title: Nitrate isotopic composition reveals nitrogen deposition and transformation dynamics along the canopy-soil continuum of a suburban forest in Japan.
abstract: BACKGROUND
  Heavy nitrogen (N) deposition often causes high nitrate (NO3(-)) accumulation in soils in temperate forested ecosystems. To clarify the sources and production pathways of this NO3(-), we investigated NO3(-) isotope signatures in deposition processes along the canopy-soil continuum of a suburban forest in Japan.
  
  
  METHODS
  The stable isotopes of N and oxygen (O) were used to trace the source and transformation dynamics of nitrate (NO3(-)) in two forest stands: a plantation of Cryptomeria japonica (coniferous tree; CJ) and a natural secondary forest of Quercus acutissima (broadleaf, deciduous tree; QA). The NO3(-) and ammonium (NH4(+)) concentrations were measured, as well as the δ(15)N and δ(18)O values of NO3(-), in rainfall, throughfall, stem flow, litter layer water, and soil water (10, 30, and 70 cm depths).
  
  
  RESULTS
  Seasonal variations were observed in the δ(15)N values of throughfall and stem flow NO3(-) at both sites, and in the δ(18)O values of throughfall and stem flow NO3(-) at the QA site. The range in the δ(18)O values of rainfall and throughfall NO3(-) was large (65-70‰) but decreased dramatically to 2-5‰ in soil water at both sites. At the QA site, the δ(18)O values of stem flow NO3(-) decreased to 40‰ during several rain events, especially in the growing season.
  
  
  CONCLUSIONS
  NO3(-) from atmospheric deposition was replaced by microbially generated NO3(-) mainly in the organic horizon and surface portion of the mineral soil under excess N deposition in this suburban forest. Microbial activity, including both immobilization and nitrification in organic-rich horizons near the surface, contributed to incorporating atmospheric NO3(-) quickly into the internal microbial N cycle. We also found evidence of microbial nitrification in the canopy of the QA stand during the growing season.
authors.affiliation: Department of Forest Science, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Shi
authors.forename: Nobuhito
authors.initials: N
authors.lastname: Ohte
authors.forename: Naoko
authors.initials: N
authors.lastname: Tokuchi
authors.forename: Naohiro
authors.initials: N
authors.lastname: Imamura
authors.forename: Miyuki
authors.initials: M
authors.lastname: Nagayama
authors.forename: Tomoki
authors.initials: T
authors.lastname: Oda
authors.forename: Masakazu
authors.initials: M
authors.lastname: Suzuki
mesh_terms: D064751:Ammonium Compounds / Q000032:analysis / Q000737:chemistry; D002853:Chromatography, Liquid; D065928:Forests; D007564:Japan; D013058:Mass Spectrometry; D009566:Nitrates / Q000032:analysis / Q000737:chemistry; D058465:Nitrification; D009584:Nitrogen / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000032:analysis* / Q000737:chemistry; D010103:Oxygen Isotopes / Q000032:analysis / Q000737:chemistry; D012987:Soil / Q000737:chemistry*; D014197:Trees
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D012987:Soil; D009584:Nitrogen