Severe Wildfires Reduce Soil Microbial Exoenzyme Production and Fungal Abundances in the Southern Appalachian Mountains
Climate change has increased drought frequency and duration globally. This effect has, and will continue, to increase fire occurrence across many regions of North America. In the Southern Appalachian Mountains, wildfires with high severity occurred in 2016 due to increased drought and human activity. Surface soils were collected from two sites: the Great Smoky Mountains National Park (GRSM) in Tennessee and the Nantahala National Forest (NNF) in North Carolina, USA spanning light, moderate, and severely burned areas and unburned controls. Surface soil samples to a depth of 15 cm were collected at three time points between 2017 and 2019 (~0.5 years, ~1 year, and 2.5 years post-fire) among burn severity plots. Microbial extracellular enzyme activities of carbon acquiring enzymes beta-glucosidase (BG), cellobiohydrolase (CBH), and beta-xylosidase (BX) were measured. Nitrogen acquiring enzymes beta-N-acetylglucosaminidase (NAG), leucyl aminopeptidase (LAP), and phosphorus acquiring enzyme acid phosphatase (AP) were measured using high-throughput fluorometric assays in addition to microbial population sizes (16S rRNA (ribosomal ribonucleic acid) for bacteria and ITS2 (internal transcribed spacer 2) genes for fungi) via quantitative-PCR. Total hydrolytic enzyme production varied over time with severe plots having significantly lower enzyme production at 2.5 years post wildfire. Individual enzymes varied among burn severity treatments and time. Light burns elicited greater carbon-specific (BG, BX) and P-specific (AP) activities six months post-fire, but this effect was transient. After 2.5 years post-burn, BX and AP were lower in severe or moderate burns relative to controls. In contrast LAP was lowest in severe burns six months post-fire, but by 2.5 years, was lowest in light burns. Fungal:bacterial ratios declined with burn severity indicative that fungi are sensitive or less resilient to wildfire burn severity during recovery. These results suggest wildfires alter the trajectories of recovery for soil microbial structure and function within a 2.5-year timeframe and potentially have long-term impacts on biogeochemical cycling.