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

Abstract

The biological and physical controls on microbial processes that produce and consume N 2O in soils are highly complex. Isotopomer ratios of N 2O, with abundance of 14N 15N 16O, 15N 14N 16O, and 14N 14N 18O relative to 14N 14N 16O, are promising for elucidation of N 2O 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 N 2O via hydroxylamine oxidation and N 2O 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 N 2O concentration in groundwater varied greatly according to groundwater chemistry, i.e. NO 3 −, DOC, and DO, although apportionment of N 2O production to nitrification or denitrification was ambiguous. Our isotopic analysis (δ 15N and δ 18O) of NO 3 − and N 2O implies that denitrification is the dominant production process of N 2O, but definitive information is not derived from δ 15N and δ 18O analysis because of large variations in isotopic fractionations during production and consumption of N 2O. However, the N 2O site preference and the difference in δ 15N between NO 3 − and N 2O indicate that nitrification contributes to total N 2O production and that most measured N 2O has been subjected to further N 2O reduction to N 2. The implications of N 2O biogeochemistry derived from isotope and isotopomer data differ entirely from those derived from conventional concentration data of DO, NO 3 −, and N 2O. That difference underscores the need to reconsider our understanding of the N cycle in the oxic–anoxic interface.