clapp, USDA-ARS, Upper Buford Circle, 439 Borlaug Hall, St. Paul, MN 55108, and C.G. Carlson, South Dakota State Univ, Plant Science Dept, Brookings, SD 57007.
The 13C natural abundance approach for determining soil organic C (SOC) stability and turnover has been used to determine SOC mineralization kinetics. These calculations often assume that 13C fractionation during relic SOC and non-harvested biomass mineralization is insignificant. The objective of this paper 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 over a 22 year period. Associated with this decrease was an increase in the soil δ13C value of 0.72‰ (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 and δ13C decreased 10% (2.8 ±1.8 g kg-1) and increased 3.2 % (0.548 ±0.332 ‰) over a 5 year period, respectively. The Rayleigh fractionation constant for this experiment was -6.94 ‰ (± 4.74‰). In a separate experiment the δ13C value of corn (Zea mays) and soybean (Glycine max) residue remained unchanged after 4 months. The impact of 13C enrichment during relic C mineralization on calculated C-budgets depends on type of residue returned to the soil. For systems where C4 residue are returned to soil derived from C3 and C4 plants, 13C enrichment of relic SOC during mineralization decreases the difference between the total (relic + new C incorporated into SOC) and relic SOC δ13C at the end of the experiment. Analysis showed that if this 13C enrichment is not considered, then the SOC half-life can be underestimated by 50%. To account for fractionation during mineralization experiments should contain no-plant controls where the relic C ε value is measured.