Precipitation-dependent nonlinear accumulation of soil organic carbon in response to warming via microbial residues
Climate change reshapes global terrestrial ecosystems via rising temperatures and shifting precipitation, jointly regulating soil organic carbon (SOC) dynamics. Multi-factorial experiments are vital for unraveling climatic interactions, yet most only apply one warming level, leaving ecosystem nonlinear responses poorly explored. Based on a five-year field experiment in an alpine meadow, we examined the interactive effects of multi-level warming (+0, +1, +2 and +4 °C) and increased precipitation (+50%) on SOC and its nonlinear responses to warming. We found that +4 °C warming combined with increased precipitation led to a 55% increase in SOC in the topsoil (0–10 cm), whereas warming alone had negligible effects. This SOC increase is primarily attributed to the accumulation of mineral-associated organic carbon linked to fungal residues. Lower warming levels (+1 °C and +2 °C) had little impact on mineral-associated organic carbon, fungal residues, and bacterial biomass, whereas these variables increased markedly at +4 °C only under increased precipitation. This consistent gradient trend revealed a precipitation-dependent nonlinear response of the three belowground variables to warming, with identified functional thresholds falling within the 2–4 °C range. These findings highlight the need to account for interactive climate drivers and potential nonlinear ecosystem responses to refine carbon–climate feedback projections.