Drought legacy interacts with wildfire to alter soil microbial communities in a Mediterranean climate-type forest.

Mediterranean forest ecosystems will be increasingly affected by hotter drought and more frequent and severe wildfire events in the future. However, little is known about the longer-term responses of these forests to multiple disturbances and the forests' capacity to maintain ecosystem function. This is particularly so for below-ground organisms, which have received less attention than those above-ground, despite their essential contributions to forest function. We investigated rhizosphere microbial communities in a resprouting Eucalyptus marginata forest, southwestern Australia, that had experienced a severe wildfire four years previously, and a hotter drought eight years previously. Our aim was to understand how microbial communities are affected over longer-term trajectories by hotter drought and wildfire, singularly, and in combination. Fungal and bacterial DNA was extracted from soil samples, amplified, and subjected to high throughput sequencing. Richness, diversity, composition, and putative functional groups were then examined. We found a monotonic decrease in fungal, but not bacterial, richness and diversity with increasing disturbance with the greatest changes resulting from the combination of drought and wildfire. Overall fungal and bacterial community composition reflected a stronger effect of fire than drought, but the combination of both produced the greatest number of indicator taxa for fungi, and a significant negative effect on the abundance of several fungal functional groups. Key mycorrhizal fungi, fungal saprotrophs and fungal pathogens were found at lower proportions in sites affected by drought plus wildfire. Wildfire had a positive effect on bacterial hydrogen and bacterial nitrogen recyclers. Fungal community composition was positively correlated with live tree height. These results suggest that microbial communities, in particular key fungal functional groups, are highly responsive to wildfire following drought. Thus, a legacy of past climate conditions such as hotter drought can be important for mediating the responses of soil microbial communities to subsequent disturbance like wildfire.