Time amplifies microbial necromassc contribution to soil organic carbon under nutrient addition

The dynamic equilibrium of soil organic carbon (SOC) pools is critical for mitigating global climate change and ensuring food security, with microbial necromass carbon identified as a core contributor to SOC formation and stability. As a key driver of global change, nutrient input has been shown to regulate SOC accumulation and microbial necromass turnover. However, the contribution of microbial necromass to SOC under nutrient enrichment, and particularly its temporal evolution, remains controversial and poorly understood. Therefore, we synthesized 303 datasets from 50 global studies measuring both microbial necromass and SOC to quantify how the effect sizes of total (TNC), fungal (FNC), and bacterial (BNC) necromass carbon on SOC change with experimental duration. We found that the effect size of microbial necromass was the most critical factor driving the SOC effect size. The effect size of microbial necromass increased at a substantially faster rate than that of SOC, supporting the Microbial Carbon Pump hypothesis. Interestingly, the contribution of the BNC effect size to the SOC effect size was time-independent and driven by nutrient type, whereas the contribution of the FNC effect size showed a convergent trend over time regulated by ecosystem type. In contrast, experimental duration significantly amplified the contribution of the TNC effect size to the SOC effect size. These findings not only provide key evidence to refine the dynamic prediction framework of the soil microbial carbon pump but also challenge the static parameter assumptions regarding microbial turnover in current Earth system models, offering new theoretical guidance for accurately assessing long-term soil carbon sequestration under global change.