Measuring diel and spatial variation in biogenic N₂ 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 N 2 and N 2 O production. We explicitly consider biogenic, groundwater-derived N 2 inputs to a stream, investigate patterns of diel and spatial variability in stream N 2 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 N 2 inputs from groundwater and stream sediments to the stream were biogenic, with the remainder coming from atmospheric N 2 in groundwater recharge. Of biogenic N 2 (bN 2 ) 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 O 2 ), has a negligible atmospheric concentration. This provides more consistent estimates of the evasion coefficient K (min 21 ) and lower uncertainty than argon. Furthermore, the relative constancy of N 2 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 bN 2 at many points within a stream system and estimation of watershed-scale bN 2 fluxes from stream beds and inflowing groundwater.