Climate warming can trigger abrupt vegetation shifts, potentially altering soil organic carbon (SOC) dynamics and reshaping ecosystem feedbacks to the global climate. However, due to the challenge of capturing slow SOC processes across diverse biomes, the underlying mechanisms remain less explored.

Elevational gradients in mountains provide natural experimental settings to enhance our understanding of how climate-induced vegetation changes influence belowground processes. Here, we investigated the patterns and thresholds of bulk soil carbon and its key fractions along nearly a 2000-meter elevational gradient, where vegetation transitions from subtropical forests to subalpine shrublands and alpine grasslands.

We found that bulk soil carbon concentration increased by 5.11% in the topsoil and 2.11% in the subsoil per 1000 meters of elevation rise below the treeline, but plateaued beyond it. This plateau was driven by a decline in particulate organic matter carbon (POM-C), which offset the continued increases in mineral-associated organic matter carbon (MAOM-C), especially Fe/Al oxide-associated carbon (Fe/Al-C). Our findings further revealed that the distinct responses of POM-C, MAOM-C and Fe/Al-C were closely linked to abrupt changes in plant, microbial and soil geochemical properties near the treeline. The decline in topsoil POM-C could be primarily attributed to reduced above-ground litter input as vegetation transitioned to subalpine shrublands, while the subsoil POM-C decline was more strongly associated with a rapid decrease in subsoil fine-root biomass above the treeline. In contrast, the accumulation of stable soil carbon fractions, MAOM-C and its components Fe/Al-C, was primarily regulated by poorly crystalline Fe/Al oxides, with additional modulation by fine-root biomass and microbial biomass.

Synthesis. Our results identified critical thresholds in soil biotic and abiotic factors, highlighting the dynamics of soil carbon storage along elevational gradients. These results provide new insights into the links between soil ecological processes and vegetation succession under climate change. Mountain ecosystems, particularly treeline ecotones, are highly sensitive to climate changes. This underscores the urgent need for careful land management in these regions to prevent disturbances that could trigger unexpected ecosystem shifts.