Fire regime impacts on soil microbes, soil organic carbon and ground cover in an Australian tropical savanna

Background Soil microbes drive the carbon cycle, yet are understudied in relation to long-term fire regimes in tropical savannas. Aim Explore the impact of fire regime on soil microbes and organic carbon. Methods We sampled topsoils (0–10 cm) of a tropical savanna near Darwin, Australia, where the frequency and season of fire had been experimentally managed for 17 years. We measured the effects of fire regime on microbial abundance, fungal-to-bacterial (F:B) ratio, soil physicochemistry (organic carbon, total nitrogen, C:N ratio, pH) and vegetative ground cover (grasses, leaf litter). Key results Microbial abundance was most influenced by fire season, minimally affected by fire frequency and reduced by both grass and litter cover; the magnitude of grass cover effect differed among paleoecological groups (i.e. ancient and modern). Soil organic carbon was not affected by fire treatments, nor was the F:B ratio. Conclusions Our data indicate that soil organic carbon, microbial abundance and F:B ratio are slow to change in a tropical savanna despite 17 years of imposed fire regimes. Implications Soil microbes in savanna ecosystems may have evolved resilience to variable fire regimes and the potential for soil carbon sequestration in Australia from fire suppression is likely limited within human timescales.