Microbial inoculants addition increases microbial necromass but decreases plant lignin contribution to soil organic carbon in rice paddies

Microbial inoculants have the potential to facilitate the degradation of crop straw, thereby significantly affecting soil carbon (C) cycling and storage in agricultural ecosystems. However, the specific mechanisms by which microbial inoculants affect the accumulation of microbial- and plant -derived C in agricultural soils remain inadequately understood. To address this knowledge gap, field trials involving microbial inoculants were conducted at two experimental sites located in the Jianghan Plain and Songnen Plain. The objective was to investigate changes in soil microbial communities, extracellular enzyme activities, microbial biomarkers, plant biomarkers, as well as mineral-associated organic carbon (MAOC) and particulate organic carbon (POC). The addition of microbial inoculants increased substantially the soil bacterial abundance and associated extracellular enzyme activities. Additionally, the addition of microbial inoculants accelerated the degradation of lignin phenols in the soil while reducing the contribution of plant lignin to soil organic carbon (SOC). Moreover, they elevated both microbial necromass C content and its contribution to SOC. Although the addition of microbial inoculants had no significant effect on POC content, it increased the MAOC content. Notably, at the Songnen Plain, the addition of microbial inoculants led to a significant increase in SOC content. In contrast, no comparable effect on SOC was observed at the Jianghan Plain. Furthermore, the contribution of microbial necromass C to SOC was greater at the Jianghan Plain compared to the Songnen Plain, which was attributable to the higher mean annual temperature at the Jianghan Plain. Overall, the addition of microbial inoculants facilitated the degradation of plant lignin by enhancing soil microbial biomass and extracellular enzyme activities while concurrently increasing microbial necromass C and driving dynamic alterations in SOC. These findings underscore the potential implications of microbial inoculants for sustainable agricultural practices aimed at improving soil health.