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- Title
Nitrification in a Subterranean Estuary: An Ex Situ and In Situ Method Comparison Determines Nitrate Is Available for Discharge.
- Authors
Wilson, Stephanie J.; Song, Bongkeun; Anderson, Iris C.; Tobias, Craig R.
- Abstract
Subterranean estuaries (STEs) form in the subsurface where fresh groundwater and seawater meet and mix. Subterranean estuaries support a variety of biogeochemical processes including those transforming nitrogen (N). Groundwater is often enriched with dissolved inorganic nitrogen (DIN), and transformations in the STE determine the fate of that DIN, which may be discharged to coastal waters. Nitrification oxidizes ammonium (NH4+) to nitrate, making DIN available for N removal via denitrification. We measured nitrification at an STE, in Virginia, USA using in situ and ex situ methods including conservative mixing models informed by in situ geochemical profiles, an in situ experiment with 15NH4+ tracer injection, and ex situ sediment slurry incubations with 15NH4+ tracer addition. All methods indicated nitrification in the STE, but the ex situ sediment slurries revealed higher rates than both the in situ tracr experiment and mixing model estimations. Nitrification rates ranged 55.0–183.16 μmol N m−2 d−1 based on mixing models, 94.2–225 μmol N m−2 d−1 in the in situ tracer experiment, and 36.6–109 μmol N m−2 d−1 slurry incubations. The in situ tracer experiment revealed higher rates and spatial variation not captured by the other methods. The geochemical complexity of the STE makes it difficult to replicate in situ conditions with incubations and calculations based on chemical profiles integrate over longer timescales, therefore, in situ approaches may best quantify transformation rates. Our data suggest that STE nitrification produces NO3−, altering the DIN pool discharged to overlying water via submarine groundwater discharge. Plain Language Summary: Groundwater mixes with coastal waters in subterranean estuaries that form in the subsurface along coastlines. Groundwater supplies compounds to these systems including nutrients such as nitrogen. Within the subterranean estuary, nitrogen may be transformed, influencing its availability to microbes for removal, its flow through sediments to coastal waters, and its availability to primary producers. Here we measured nitrification, a critical nitrogen cycle process that transforms ammonium to nitrite/nitrate in a subterranean estuary. We used several methods to measure nitrification including mixing models that rely on measured nitrogen concentrations across the groundwater salinity gradient, an isotope labeled nitrogen experiment that allows for tracing of nitrogen in the subsurface over time, and laboratory incubations. All methods indicated active nitrification in the STE, but the laboratory incubations revealed higher rates than both the tracer injections and mixing model estimations. Our data suggest that tracer approaches field conditions and heterogeneity, but that incubations and mixing model estimates are useful for determining control points and net impacts of the subterranean estuary on groundwater, respectively. Nitrification alters the form of nitrogen that is transported to overlying water via submarine groundwater discharge and, therefore, may influence the impact of groundwater derived nitrogen on coastal biogeochemistry. Key Points: Nitrification in the subterranean estuary was observed with both ex situ and in situ measurement methodsEx situ incubation experiments revealed higher nitrification rates than in situ tracer or mixing model calculationsNitrate produced in the subterranean estuary may be exported to overlying coastal waters via submarine groundwater discharge
- Subjects
VIRGINIA; NITRIFICATION; ESTUARIES; GROUNDWATER; TERRITORIAL waters; NITROGEN cycle; COASTAL sediments; GEOCHEMICAL modeling; GROUNDWATER analysis
- Publication
Journal of Geophysical Research. Biogeosciences, 2024, Vol 129, Issue 6, p1
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2023JG007876