Soil respiration related to the molecular composition of soil organic matter in subtropical and temperate forests under soil warming

The chemical composition and degradability of soil organic matter (SOM) are among the most important factors influencing the feedback between soil CO₂ emissions and climate warming. We hypothesized that the response of soil respiration to long-term warming in various forest ecosystems depends on how soil warming alters the chemical composition of SOM. Therefore, we compared the effects of long-term soil warming on soil respiration, SOM molecular structure, and bacterial and fungal diversity in two forest ecosystems in the southern subtropical and warm temperate zones of China. In the subtropical forest, soil warming did not affect soil respiration in the short term (2–3 years) but decreased it in the longer term (10 years, −10%). The decline in soil respiration was associated with an increased aliphaticity of SOM and lower O-alkyl C content, along with an increased abundance of microbial K-strategists over time. In the warm temperate forest, soil warming significantly stimulated soil respiration by 35% in the short term and 30% in the long term. The sustained positive response to warming was likely related to the increased decomposability of SOM owing to increased root C input. Our results suggest that the molecular composition of SOM is affected by warming and in turn feeds back to longer-term soil respiration responses. The different responses at the two study sites suggest considerable variation in the feedback within different forest ecosystems.