Soil warming did not enhance leaf litter decomposition in two subtropical forests

Climate warming is expected to increase leaf litter decomposition rates. However, higher temperature may not uniformly affect the factors that influence decomposition, resulting in unexpected decomposition patterns. Over a period of two years, we studied litter decomposition in response to soil warming in a native Castanopsis kawakamii forest and a Cunninghamia lanceolata (Chinese fir) plantation. Our study aimed to determine how soil warming affects litter mass loss, extracellular enzymatic activities (EEAs), litter chemistry, and microbial biomass. In the C. kawakamii forest, soil warming caused variation in litter mass loss with time but had no net effect on the litter decomposition constant. In the Chinese fir forest, however, soil warming had a consistent negative effect on litter mass loss, leading to a significant decrease in the decomposition constant by the end of the study. Additionally, there was a significantly slower mass loss of the acid-insoluble C fraction in Chinese fir litter in response to warming. Soil warming did not affect the microbial biomass of either litter, and its effect on EEAs varied with time. Litter moisture was significantly reduced by soil warming, with a greater reduction in Chinese fir litter (−9.2%) than in C. kawakamii litter (−4.1%). Applying an ANCOVA model, we found a significant correlation between litter mass loss rate and litter moisture, but not with other litter characteristics. Our results suggest that litter drying in soil warming does not affect microbial activities or litter microbial decomposition, but may diminish the physical leaching of soluble compounds from the litter into the underlying soil horizons. This may be a mechanism that explains the soil warming effect on litter decomposition in subtropical forests. • No net warming effect on litter decomposition in the Castanopsis kawakamii forest. • Warming significantly decreased litter decomposition in the Chinese fir plantation. • Warming didn't affect microbial biomass in two litter types. • Warming effect on extracellular enzymatic activities in two litters varied with time. • Litter mass loss rates were closely correlated with litter moisture.