Wildfires, which are increasing in frequency and severity in the western U.S., play an important role in biogeochemical cycling. Such events can cause physical, chemical, and biological changes that impact downstream water quality. Beaver activity within a fire-impacted river can lead to changes in ecosystem function, allowing beaver ponds the potential to transform electron donors and acceptors such as carbon, nitrogen, sulfur, and redox-active metals. Therefore, we propose that the convergence of wildfires and beaver ponds could have important consequences for biogeochemical cycling within the ponds and for downstream water quality. Here, we study the potential changes to carbon and nitrogen cycling due to both severe wildfire activity and beaver pond presence within the Medicine Bow-Routt National Forest, Colorado, USA, which burned during the 2018 Ryan Fire. Samples were collected from a sequence of beaver ponds and interconnecting free-flowing channels starting above the burn during the summer of 2019. We observed lower freshness and fluorescence indices within the fire-impacted beaver ponds relative to upgradient stream water. Fourier transform ion cyclotron resonance mass spectrometry analysis displayed additional differences between the beaver ponds and stream water sampling sites, with more N-containing compounds, higher N/C, lower O/C, and increased aromaticity indices within the ponds, suggesting increased recalcitrance due to lower redox potentials. The effects appear to be generally localized, as the proportion of aromatic-like CHNO compounds rapidly decreases within the faster flowing stream water. While these results demonstrate that the effects of reducing conditions on fire-derived CHNO compounds are generally confined to the beaver ponds, these features are potentially important controls for microbial nitrogen cycling.

Impact of beaver ponds on biogeochemical cycling of organic nitrogen within a fire-impacted watershed

ACS Spring Meeting 2021

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