Variations of the effects of reduced precipitation and N addition on microbial diversity among different seasons in a temperate forest

Changes in microbial diversity affect biogeochemical cycles and ecosystem functioning. Increasing nitrogen (N) deposition and reducing precipitation significantly affect microbial diversity during the growing season. Furthermore, the variations of the effects of reduced precipitation and N addition on microbial diversity among different periods is still unclear. In order to reveal these effects, an 8-year field experiment was set up in a temperate old growth forest of northeast China, including four treatments: control, reduced precipitation (−30% of through-fall), N addition (50 kg N ha−1 yr−1), and their combined effect (−30% of through-fall and 50 kg N ha−1 yr−1), respectively. To reach a better understanding, microbial diversity among different periods was measured. The results showed that bacterial and fungal diversity significantly decreased in the growing season with N addition but significantly increased in winter. These positive or negative effects of reduced precipitation and N addition on bacterial and fungal diversity were attributed to changes in diversity of the main phyla of bacteria and fungi. Moreover, the positive impact of reduced precipitation on fungal diversity increased with N addition in the winter, while it decreased with N addition in the growing season. Our structural equation modeling showed that the negative impacts of reduced precipitation on soil moisture led to increased bacterial and fungal diversity. Our results illustrate that the direction of effect of reduced precipitation and N addition on microbial diversity may be shifted with seasonal changes and microbial taxonomic level, which provides new insight and framework for understanding and modeling responses of microbial diversity to N deposition and changes in precipitation pattern during different seasons.