Improving the application of open channel methods to estimate watershed-scale denitrification
In many watersheds, only a small fraction of net anthropogenic nitrogen inputs (NANI) is represented by river and stream discharge. The remainder, known as the “missing N”, has two possible fates: (1) storage within the watershed and (2) transformation into biogenic N gases such as N2, N2O, and NO, followed by evasion to the atmosphere. One promising approach to determining the fate of the “missing N” in terms of gas losses is to quantify biogenic N gases escaping from stream surfaces within a watershed. The goal of this project was to improve the application of open channel methods of measuring gas evasion from stream surfaces. Specifically, we aimed to distinguish physical and biological processes that could contribute to elevated N2 concentrations in stream water and explore the use of 222Rn as a natural tracer of gas exchange in gaining streams. We conducted two open-channel studies in a channelized stream on the Eastern Shore of Maryland, one lasting 12 hours and one lasting 24 hours. We used membrane-inlet mass spectrometry to measure the concentrations of O2, N2, and Ar, and we used N2/Ar relative to solubility to estimate excess N2.
Both physical excess N2 from low-temperature recharge and biogenic excess N2 were present in the stream. Biogenic N2 delivered by groundwater was the predominant source compared to in-stream production in sediments. 222Rn was an effective tracer of gas exchange because it is naturally added to gaining streams by groundwater inflow and has a negligible background level. Another advantage is that 222Rn can be measured continuously in the field at intervals of 10-15 minutes using a RAD7 meter. Evasion of biogenic N2 gas to the atmosphere showed diel variability and was approximately 20% higher in daytime hours, when stream temperatures were rising, compared to nighttime hours with falling stream temperatures. The results suggest that the fate of a large fraction of the missing N is biological reduction to N2, followed by discharge of high-N2 groundwater into streams and evasion to the atmosphere. . Additionally, we propose a streamlined, radon-based methodology for estimation of biogenic N gas loss from gaining streams that could be applied at larger spatial scales. This method has the potential to allow direct quantification of biogenic N gas loss from stream surfaces at the whole-watershed scale, representing an important step forward in understanding watershed N biogeochemistry.