Necromass carbon but not microbes constrain soil carbon release in restoration of degraded alpine grassland

Alpine grassland restoration, a critical strategy for enhancing soil organic carbon (SOC) sequestration in high-altitude ecosystems, profoundly influences plant‒soil‒microbe interactions that govern the magnitude of carbon (C)-climate feedback. However, mechanisms driving plant and microbial regulation of SOC mineralization (i.e., soil CO2-C release) during degraded alpine grassland restoration remain unresolved, limiting predictions of SOC cycling in these vulnerable ecosystems. Here, by integrating passive and active restoration experiments with aerobic incubation, high-throughput sequencing, and biomarker analyses, we disentangled how restoration-induced shifts in SOC composition (plant- and microbial-derived C) and microbial activity and diversity regulate soil CO2-C release in degraded alpine grassland on the Qinghai-Tibetan Plateau. Our results showed that soil CO2-C release increased significantly with restoration progression under both passive and active approaches. Alpine grassland restoration markedly enhanced plant-derived C accumulation and its SOC contribution, while microbial-derived C remained unchanged due to reduced necromass accumulation coefficients. Notably, although active restoration accelerated plant-derived C accumulation, its oxidation decomposition degree was lower compared to passive restoration and even to unrestored heavily degraded grasslands, increasing the SOC pool lability. Fungal community restructuring, particularly in the saprophytic fungal community, emerged as a hallmark of restoration. More importantly, we found that elevated soil CO2-C release during degraded alpine grassland restoration was not primarily mediated by microbial activity and diversity shifts but strongly linked to divergent plant- and microbial-derived C accumulation patterns, especially the dynamics of plant-derived C. These insights underscore the critical roles of plant- and microbial-derived C redistribution in grassland restoration and suggest new mechanisms for restoration-induced soil C dynamics.