Soil extracellular enzyme activity and stoichiometry in China’s forests

1. Ecoenzymatic stoichiometry links microbial decomposition with nutrient mineralization and improves our understanding of nutrient cycling in terrestrial ecosystems. Microbial C:N:P acquisition in the topsoil converged at a ratio of 1:1:1 in global ecosystems. However, whether the ratio of microbial acquisition is stable in forest soils, and is applicable among different soil depths remain unknown.

2. Based on large-scale soil sampling in China’s forests, we examined the patterns and environmental drivers of the eight most-widely measured enzyme activities and the relevant stoichiometry.

3. We found that the ratio of C:N:P acquisition significantly deviated from 1:1:1. The specific enzyme activities (g SOC-1) did not change significantly with latitude except those for xylosidase and acid phosphatase. Similarly, only the C:P acquisition ratio increased with latitude. Vertically, the specific activities of C-acquiring enzymes mainly increased, N-acquiring enzymes decreased, and P-acquiring enzymes did not change with soil depth. Moreover, all ratios of microbial acquisition decreased, and the percentage of recalcitrant C increased significantly with increasing depth. Our study also showed that temperature and soil C:N ratio were the important factors in explaining the variations in specific enzyme activities and microbial nutrient acquisition.

4. Our results indicated that no constant microbial C:N:P acquisition ratio can be widely recognized, and that SOC quality changed from labile to recalcitrant with depth. We highlight that depth-dependent enzymatic processes should be considered in future SOC dynamic models.