The Effects of Hurricane Flooding on Ammonium Oxidation in Agricultural Riparian Zones

Extensive flooding in North Carolina due to the landfall of Hurricane Florence in September 2018 had major implications for the biogeochemical conditions of the Neuse River watershed. This watershed is characterized by an extensive number of concentrated animal feeding operations (CAFOs), which produce vast amounts of waste and runoff in the form of nitrogen pollution. Flooding due to hurricane precipitation, which exceeded 20 inches over the course of three days in some areas of the watershed, resulted in the widespread release of hog waste stored in open air cesspools. This research investigates how such drastic changes in the chemical loading to riparian zones can cause shifts in ammonium- and iron-cycling bacteria, with specific interest in those involved in the Feammox process. To understand the effect of CAFO waste on the watershed, a variety of sampling sites were investigated, including small tributaries, forested wetlands and the main channel of the Neuse. These sites were sampled before and after the hurricane, and one year following the initial sampling to assess the extent of ecosystem recovery. Analyses of field samples investigated the geochemistry of the water column and riparian soil, the broader microbial community and targeted bacterial species of interest. Results show a distinct change in both chemical and microbial conditions of the watershed, with an average of a 300% increase in total available iron and a 75% decrease in ammonium concentration after the storm. Shifts in the microbial community at the sampling locations can be attributed both to the change in the chemistry of the watershed and to the inundation of new upstream sediments, which host different microbial communities, to downstream sampling sites. Additionally, the presence Acidimicrobiaceae bacterium A6, which is responsible for the Feammox process, was found at all sampling sites and at all three timepoints, however the abundance of this bacterium was negatively affected by the flooding event. Finally, the role of the Feammox process in the overall ammonium cycling of the sampled ecosystem has been analyzed through laboratory incubations, allowing for a better understanding of the role of the Feammox process in nutrient cycling in the environment, as well as the effect of hurricane flooding on the activity of Feammox bacteria.