Linking the presence and function of denitrification genes to whole river estimates of denitrification in response to a large-scale nitrogen enrichment of the Kansas River (USA)

Rivers play a critical role in transporting nutrients to coastal waters; thus, understanding river N transport and transformation is of fundamental importance. Nitrate concentration, land use, hydrology, and seasons all affect rates of denitrification (conversion of nitrate to di-nitrogen gas). The linkages between riverine microbial community composition (MCC) and denitrification rates, however, are often unclear. Furthermore, there are few, if any, ecosystem-scale manipulations of the microbial community that can directly test the importance of how changing MCC can alter ecosystem-scale functions like denitrification. Thus, a large, ecosystem-scale addition of both N-enriched water and a novel N-enriched microbial community provided a unique opportunity to study the role of MCC in altering whole river N cycling rates. We asked: Does whole river denitrification change in response to a four-month addition of high nitrogen ( 600 mg N/L nitrate; 300 mg N/L ammonium) and enriched microbial community? We used a release of water from a decommissioned fertilizer plant and its resident microbial enrichment culture into the Kansas River to address our question. We measured whole river denitrification rates using the open-channel estimate of di-nitrogen gas production from samples collected hourly (for 24 hours) at a pair of sites, upstream and 5km downstream of the release point. We conducted the sampling at the end of the four-month release of the high nitrogen water and two weeks post-release. We paired whole-river estimates of denitrification with real-time nitrate and dissolved oxygen sensors, which measured concentrations on 15-min increments. We collected hourly MCC samples to measure the presence and expression of genes responsible for denitrification. Preliminary MCC analysis indicates that the fertilizer water released a unique microbial community into the river and that the effects of that release dissipated downstream. We also measured increased nitrous oxide and higher daytime nitrate concentrations downstream of the release. Quantifying whether the Kansas River transports novel microbial communities and how riverine N cycling changes in response to the additions will aid us in understanding the connection between MCC and biogeochemical cycling in aquatic ecosystems.