Altered rainfall regimes modify microbial biomass and activity in Australian drylands

Anthropogenic climate change effects are becoming increasingly noticeable, particularly concerning extreme events, such as drought. Drought constrains water availability which negatively impacts plant growth and carbon (C) inputs into the soil. Additionally, water limitation impairs nutrient availability and soil microbial activity with cascading impacts on biogeochemical cycles, which further influence plant growth. The complexity of soil, however, make it difficult to quantify microbially-driven changes in nutrient cycling during water constraints and the resulting effects on ecosystem function. The main objective of this study was to expand knowledge on belowground responses to drought and how these are governed by historical rainfall, in drylands. We explored how four years of reduced and increased rainfall influenced microbial biomass and extracellular enzyme activity under field conditions in 6 drylands in New South Wales and Queensland, Australia. The study coincided with a significant 3-year drought that impacted all 6 sites, resulting in lower plant standing biomass at all sites. We found that there was considerable variation in microbial biomass across sites, with rainfall manipulations inducing changes in fungal biomass (using the phospholipid fatty acid 18:2w6c as a marker), and an associated increase in fungal:bacterial ratios, at a subset of the sites. Similarly, microbial biomass C showed a response to altered rainfall regimes at a subset of the sites only. These responses were particularly pronounced when reduced and increased rainfall treatments were compared, with greater fungal biomass and microbial C in the increased rainfall treatments. However, there were strong treatment effects on enzyme activity across all sites for enzymes associated with nitrogen cycling, indicating that microbial activity can be greatly impacted by rainfall changes even if biomass show limited responses. This suggest that C and nutrient cycling governed by microbes is likely to be impacted by altered rainfall patterns which will have ecosystem-wide effects on dryland biochemical cycling.