Soil microbial recolonisation after a fire in a Mediterranean forest

The capacity of different microbial groups to recolonise soil after a fire event will be decisive in determining the microbial community after the fire. Microbial recovery after a wildfire that occurred in Sierra la Grana (Alicante province, southeast Spain) was tracked for 32 months after the fire. Colony forming units (CFUs) of different microbial groups, microbial biomass, soil respiration, bacterial growth (leucine incorporation) and changes in the microbial community structure (phospholipid fatty acid (PLFA) analysis) were determined directly after the fire and four times during the recovery period. Direct effects were reflected by low values of most microbiological variables measured immediately after the fire. Microbial biomass increased during the first year after the fire but was below the unburned reference site 32 months after the fire. Bacterial activity and soil respiration showed the highest values immediately after the fire, but decreased to values similar to that of the unburned reference site or even lower (respiration) 32 months after the fire. Colony forming units of bacterial groups estimated by the plate count method peaked 8 months after the fire, but then decreased, showing values similar to the unburned reference site at the end of the study, with the exception of spore formers, which were 20 times higher than the reference site 32 months after the fire. Fungal CFUs were more sensitive to the fire and recovered more slowly than bacteria. Fungi recovering less rapidly than bacteria were also indicated by the PLFA pattern, with PLFAs indicative of fungi being less common after the fire. The recovery of microbial biomass and activity was mirrored by the initially very high levels of dissolved organic carbon being consumed and decreasing within 8 months after the fire. The wildfire event had thus resulted in a decrease in microbial biomass, with a more bacteria-dominated microbial community.