The Effect of Wildfire Severity and Hillslope Gradient on Ammonia Oxidizing Bacterial Abundance and Spatial Distribution in a Ponderosa Pine Forest

Consequences of increasing frequency of severe wildfire in forest systems is a major area of concern in forest systems that are nitrogen limited. Yet the effects of severe fire on key microorganisms that regulate nitrification, a critical process within nitrogen cycling, are poorly understood. This work underlines the importance of how both burn severity and hillslope gradient act as dominant driving forces of post-fire ammonia-oxidizing bacterial abundance and distribution. The effects of the physical environment on abundance were dependent on the burn severity over the rainy season. Only pH and charcoal content were positively correlated to abundance. Prediction maps made with Empirical Bayesian Kriging showed spatial patterns over time are determined by burn severity and hillslope gradient. Immediately following wildfire, AOB abundance is significantly higher in severely burned soils than in soils exposed to low severity burns, with orders of magnitude increases in abundance over the course of the rainy season on steeper hillslopes. Findings indicate hillslope gradient, operating conjointly with correlated soil parameters and precipitation, was a dominate driving force of distribution patterns and abundance on severely burned hillslopes. This may be important for forest systems, such as those in mountainous regions, that are already nitrogen limited whose topography makes them especially vulnerable to post-fire erosional losses of nitrogen. Further research in similar systems could provide valuable insight to land managers for restoration following severe fires.