Temporal amplification of microbial necromass contribution to soil organic carbon under nutrient addition

The balance of soil organic carbon (SOC) pools is vital for mitigating global climate change and securing food supplies, with microbial necromass carbon (MNC) being identified as a primary driver of SOC formation and stability. As a key driver of global change, nutrient inputs have been shown to regulate the accumulation of SOC and microbial necromass turnover. However, the contributions of microbial necromass to SOC under nutrient enrichment, especially its changes over time, are still unclear. Therefore, we synthesized 303 datasets from 50 global studies that measured both microbial necromass and SOC, to quantify how the effect sizes of total necromass carbon (TNC), fungal necromass carbon (FNC), and bacterial necromass carbon (BNC) on SOC altered with experimental duration. We found that the effect size of microbial necromass was the most critical factor impacting that of the SOC. The microbial necromass effect size increased at a faster rate than that of SOC, which supported the microbial carbon pump hypothesis. Interestingly, the contribution of the BNC effect size to that of the SOC was temporally independent and driven by nutrient type, whereas the contribution of the FNC effect size exhibited a convergent trend over time that was regulated by ecosystem type. In contrast, the experimental duration significantly amplified the contribution of the TNC effect size to that of the SOC. These findings indicate that long term SOC sequestration is not a simple linear accumulation of specific residue components, but is better understood as an emergent system property arising from the time dependent coordination of distinct stabilization pathways. Incorporating this temporal dependence into the analytical framework deepens our understanding of microbial carbon pump dynamics and provides a stronger mechanistic basis for assessing long term soil carbon sink potential under global change.