Stronger Microbial Decay of Recalcitrant Carbon in Tropical Forests than in Subtropical and Temperate Forest Ecosystems

Soil enzymes are the most potent bioactive components of forest ecosystems. They are involved in carbon and nutrient cycling, and the ratio of ligninase to cellulase activity can be used to effectively track changes in the soil carbon (C) pools for microbial assimilation. However, our understanding of enzyme ratios and their drivers is still unclear across forest biomes. Here, we collected soil samples and analyzed enzyme properties in ten typical forest ecosystems along a forest transect of 3425 km spanning from 18°N to 45°N in China. The results showed that the average soil ligninase activity was 3.49 ± 0.79 nmol h-1 g-1, while the average soil cellulase activity was 525.26 ± 225.04 nmol h-1 g-1 across forests biomes. The average activity ratio of ligninase to cellulase in tropical forest biomes was 27.9% higher than in subtropical forests and 64.2% higher than in temperate forest ecosystems. The partial least squares regression (PLS) model demonstrated that the ligninase: cellulase activity ratio was significant negative with soil substrates (r= -0.94, p<0.001), and significant positive with microbial abundance (r= 0.38, p<0.01), respectively. The variation partitioning analysis (VPA) model further revealed that soil substrates were the most important driver, which explained 19.4% of the variation in the ratio of ligninase: cellulase activities, while the microbial abundance (fungal abundance), the subordinate driver, explained 2.8% of the variation. This study provides crucial information about the distribution of enzyme ratios along the latitude gradient and highlights the microbial utilization of recalcitrant C pools in tropical forests, providing an insight into the response of the global C cycle under a changing climate