Tracking the fate of a high concentration groundwater nitrate plume through a fringing marsh: A combined groundwater tracer and in situ isotope enrichment study

A groundwater plume enriched in 15NO3− was created upgradient of a mesohaline salt marsh. By measuring the changes in concentration and isotopic enrichment of NO3−, N2O, N2, NH4+, and particulate organic nitrogen (PON) during plume transport through the marsh, in situ rates of dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (DNF) were estimated, as well as N storage in the reduced N pools. For groundwater discharge within the top 10 cm of marsh, NO3− removal was 90% complete within the 50 cm of marsh nearest the upland border. The peak NO3− loss rate from the plume ranged from 208 to 645 µM d−1. Rates of DNRA (180 µM d−1) and DNF (387–465 µM d−1) processed 30% and 70% of the NO3− load, respectively. Terminal N2 production was approximately equal to N2 production rates during DNF. Comparison of 15N lost from the 15NO3− pool and 15N gained in each of the reduced products accounted for only 22% of the reduced 15N, thus indicating N export from the system. Despite high rates of DNRA, the NH4+ produced was not a long‐term repository for the groundwater‐derived N but was instead rapidly immobilized into marsh PON and retained on longer timescales. The small inventory of 15N in the N2 and N2 pools relative to DNF rates, coincident with an undersaturation of dissolved argon, indicated that denitrified N was exported to the atmosphere on short timescales. The relative magnitudes of DNF and DNRA in conjunction with the immobilization of NH4+ and evasion of N gases dictated the extent of export versus retention of the groundwater NO3− load.

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