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Measuring diel and spatial variation in biogenic N<inf>2</inf> delivery, production, and loss with natural tracers: Application to watershed-scale estimation of denitrification

Abstract:

Complete understanding of the nitrogen cycle is hindered by the challenges of measuring denitrification at watershed scales. Here, we refine the open-channel approach, a fundamental field method in stream biogeochemistry that can be used to quantify denitrification in streams based on N2 and N2O production. We explicitly consider biogenic, groundwater-derived N2 inputs to a stream, investigate patterns of diel and spatial variability in stream N2 fluxes, and explore the use of two potential natural tracers of gas exchange, argon and radon, in place of the artificial tracers often employed in open-channel studies. We conducted two open-channel studies, 12 h and 24 h in duration, in a channelized stream on the eastern shore of Maryland. Twenty-two to forty-three percent of total N2 inputs from groundwater and stream sediments to the stream were biogenic, with the remainder coming from atmospheric N2 in groundwater recharge. Of biogenic N2 (bN2) inputs, 37–100% came from groundwater and the remaining 0–63% were from in-stream production. Radon can be measured continuously in the field and, unlike other potential natural tracers of gas exchange (Ar and O2), has a negligible atmospheric concentration. This provides more consistent estimates of the evasion coefficient K (min−1) and lower uncertainty than argon. Furthermore, the relative constancy of N2 fluxes during nighttime hours suggested the possibility of a simplified radon-based sampling procedure requiring ∼ 6–8 h per site. This approach enables quantification of bN2 at many points within a stream system and estimation of watershed-scale bN2 fluxes from stream beds and inflowing groundwater.