Soil organic carbon decomposition from recently added and older sources estimated by δ13C values of CO2 and organic matter

The production of CO 2 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 δ 13 C values of CO 2 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 CO 2 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 δ 13 C of CO 2 purified from the admixture of atmospheric CO 2 by the Miller/Tans model and on the δ 13 C values of SOM, the contributions of younger and older C sources to CO 2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO 2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO 2 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 δ 13 C values of both SOM and of CO 2 allows not only for the quantification of the CO 2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms.