Frequent-wildfires with shortened time-since-fire affect soil microbial functional stability to drying and rewetting events

This study analysed the resistance and resilience to 1 and 4 successive drying and rewetting (D/Rw) cycles of some processes driven by soil microbes, when soils have previously suffered from frequent wildfires with both short and longer time since fire. We hypothesised that frequent and recent wildfires destroying sensitive microbial species together with resource availability (C and N) might hamper the stability of microbial functions against droughts especially narrow processes. Our findings showed that microbial respiration was stable against frequent and recent wildfires (no additional change), but not against drying and rewetting events. Indeed, microbial respiration increased after 1 D/Rw cycle mainly attributed to a loss of 18% of microbial active biomass (not resistant) and decreased fourteen days after rewetting the dry soil (not resilient). Even more, 4 D/Rw cycles immediately brought back this activity at lower level (i.e. not resistant) concomitantly with a loss of metabolic quotient (qCO2) especially with frequent and recent wildfires. Contrariwise, some specific activities such as cellulase activities and also catabolic level physiological profiles of cultivable bacteria (Biolog) were heavily impacted by the combination of frequent and recent wildfires and droughts. Alkaline phosphatases, which are synthesised by many bacterial and fungal species, increased at short term (i.e. not resistant) indicating an increase in microbial phosphorus demand for a long time (i.e. also not resilient). In this study, addition of unlimited availability of resources as cellulose and ammonium nitrate, suppressed the effect of wildfire regimes on functional stability of microbial communities. Hence, we concluded that resource availability poorly explained the stability against stresses of microbial processes, mainly driven by hydric stress, except for alkaline phosphatase activities which were depressed by an experimental fertilisation.