Previous patch-burn grazing research has largely overlooked how soil nutrients and microbial communities respond to the combined use of fire and grazing in this context. There is a perception that the use of fire with or without grazing is detrimental to soil nutrient availability in grasslands despite some evidence that their collective impacts do not negatively affect soil nutrient availability. We investigated how soil nutrient availability and microbial communities vary across the time since fire gradient created in patch-burn grazing pastures in southwestern North Dakota. We collected soil samples during the 2018 and 2019 grazing seasons in three patch-burn pastures grazed by cow-calf pairs and three patch-burn pastures grazed by sheep. We sampled four points per patch for sixteen points per pasture. We measured plant available nitrogen (ammonium and nitrate) monthly from June – September of each year and calcium, magnesium, phosphorous, potassium, total carbon, and total nitrogen in July of each year. In 2019, we surveyed microbial abundance and composition of broad taxonomic groups in June and measured relative decomposition activity using litter bags buried from June – September. We compare nutrient availability, microbial abundance, and relative decomposition activity between patches using mixed-effect regression models and Tukey post-hoc comparisons. We compare the microbial community composition with nutrient availability, soil moisture, and time since fire using multidimensional scaling with post-hoc factor and vector fitting.
Results/Conclusions
Recently burned patches had higher nitrate, total nitrogen, total carbon, potassium, relative decomposition activity, microbial biomass, and soil moisture than patches with two years since fire or patches that had not yet been burned. Time since fire significantly influenced microbial composition with patches that had not yet been burned being significantly different from recently burned patches. Soil moisture, nitrate, potassium, calcium, and total carbon significantly influenced microbial composition. This suggests that patch-burn grazing did not negatively affect the measured soil nutrients or microbial biomass.