Prescribed versus wildfire impacts on exotic plants and soil microbes in California grasslands

Prescribed burns are often used as a management tool to decrease exotic plant cover and increase native plant cover in grasslands. These changes may also be mediated by fire impacts on soil microbial communities, which drive plant productivity and function. Yet, the ecological effects of prescribed burns compared to wildfires on either plant or soil microbial composition remain unclear. Grassland fires account for roughly 80 % of global annual fires, but only roughly 12 % of research on belowground impacts of fires occurs in grasslands, limiting our understanding of aboveground belowground connections in these important habitats. Here, we took advantage of the serendipitous opportunity of a wildfire burning through the same reserve where we had previously sampled a prescribed burn. This enabled us to investigate the impacts of a spring prescribed burn versus a fall wildfire on plant cover and community composition and bacterial and fungal richness, abundance, and composition. Our California grassland sites were thus within the same reserve, limiting environmental, vegetation, or climate variation between the sites. We used qPCR of 16S and 18S to assess impacts on bacterial and fungal abundance and Illumina MiSeq of 16S and ITS2 to assess impacts on bacterial and fungal richness and composition. Wildfire had stronger impacts than prescribed burns on microbial communities and both fires had similar impacts on plants with both prescribed and wildfire reducing exotic plant cover but neither reducing exotic plant richness. Fungal richness declined after the wildfire but not prescribed burn, but bacterial richness was unaffected by either. Yet, fire exposure in both fire types resulted in reduced bacterial and fungal abundance and altered bacterial and fungal composition. Plant diversity differentially impacted soil microbial diversity, with exotic plant diversity positively impacting bacterial richness and having no effect on arbuscular mycorrhizal richness. However, the remainder of the soil microbial communities were more related to aspects of soil chemistry including cation exchange capacity, organic matter, pH and phosphorous. Our coupled plant and soil community sampling allowed us to capture the sensitivity to fire of the fungal community and highlights the importance of potentially incorporating management actions such as soil or fungal amendments to promote this critical com-munity that mediates native plant performance.