Effect of prescribed fire on microbial indicators of soil biogeochemistry in a mixed-grass prairie

Prescribed fire is a widely used management tool for restoring grassland ecosystems. This study assessed the effects of prescribed fire on soil microbial communities and physicochemical properties in both tame (non-native forage) and native pastures within a northern mixed-grass prairie in southwestern Saskatchewan, Canada. Soil samples (0–10 cm depth) were collected from burned and adjacent unburned areas in 2018 (first growing season post-fire) and in 2019 (1-year post-fire). Microbial abundance and community composition were analyzed using phospholipid fatty acid (PLFA) profiling. Prescribed fire caused minimal short-term changes in soil microbial communities and properties, with responses varying by pasture type. In the tame pasture, fire initially increased fungal abundance and the Gram-negative to Gram-positive bacterial ratio (GN:GP), while reducing microbial biomass C and N, and total C and N, indicating organic matter loss and nutrient volatilization. Although fungal abundance recovered by the following year, the elevated GN:GP ratio persisted, suggesting that post-fire conditions favored fast-growing GN bacteria over spore-forming GP bacteria. Microbial community composition in both pasture types was strongly correlated with the relative abundances of fungi, total bacteria, and Gram-type bacterial groups. Native pastures showed greater microbial and nutrient stability across both sampling years. Overall, seasonal and annual variability exerted a stronger influence on microbial communities and soil nutrients than fire. These findings demonstrate the resilience of soil microbial communities to low-intensity prescribed fire in mixed-grass prairie ecosystems and suggest that fire, under controlled conditions, can be a viable management tool with short-term disruption to soil biological function.