title: Procedure for rapid determination of δ15N and δ18O values of nitrate: development and application to an irrigated rice paddy watershed.
abstract: The dual isotope approach using the stable isotope ratios of nitrate nitrogen (δ(15)N(NO3)) and oxygen (δ(18)O(NO3)) is a strong tool for identifying the history of nitrate in various environments. Basically, a rapid procedure for determining δ(15)N(NO3) and δ(18)O(NO3) values is required to analyze many more samples quickly and thus save on the operational costs of isotope-ratio mass spectrometry (IRMS). We developed a new rapid procedure to save time by pre-treating consecutive samples of nitrous oxide microbially converted from nitrate before IRMS determination. By controlling two six-port valves of the pre-treatment system separately, IRMS determination of the current sample and backflush during the next sample pre-treatment period could be conducted simultaneously. A set of 89 samples was analyzed precisely during a 25-h continuous run (17 min per sample), giving the fastest reported processing time, and simultaneously reducing liquid nitrogen and carrier helium gas consumption by 35%. Application of the procedure to an irrigated rice paddy watershed suggested that nitrate concentrations in river waters decreased in a downstream direction, mainly because of the mixing of nitrate from different sources, without distinct evidence of denitrification. Our procedure should help with more detailed studies of nitrate formation processes in watersheds.
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Saeko
authors.initials: S
authors.lastname: Yada
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Nakajima
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Sunao
authors.initials: S
authors.lastname: Itahashi
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Kei
authors.initials: K
authors.lastname: Asada
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Seiko
authors.initials: S
authors.lastname: Yoshikawa
authors.affiliation: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Japan E-mail: helios02@affrc.go.jp.
authors.forename: Sadao
authors.initials: S
authors.lastname: Eguchi
mesh_terms: D057914:Agricultural Irrigation; D058440:Denitrification; D005618:Fresh Water / Q000032:analysis; D013058:Mass Spectrometry / Q000379:methods; D009566:Nitrates / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry*; D009587:Nitrogen Isotopes; D009609:Nitrous Oxide; D012275:Oryza; D010100:Oxygen / Q000737:chemistry*; D010103:Oxygen Isotopes / Q000032:analysis; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D014874:Water Pollutants, Chemical; D009609:Nitrous Oxide; D009584:Nitrogen; D010100:Oxygen