Soil Microbial Nitrogen Cycling Responses to Wildfire in a High Elevation Forested Catchment in Jemez Mountains, NM

Abstract:

Microbial communities mediate major ecosystem processes such as nutrient cycling, and their recovery after disturbances plays a substantial role in overall ecosystem recovery and resilience. Disturbances directly shift microbial communities and their related processes, and the severity of impact typically varies significantly with landscape position, depth, and hydrological conditions such that different conditions indicate that a specific process will be dominant. Wildfires in the southwest US are a major source of landscape-scale disturbance, and are predicted to continue increasing in size and intensity under climate change. This study investigates changing nitrogen cycling across a post-wildfire catchment within the Jemez River Basin Critical Zone Observatory. This site experienced a mixed (intermediate to high) burn severity wildfire in June 2013. Nitrogen cycling was investigated by profiling via qPCR the abundance of five key genes involved in microbial nitrogen cycling (nifH, amoA, nirS, nirK, nosZ), at points along and within the catchment. These results are being analyzed in the context of broader microbial community data (enzyme assays, microbial cell counts and biomass, and 16S rRNA gene amplicons surveys) and biogeochemical data (total organic carbon, total nitrogen, pH, graviametric water content, etc). W 22 sites along the sides of the basin (planar zones) and within the hollow (convergent zone) were sampled at 13 days, one, and two years post-fire, at six discrete depth increments from 0 to 40 cm from the surface. We attribute significance of variation in gene abundance in planar versus convergent zones, and among depths, to the strong correlation of nitrogen cycling processes (i.e., nitrification and denitrification) with specific C:N ratios, total organic carbon content, and other biogeochemical and soil edaphic parameters that vary with landscape position and wildfire. Data were also interrogated for evidence of multi-year patterns in nutrient-cycling recovery post-fire, which are expected to correlate with patterns of nitrogen cycling post-wildfire seen in prominent literature.