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

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 δ13C values of CO2 and of soil organic matter (SOM) after a C3 to C4 vegetation change will enable to calculate the relative availability of younger (C4-derived) and older C (C3-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 C3 reference (wheat), a C4/fallow (fallow after one year of maize cropping), and a C4/C4 (two years of maize cropping). Based on the δ13C of CO2 purified from the admixture of atmospheric CO2 by the Miller/Tans model and on the δ13C 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 (C4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C3-derived). We concluded that simultaneous analysis of the δ13C 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.