Tropical forest soil under long-term ecosystem warming differentially accumulates organic carbon in different soil fractions over time

The fate of tropical forest soil organic carbon (SOC) under warming is crucial to predicting carbon-climate feedbacks, yet remains highly uncertain. Here, based on a 9-year ecosystem-level warming experiment (+0°C, +1.0°C, and +2.1°C), we observed two phases of change in SOC and its fractions under warming: (1) an initial net SOC loss in years 2-4 due to a reduction in mineral-associated organic carbon in topsoil; and (2) a subsequent net SOC accumulation in years 6-9 due to an increase in particulate organic carbon in both topsoil and subsoil. This phased variation was mainly driven by the sustained increase in plant carbon input and the attenuated SOC decomposition (soil heterotrophic respiration, microbial carbon use efficiency, and enzyme activities under warming returned to the same level as in the +0°C treatment). Considering changes in plant carbon input and microbial thermal adaptation, tropical forest soils may accumulate more plant-derived carbon under future warming.