title: Isotopic signals of summer denitrification in a northern hardwood forested catchment.
abstract: Despite decades of measurements, the nitrogen balance of temperate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ(15)NNO3 and δ(18)ONO3) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853; School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom; s.wexler1@uea.ac.uk clg33@cornell.edu.
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Wexler
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853; s.wexler1@uea.ac.uk clg33@cornell.edu.
authors.forename: Christine L
authors.initials: CL
authors.lastname: Goodale
authors.affiliation: Department of Forest Resources and Environmental Conservation, Virginia Water Resources Research Center, Virginia Tech, Blacksburg, VA 24061;
authors.forename: Kevin J
authors.identifier: http://orcid.org/0000-0001-5751-3956
authors.initials: KJ
authors.lastname: McGuire
authors.affiliation: US Forest Service, Northern Research Station, North Woodstock, NH 03262; and.
authors.forename: Scott W
authors.initials: SW
authors.lastname: Bailey
authors.affiliation: Cary Institute of Ecosystem Studies, Millbrook, NY 12545.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Groffman
mesh_terms: D064751:Ammonium Compounds / Q000032:analysis; D002980:Climate; D003237:Connecticut; D058440:Denitrification / Q000502:physiology; D065928:Forests; D005618:Fresh Water / Q000032:analysis; D060587:Groundwater / Q000032:analysis; D009566:Nitrates / Q000032:analysis; D009573:Nitrites / Q000032:analysis; D009587:Nitrogen Isotopes / Q000378:metabolism*; D010103:Oxygen Isotopes / Q000378:metabolism*; D045483:Rivers; D012621:Seasons; D012987:Soil / Q000737:chemistry; D014197:Trees / Q000378:metabolism*
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D009573:Nitrites; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D012987:Soil
keywords: denitrification; forested watershed; nitrogen cycle; stable isotopes; streamwater chemistry