Synergistic Interactions Between Warming and Elevated Atmospheric CO2 Increase Soil Organic Carbon Sequestration.

Warming and elevated CO2 (eCO2) are two potentially opposing climate-carbon (C) feedback mechanisms that modulate the magnitude of the land C sink, with warming decreasing and eCO2 increasing C sequestration. However, their net effect on soil organic C (SOC)-the largest terrestrial C stock-remains uncertain. Here, we quantify how warming, eCO2, and their interactions influence SOC by using 5558 paired observations from 1392 global studies across ecosystem types. Our study shows that warming reduces SOC by 8.0%, primarily by suppressing aboveground C inputs (-1.0%) and decreasing microbial C use efficiency (-9.9%). Concurrent warming and eCO2 increase SOC by 7.6%-a synergistic effect larger than eCO2 alone (+4.8%), primarily contributed by croplands. This outcome may result from increases in plant C inputs and soil nitrogen availability under eCO2, which facilitate microbial C assimilation and necromass formation (+3.9%). These processes promote the accumulation of mineral-associated C (+9.9%) and offset the negative effects of warming. The combined effect of warming and eCO2 is projected to increase SOC by 27.4 Pg C by 2100. Our findings highlight that synergistic interaction between warming and eCO2 increases SOC sequestration and enhances SOC stability under future climate change.