Influence of cropping system management and crop residue addition on soil carbon turnover through the microbial biomass

The fate and turnover of microbial carbon (C) in an arable soil following crop residue addition likely depends on the quality of both native soil organic matter (SOM) and residues. We labeled the microbial biomass with C-13-glucose and followed the microbial C-13 turnover into different SOM pools under the influence of three plant amendments (mature wheat, immature wheat, and vetch) in a laboratory incubation experiment using a soil with two different contents of organic C (0.9 and 1.3 %) owing to different soil management. At the end of incubation, more labeled glucose C was assimilated into microbial biomass in amended samples compared to an unamended control. The addition of plant residues also caused a positive priming effect, enhancing mineralization of soil organic C, which led to overall less glucose-derived C being incorporated into soil C pools. This was more pronounced in the soil with lower soil organic C content. Recovery of microbial C as recalcitrant C ranged between 6.0 and 7.1 %, with no significant effect of initial SOM content and addition of plant materials. Although the short-term fate of C present in microbial biomass was clearly affected by residue additions and initial native SOM, the extent to which it was stabilized as recalcitrant C during this time was not affected and must therefore be controlled by other factors than soil amendments and land management.