Drought stress induced increase of fungi:bacteria ratio in a poplar plantation

Soil microbial communities are key to ecosystem processes in terrestrial ecosystems. Although droughts are projected to be more frequent with the intensifying effects of climate change, our understanding of the responses of soil microbial communities to drought remains incomplete. For this study, we employed 30% and 50% throughfall reduction manipulation experiments to simulate different drought intensities, and collected soil samples at three depths (0-15, 15-30 and 30-45 cm) for each season in a poplar plantation. We analyzed the physical and chemical properties of the soil samples (e.g., soil moisture, pH, total C, total N, and labile organic C and N contents represented by extractable organic C and N contents, microbial biomass C and N contents) and microbial communities, via phospholipid fatty acid (PLFA) and high-throughput sequencing methods, from November 2018 to August 2019. We found that drought significantly decreased soil moisture, the content of soil labile organic C and N, and total microbial biomass; however, it increased soil pH and microbial biomass C:N across all soil depths. Furthermore, fungi:bacteria and Gram+:Gram- bacterial ratios increased with drought treatments across all three soil depths. Drought reduced the relative abundance of Proteobacteria, but increased the relative abundance of Acidobacteria across the three soil depths. In contrast, drought had no significant influence on the relative abundance of fungal phyla. The changes in soil properties and microbial communities increased with drought intensity. Regression analysis demonstrated that changes in the total PLFA, fungi:bacteria and Gram+:Gram- bacterial ratios were significantly correlated with soil moisture, pH, and the soil labile organic C and N content. Redundancy analysis revealed that soil pH, extractable organic C and extractable organic N accounted for the majority of the variabilities in the bacterial communities. The patterns of fungal community structures in the drought treatment plots were not distinct from the control groups across three soil depths. Our results suggested that droughts induce significant increases in fungi:bacteria and Gram+:Gram- bacterial ratios in forest soils, primarily through changes in soil pH, moisture, and soil labile organic C and N content.