Biogeochemistry of nitrous oxide in groundwater in a forested ecosystem elucidated by nitrous oxide isotopomer measurements

The biological and physical controls on microbial processes that produce and consume N2O in soils are highly complex. Isotopomer ratios of N2O, with abundance of 14N15N16O, 15N14N16O, and 14N14N18O relative to 14N14N16O, are promising for elucidation of N2O biogeochemistry in an intact ecosystem. Site preference, the nitrogen isotope ratio of the central nitrogen atom minus that of the terminal nitrogen atom, is useful to distinguish between N2O via hydroxylamine oxidation and N2O via nitrite reduction.We applied this isotopomer analysis to a groundwater system in a temperate coniferous-forested ecosystem. Results of a previous study at this location showed that the N2O concentration in groundwater varied greatly according to groundwater chemistry, i.e. NO3−, DOC, and DO, although apportionment of N2O production to nitrification or denitrification was ambiguous. Our isotopic analysis (δ15N and δ18O) of NO3− and N2O implies that denitrification is the dominant production process of N2O, but definitive information is not derived from δ15N and δ18O analysis because of large variations in isotopic fractionations during production and consumption of N2O. However, the N2O site preference and the difference in δ15N between NO3− and N2O indicate that nitrification contributes to total N2O production and that most measured N2O has been subjected to further N2O reduction to N2. The implications of N2O biogeochemistry derived from isotope and isotopomer data differ entirely from those derived from conventional concentration data of DO, NO3−, and N2O. That difference underscores the need to reconsider our understanding of the N cycle in the oxic–anoxic interface.