Rhizosphere as a hotspot for microbial necromass deposition into the soil carbon pool

Microbial leftovers, known as necromass, are key players in storing carbon in the soil around plant roots (i.e. rhizosphere), a zone characterized by high-efficiency microbial anabolism. Yet, the extent and mechanisms through which the rhizosphere contributes to soil organic carbon (SOC) via microbial necromass, especially under changing environments remain unclear. We aimed to evaluate the contributions of microbial necromass to SOC and influencing factors from the rhizosphere perspective. We collected the rhizosphere and bulk soil from 39 alpine coniferous forest sites on the eastern Tibetan Plateau to assess the extent of microbial necromass contribution to SOC in the rhizosphere from a dynamic perspective by calculating the ratio of increased amino sugars (AS) to increased SOC in the rhizosphere relative to that in bulk soil (R_AS/SOC). We also collected climate data and determined nutrient concentrations and microbial physiological traits in rhizosphere soil to elucidate the factors affecting R_AS/SOC. The results showed that across all sampling sites, the average concentrations of SOC-normalized AS in the rhizosphere were significantly higher than those in the bulk soil. Furthermore, the average R_AS/SOC was greater than 1, indicating a faster microbial necromass accumulation than SOC accumulation in the rhizosphere. These results implied that the rhizosphere sustains a greater capacity for microbial necromass contribution to the SOC pool than the bulk soil does. Soil nutrient availability was the primary factor affecting R_AS/SOC, and precipitation indirectly affected microbial anabolism and R_AS/SOC by changing soil nutrient status. Additionally, with increasing rhizosphere soil nutrient availability, microbial carbon-use efficiency and growth rate increased but the biomass-specific enzyme activity declined, indicating that microorganisms tended to exhibit high-yield strategies with increasing soil nutrient availability. Synthesis. Our findings underpin the vital effect of microbial necromass in SOC accumulation from the rhizosphere perspective and offer valuable insights into mechanisms underlying microbial C metabolic processes in rhizosphere SOC accumulation under changing environments.