Differential seasonal effects of water addition and nitrogen fertilization on microbial biomass and diversity in a temperate desert

We investigated seasonal changes of soil microbial respiration (MR), biomass carbon (MBC) and nitrogen (MBN) and bacterial and fungal community composition in response to manipulative water addition and N fertilization over three years in the Gurbantunggut Desert, Northwestern China. Water addition significantly interacted with season to affect MR, MBC and MBN, with significant promotions of MR by 59.4-107.7%, MBC by 43.1-43.5% and MBN by 56.3-91.4% in summer and autumn, and no impacts in spring and winter. Water addition increased bacterial biomass in summer and autumn, bacterial and fungal diversity and abundance in summer, and altered microbial community composition by increasing the relative abundance of Proteobacteria phylum and decreasing Ascomycota phylum. Nitrogen fertilization increased microbial biomass by 14.0% and fungal biomass by 30.0% in winter, and altered microbial community composition in winter and spring by increasing the relative abundance of Ascomycota phylum and decreasing Actinobacteria phylum. Our results indicate the high sensitivity of soil microbial communities to water and nitrogen availability in a temperate desert. The projected increase in precipitation and N deposition in the desert may induce corresponding changes in ecological processes that involve microbes. The results also highlight the importance of seasonal effects on the responses of soil microbial community, which should be considered in ecosystem models.