Variation of microbial residue contribution to soil organic carbon sequestration following land use change in a subtropical karst region

Microbial residues are an important constituent of persistent soil organic carbon (SOC). Nevertheless, how microbial residue contribution to bulk SOC sequestration changes following land use conversion remains poorly understood. Here, we investigated the variation of soil microbial residue C and its contribution to SOC following conversion of maize-soybean field to mulberry, forage grass and sugarcane fields in southwest China, with forest used as reference. Microbial residue C contents were estimated based on microbial cell wall biomarker amino sugars. The contents of fungal, bacterial and total microbial residue C were lowest in the sugarcane, maize-soybean and mulberry fields, intermediate in the forage grass field and highest in the forest. Fungal to bacterial residue C ratios were comparable among the five land use types, implying that the relative contribution of fungi and bacteria residues to SOC sequestration remained unchanged. Conversion of maize-soybean field to mulberry and sugarcane fields had no significant effect on microbial residue contribution to SOC sequestration. Nevertheless, the contribution of total microbial residues to SOC sequestration was marginally (p < 0.1) and significantly (p < 0.05) higher, respectively, in the forage grass field and forest than in the maize-soybean field. Levels of SOC and living microbial biomass were the major factors determining the dynamics of microbial residue contents. Stoichiometric imbalance of carbon: nitrogen ratios between soil organic matter and microbial biomass (i.e., C:N imbalance) was found to negatively correlate with the contribution of microbial residues to SOC sequestration. Our results suggest that increase of C:N imbalance may explain lowered microbial residue contribution to SOC sequestration following land use change.