Abstract
Nitrous oxide (N2O) has 265 times greater greenhouse potential than carbon dioxide and its atmospheric concentration has increased by about 20% since industrialization; however, N2O production and emissions from aquatic systems are poorly constrained. To evaluate N2O fluxes associated with meteoric groundwater discharge to coastal zones, we measured N2O concentrations in May and October 2011 from two discharge points of the Upper Glacial aquifer on Long Island, NY, USA. One coastal zone contains only fresh water and the other contains an upper saline zone. N2O concentrations decreased by around 40% for the fresh water and a factor of two for the salt water from May to October, 2011. Fluxes were around 100 to 200 times greater from the freshwater (246 to 448 µmol/m shoreline/day) than saltwater aquifer (26.1 to 26.5 µmol/m shoreline/day). N2O concentrations correlate positively with NO3− and dissolved oxygen concentrations and negatively with salinity, dissolved organic carbon (DOC) and N2 denitrification concentrations. The smaller saltwater N2O export resulted from DOC enrichment in the upper saline zone, which appears to have driven denitrification to completion, removed N2O, and increased N2 denitrification. DOC concentrations should be considered in global N2O flux estimates for coastal aquifers.
Highlights
Nitrous oxide (N2O) is a powerful greenhouse gas with a greenhouse potential approximately 265 times that of CO2 [1]
In a study of river sediments, N2O production resulted from only 0.01% of the nitrate reduction when sediments were amended with carbon [50], and 6% of NO3− reduced formed NO2− with the balance of NO3− reduction continuing to form N2 or NH4+. Similar to these river settings, we find N2O production to be negatively correlated with dissolved organic carbon (DOC) concentration at SBH, where DOC is entrained in the aquifer in the upper saline zone (USZ) (Figure 4)
Production of N2O in the coastal aquifers of PJH and SBH was regulated by high concentrations of NO3−, but N2O concentrations in shallow porewater were altered in the presence or absence of a DOC rich USZ
Summary
Nitrous oxide (N2O) is a powerful greenhouse gas with a greenhouse potential approximately 265 times that of CO2 [1]. The increase in atmospheric N2O concentrations largely results from anthropogenic activities that increase reactive N, including fixation of atmospheric N2 through the Haber-Bosch process, increased cultivation of biological nitrogen fixers, such as legumes and rice, and burning of fossil fuels [2,3,4]. About two thirds of the uncertainty of the N2O budget originates from limited information about N2O emissions from agricultural leaching and runoff, human sewage, and atmospheric deposition, according to the Intergovernmental Panel on Climate Change (IPCC) 5th assessment report [6]. According to this methodology leaching and runoff comprises about half of the uncertainty in calculating the total agricultural source. Uncertainty around the emission factors from estuaries, rivers, and groundwater stems from (1) small number of studies; (2) variability in study methodology and (3) differences in biogeochemical reactions within each setting
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