Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2006
Publication Date: 5/17/2007
Citation: Clay, D.E., Clapp, C.E., Kleinjan, J., Liu, Z., Woodard, H., Bly, A. 2007. 13C fractionation during relic soil organic C mineralization on carbon budgets and half-lives calculated using the stable isotope approach. Soil Science Society of America Journal. 71:1003-1009. Interpretive Summary: Agricultural management practices can play a significant role in the storage and release of carbon and hence have an important part in determining atmospheric carbon dioxide concentrations. We are attempting to understand some of the aspects of soil carbon dynamics in a followed silt loam soil in south central Minnesota, compared with a fallowed loam soil in South Dakota. We analyzed the carbon content and the naturally occurring stable carbon isotope ratio (C-13/C-12) of soil samples taken at the end of 22 years in Minnesota. The soil organic carbon (SOC) in the surface 30 cm decreased by 20 and 10% at the respective sites. The isotope C-13 ratio increased by 3.8 and 3.2% for the same sites. This enrichment in C-13 must be considered in calculations of the half-life of SOC over time. Findings on the SOC storage and its turnover rate estimated from the current carbon provide valuable information to environmental scientists, engineers and consultants concerning present global carbon budgets. Tillage and crop residue management can influence carbon storage in US croplands and can have a major impact on global warming, but long-term field experiments are needed to project carbon storage amounts and controlling conditions.
Technical Abstract: 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 delta 13C value of 0.72‰ (from -18.97 to -18.25‰). Based on these values, the Rayleigh fractionation constant (epsilon) of relic SOC was -3.45‰. At the South Dakota site, SOC and delta 13C 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 delta 13C 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. In a simulation study, the effect of 13C discrimination during relic SOC mineralization was evaluated. For systems where C4 residue are returned to soil derived from C3 and C4 plants, 13C enrichment of relic SOC during mineralization decreased the difference between the total SOC (relic + new C incorporated into SOC) at the end of the study and relic SOC delta 13C remaining in the soil after mineralization. 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 epsilon value is measured.