Soil microbial community resistance to drought and links to C stabilization in an Australian grassland

Drought is predicted to increase in many areas of the world, which can greatly influence soil microbial community structure and C stabilization. Increasing soil carbon (C) stabilization is an important strategy to mitigate climate change effects, but the underlying processes promoting C stabilization are still unclear. Microbes are an important contributor of C stabilization through the adsorption of microbial-derived compounds on organo-mineral complexes. Management practices, such as addition of organic amendments might increase soil C stock and mitigate drought impacts, especially in agro-ecosystems where large losses of C have been reported. Here, we conducted a drought experiment where we tested whether the addition of organic amendments mitigates drought effects on soil C stabilization and its links to microbial community changes. In a semi-natural grassland system of eastern Australia, we combined a management treatment (compost vs. inorganic fertilizer addition) and a drought treatment using rainout shelters (half vs. ambient precipitation). We measured soil moisture, soil nitrogen and phosphorus, particulate organic C (Pom-C) and organo-mineral C (Min-C). Microbial community composition and biomass were assessed with PLFA analyses. A structural equation modeling (SEM) approach was used to examine the controls of soil moisture, Pom-C and nutrients on soil microbial biomass and community structure and changes in Min-C. Overall, the drought treatment did not affect microbial community structure and Min-C, while fertilizer only marginally increased Min-C, highlighting the resistance to these treatments in this grassland soil. In the surface soil (0-5 cm) Min-C was strongly associated with fungi that may have been stimulated by root exudates, and by gram-negative bacteria in the deep soil (5-15 cm) that were more affected by Pom-C and soil moisture.. We conclude that the grassland microbial community and its effect on Min-C at our field-site were non-responsive to our drought treatment, but sensitive to variability in soil moisture and microbial community structure. Our findings also show that surface compost application can moderately increase soil C stabilization under drought, representing a useful tool for improving soil C stability.