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

Pausch, Johanna, Kuzyakov, Yakov
Soil biology & biochemistry 2012 v.55 pp. 40-47
carbon dioxide, soil organic carbon, models, fallow, corn, microorganisms, soil depth, wheat, soil sampling, roots, vegetation, crops
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.