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This article in AJ

  1. Vol. 98 No. 3, p. 443-450
    Received: Mar 4, 2005

    * Corresponding author(s): david.clay@sdstate.edu
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Theoretical Derivation of Stable and Nonisotopic Approaches for Assessing Soil Organic Carbon Turnover

  1. D. E. Clay *a,
  2. C. G. Carlsona,
  3. S. A. Claya,
  4. C. Reesea,
  5. Z. Liua,
  6. J. Changa and
  7. M. M. Ellsburyb
  1. a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
    b USDA-ARS, Northern Grain Insect Research Lab., Brookings, SD 57006


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/SOCinitial = [1/(SOC × 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) SOCretained = [SOCfinalsoil final − ΔPCR)/(ΔSOCretained − ΔPCR)]; (ii) ΔSOC retained = Δsoc initial − [ε ln(SOCretained/SOCinitial)], (iii) ΔPCR = ΔNHC − [ε ln(PCR/NHC)]; and (iv) SOCfinal = SOCretained + PCRincorp, where ε is the Rayleigh fractionation constant, PCRincorp is the amount of NHC incorporated into SOCfinal with ΔPCR being the associated 13C discrimination (Δ) value, and SOCretained is the amount of SOCinitial retained in the soil after mineralization with ΔSOC retained being the associated Δ 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 Δ. 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 (ε = 0), then 14.9 and 7.6% of the SOC measured in 1995 (SOCinitial) was mineralized, and 7420 and 2780 kg C ha−1 of NHC were incorporated into SOCfinal in the 523.4- to 527.3- and 527.3- to 529.2-m elevation zones, respectively. If 13C discrimination occurred (ε = −2.52‰) during SOC mineralization, then the calculated amount of SOC mineralized and the amount of new C incorporated into SOC were reduced.

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