Divergent effects of biomass-derived carbon dots application and sweetpotato planting on accumulations of soil microbial necromass carbon in Vertisol

Biomass-derived nano carbon dots (CDs) application and sweetpotato (SP) planting can alter soil microbial community structure. However, the impact of these treatments on soil microbial necromass carbon (MNC) and associated mechanisms remains unclear. In this study, we combined pot experiments and laboratory analyses to assess soil physicochemical properties, microbial community characteristics, metabolic enzyme activity and multivariate correlations, aiming to explore the determinants of soil MNC accumulation in Vertisol following CDs application and SP planting. The results showed inconsistent effects of CDs application on soil bacterial and fungal necromass C content. Although CDs application increased bacterial richness and the relative abundances of Proteobacteria, Firmicutes and Cyanobacteria in the absence of SP planting, it failed to enhance bacterial necromass C accumulation. In contrast, CDs application improved soil fungal necromass C content regardless of SP planting. Besides directly increasing soil organic carbon (SOC) concentrations, both CDs application and SP planting enhanced the contributions of fungal necromass C to SOC. However, SP planting neither increased soil dissolved organic carbon (DOC) nor altered the compound contents in DOC solution. Extracellular enzymes related to C-cycling (e.g., β-α-cellobiohydrolase and β-1,4-xylosidase) also significantly diminished under SP planting without CDs application. Linear discriminant analysis (LDA) identified distinct bacterial and fungal genera between the CDs application and SP planting treatments. Structural equation models (SEMs) revealed that the reduced accumulation of bacterial necromass C was primarily driven by increased β-1,4-glucosidase activity and shifted in bacterial community composition, which limited microbial substrate utilization and growth. The increased fungal necromass C accumulation was attributed to altering fungal community structure and decreasing α-diversity, which promoted necromass formation through sequential assimilation, synthesis, and turnover of CDs and SP carbon inputs. These results highlight the differential responses of bacterial and fungal necromass accumulation to CDs application and SP planting, providing novel insights into the regulatory roles of nano CDs and plant-microbe interactions in SOC sequestration processes in Vertisol.