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

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.