Mild Decade‐Warming Shifts Soil Organic Carbon Composition Without Altering Total Stock in Temperate and Subtropical Forests

Although soil respiration is known to increase with warming, the long‐term stability of total forest soil organic carbon (SOC) stocks and their compositional changes remain uncertain. Through a 12‐year soil warming experiment (+1.21°C) in temperate and subtropical forests, we found that SOC content remained stable in both 0–10 cm and 10–20 cm layers. A supporting global meta‐analysis further indicated that substantial SOC loss in forests occurs only under sustained warming exceeding 2°C. However, after 12 years, warming induced distinct compositional shifts. In the temperate forest, mineral‐associated C and the aromatic C/O‐alkyl C ratio declined, suggesting weakened mineral protection and lignin decomposition. Conversely, in the subtropical forest, alkyl C and the alkyl C/O‐alkyl C ratio increased, indicating accumulation of recalcitrant compounds. These results demonstrate that while mild long‐term warming below 2°C can maintain overall SOC stocks, it drives ecosystem‐specific compositional transformations that may influence future soil C persistence under continued climate change.