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Soil and Crop Management

Theoretical Derivation of Stable and Nonisotopic Approaches for Assessing Soil Organic Carbon Turnover

^a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
^b USDA-ARS, Northern Grain Insect Research Lab., Brookings, SD 57006

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

Received for publication March 4, 2005. Techniques for measuring soil organic C (SOC) turnover in production fields are needed. The objectives of this study were to propose and test nonisotopic and ¹³C stable isotopic techniques for assessing SOC turnover. Based on SOC equilibrium and mass balance relationships, an equation was derived: NHC/SOC_initial = [1/(SOC x k_NHC)](dSOC/dt) + k_SOC/k_NHC, where dSOC/dt is the annual change in SOC, NHC is nonharvested C returned to soil, k_SOC is the annual mineralization rate of SOC, and k_NHC is the annual mineralization rate of NHC. This equation was used to calculate maintenance rates. An isotopic approach based on simultaneously solving the equations was developed to determine C budgets: (i) SOC_retained = [SOC_final (_{soil final} – _PCR)/(_SOCretained – _PCR)]; (ii) _{SOC retained} = _{soc initial} – [ ln(SOC_retained/SOC_initial)], (iii) _PCR = _NHC – [ ln(PCR/NHC)]; and (iv) SOC_final = SOC_retained + PCR_incorp, where is the Rayleigh fractionation constant, PCR_incorp is the amount of NHC incorporated into SOC_final with _PCR being the associated ¹³C discrimination () value, and SOC_retained is the amount of SOC_initial retained in the soil after mineralization with _{SOC retained} being the associated value. Isotopic and nonisotopic approaches were tested on a production field where aboveground corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields were measured with a yield monitor and soil samples collected from a slightly offset grid (30 m) in 1995 and 2003 were analyzed for organic C and . The nonisotopic approach showed that maintenance rates increased with SOC and that an accurate measure of NHC was required to calculate maintenance requirements. Sensitivity analysis of the isotopic approach showed that calculated budgets were sensitive to ¹³C discrimination during SOC mineralization. If ¹³C discrimination during SOC and NHC mineralization did not occur ( = 0), then 14.9 and 7.6% of the SOC measured in 1995 (SOC_initial) was mineralized, and 7420 and 2780 kg C ha^–1 of NHC were incorporated into SOC_final in the 523.4- to 527.3- and 527.3- to 529.2-m elevation zones, respectively. If ¹³C discrimination occurred ( = –2.52) during SOC mineralization, then the calculated amount of SOC mineralized and the amount of new C incorporated into SOC were reduced.

Abbreviations: , ¹³C discrimination value • NHC, nonharvested C • PCR, plant biomass C remaining in the soil • SOC, soil organic C

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