Prescribed burns drive lasting changes in soil nitrogen cycling and microbial function.

Abstract

Fire is a major pulse disturbance to soil microbial communities, with broad implications for nutrient cycling; however, regular burning is also a natural and often-essential process maintaining biodiversity in unique and imperiled fire-dependent ecosystems. Prescribed fire is widely used to promote this biodiversity and simultaneously reduce wildfire risk. Although such repeated burning is known to alter surface biodiversity, belowground soil geochemistry, and soil microbial community structure, the functional consequences (i.e., the metabolic capabilities that underlie the ecosystem services soil microorganisms provide) remain underexplored. Here, we examined the effects of 30 years of repeated prescribed fire at the Albany Pine Bush-a fire-dependent, inland pitch pine barren ecosystem of the northeastern United States. Compared with the control stands, we observed that this long-term fire management has led to substantial depletion of inorganic soil nitrogen, specifically nitrate. We found no meaningful differences in the higher-level taxonomic composition of soil prokaryotic or fungal communities; however, analysis of metagenome-assembled genomes assembled from these soils revealed several differentially abundant populations. Furthermore, our metagenomic analysis revealed significant changes in the nitrogen-cycling functional potential, specifically decreased dissimilatory nitrate reduction and denitrification potential in repeatedly burned soils. These functional shifts have important implications for both nutrient cycling and emissions of trace nitrogen gases from these soils. Our results suggest that functionally meaningful changes in the soil microbiome can persist between burn events, even when higher-order community membership appears stable. This may imply that repeated fire can deplete reactive nitrogen emissions from soils by lowering the functional capacity of nitrogen-reducing microbes.IMPORTANCEPrescribed fire is widely used by land managers to reduce wildfire risk and promote biodiversity. While the effects of fire on aboveground processes are well understood, much less is known about how repeated burning influences soil biological properties-including the functional role that soil microorganisms play in nutrient cycling and greenhouse gas production. We addressed this gap by studying soils from the Albany Pine Bush, a rare and endangered ecosystem that has experienced regular prescribed fires for 30 years. Long-term fire management significantly altered soil chemistry, specifically lowering the amount of nitrogen in the soil. In addition, we found that fire management decreased the genetic potential of the soil microbial community to produce nitrogen oxides-potent contributors to climate change. Thus, prescribed fire's contribution to greenhouse gas emissions may involve a complex relationship between direct fire-driven emissions, increased fire resilience of promoted vegetation, and-as suggested by our results-the reduced ability of soil microbes to produce greenhouse gases.