Changes in soil organic carbon status and microbial community structure following biogas slurry application in a wheat-rice rotation

Biogas slurry is widely used as a crop fertilizer due to its available nitrogen content. However, it remains unclear how biogas slurry application affects soil organic carbon (SOC) status and soil microbial community under typical agricultural systems. Here, under a wheat-rice field experiment, we examined the responses of SOC and soil bacterial and fungal communities to biogas slurry application, both with (BSS) and without (BS) straw return, relative to chemical nitrogen fertilizer with (CFS) and without (CF) straw return. The BS treatment significantly increased total organic carbon (TOC) at all soil depths (0-60 cm), compared to CF. Greater TOC occurred at 20-40 cm depth under BSS relative to all other treatments. However, straw return had no impact on soil TOC content under the CF and CFS treatments. Labile organic carbon (LOC) in the topsoil and recalcitrant organic carbon (ROC) at 20-60 cm depth was significantly greater under BS relative to CF. The bacterial class Gammaproteobacteria and family Hyphomicrobiaceae were found to be specifically abundant under biogas slurry application after one year of wheat-rice double cropping. Network analyses showed that the soil bacterial community under biogas slurry application was more complex than under chemical fertilizer application, while the opposite was true for the fungal community. Correlations between network modules and the SOC fractions indicated that biogas slurry application stimulated soil bacteria and fungi to participate in SOC cycling. The module functionality supports our speculation that soil microorganisms degraded the biogas slurry derived-ROC in the topsoil. Overall, we conclude that substitution of chemical fertilizer with biogas slurry can be beneficial for increasing SOC stocks and, in systems with straw return, enhancing straw decomposition.