Soil organic carbon decomposition from recently added and older sources estimated by delta C-13 values of CO2 and organic matter

Abstract

The production of CO2 in soil strongly depends on the availability of organic carbon (C) for microorganisms. It is obvious, that C that entered the soil recently is more easily available for microorganisms in comparison to older C. However, only very few approaches allow for a quantitative estimation of the availability of C in relation to the time it is entering the soil. We hypothesized that delta C-13 values of CO2 and of soil organic matter (SOM) after a C-3 to C-4 vegetation change will enable to calculate the relative availability of younger (C-4-derived) and older C (C-3-derived) sources for microorganisms. Soil CO2 was sampled over one vegetation period at depths of 10, 40-50 and 60-70 cm at three treatments: a C-3 reference (wheat), a C-4/fallow (fallow after one year of maize cropping), and a C-4/C-4 (two years of maize cropping). Based on the delta C-13 of CO2 purified from the admixture of atmospheric CO2 by the Miller/Tans model and on the delta C-13 values of SOM, the contributions of younger and older C sources to CO2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO2 and SOM, we found that the relative availability of organics recently introduced into the soil (C-4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C-3-derived). We concluded that simultaneous analysis of the delta C-13 values of both SOM and of CO2 allows not only for the quantification of the CO2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms. (C) 2012 Elsevier Ltd. All rights reserved.