Contribution of soil Microbial Necromass Carbon to Soil Organic Carbon fractions and its influencing factors in different grassland types

Abstract. Microbial necromass carbon(MNC) is a significant source of soil organic carbon (SOC), the quantitative contribution of MNC to distinct SOC fractions and its regulatory mechanisms across various grassland types remain largely unexplored. This study through a comprehensive investigation of soil profiles (0–20 cm, 20–40 cm, and 40–100 cm) across four grassland types in Ningxia, China, encompassing meadow steppe (MS), typical steppe (TS), desert steppe (DS), and steppe desert (SD). We quantified mineral-associated organic carbon (MAOC), particulate organic carbon (POC), and their respective microbial necromass components, including total microbial necromass carbon (TNC), fungal necromass carbon (FNC), and bacterial necromass carbon (BNC), and analyzed the contributions to SOC fractions and influencing factors. Our findings reveal three key insights. First, the contents of MAOC and POC in the 0–100 cm soil layer were in the following order of magnitude: Meadow steppe (MS) >Typical steppe (TS) > Desert steppe (DS) > Steppe desert (SD), with the average content of POC was 9.3 g/kg, which was higher than the average content of MAOC (8.73 g/kg). Second, the content of microbial TNC in MAOC and POC decreased with the depth of the soil layer, the average content of FNC was 3.02 g/kg and 3.85 g/kg, which was higher than the average content of BNC (1.64 g/kg and 2.08 g/kg). FNC dominated both MAOC and POC, and its contribution was higher than the contribution of BNC. Thid, through regression analysis and random forest modeling, we identified key environmental drivers of MNC dynamics: mean annual rainfall (MAP), electrical conductance (EC), and soil total nitrogen(TN) emerged as primary regulators in surface soils (0–20cm), while available potassium(AK), SOC, and mean annual temperature (MAT) dominated deeper soil layers (20–100 cm). This research by: 1) establishing the vertical distribution patterns of MNC and SOC fractions in soil profiles; 2) quantifying the relative contributions of MNC to SOC fractions across different grassland ecosystems soil profiles and elucidating their environmental controls, offering a deeper understanding of the mechanisms driving MNC to soc fractions accumulation in diverse grassland ecosystems, and provide data support for further research on the microbiological mechanisms of soil organic carbon formation and accumulation in arid and semi-arid regions.