Nitrogen cycling and trace gas dynamics in shallow coastal aquifers

Abstract

Since the industrial revolution, human activity has doubled the amount of carbon in Earth’s atmosphere and increased the rate of nitrogen supply to its biosphere by 40%. As a result, the climate is warming, sea level is rising, and fresh and saltwater resources are being threatened by eutrophication. Coastal areas are among the most vulnerable to these environmental challenges, and are economically critical zones that support tourism, fisheries, and half of the world’s population. Primary production in coastal waters tends to be nitrogen limited, so anthropogenic nitrogen inputs results in eutrophication and degraded coastal water quality. Groundwater discharge is the dominant source of nitrogen to the South Atlantic Bight and rivals riverine nitrogen loading in many coastal ecosystems globally. However, groundwater must transit coastal aquifers prior to discharge where microbial communities have the potential to remove bioavailable nitrogen via coupled nitrification and denitrification or anammox. This microbial activity also generates methane and nitrous oxide, which are potent greenhouse gases. Thus, coastal groundwater influences both water quality and atmospheric chemistry. The goal of this work was to quantify patterns and rates of microbial processes in shallow coastal aquifers that contribute to the production and consumption of bioavailable nitrogen and the greenhouse gases nitrous oxide and methane. High nitrification rates (0.78 ± 0.26 mmol m-2 day-1) were found in shallow beach sand on a barrier island in coastal Georgia, USA. High nitrous oxide concentrations were also observed at this location (median = 282 nM, n = 32). Nitrous oxide production was supported by a nitrate loss rate of 11 mmol m-2 day-1, which in turn, was too high to be supported by the observed nitrification rate alone, suggesting that nitrogen fixation was also important at this site. A hotspot of methane was observed in the freshwater lens near the center of Cabretta Island (median = 587 µM) supported by high rates of methanogenesis (22.2 ±10.6 mmol m-2 day-1). However, most of this methane was consumed before it could be exported to the ocean due to active methanotrophy (18 ± 2.1 mmol m-2 day-1) in surficial beach sand. Finally, submarine groundwater discharge was shown to export roughly the same quantity of greenhouse gases from salt marsh soils as direct efflux to the atmosphere, the only export pathway recognized previously in the literature.