title: Stream denitrification across biomes and its response to anthropogenic nitrate loading.
abstract: Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. mulhollandpj@ornl.gov
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Mulholland
authors.forename: Ashley M
authors.initials: AM
authors.lastname: Helton
authors.forename: Geoffrey C
authors.initials: GC
authors.lastname: Poole
authors.forename: Robert O
authors.initials: RO
authors.lastname: Hall
authors.forename: Stephen K
authors.initials: SK
authors.lastname: Hamilton
authors.forename: Bruce J
authors.initials: BJ
authors.lastname: Peterson
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Tank
authors.forename: Linda R
authors.initials: LR
authors.lastname: Ashkenas
authors.forename: Lee W
authors.initials: LW
authors.lastname: Cooper
authors.forename: Clifford N
authors.initials: CN
authors.lastname: Dahm
authors.forename: Walter K
authors.initials: WK
authors.lastname: Dodds
authors.forename: Stuart E G
authors.initials: SE
authors.lastname: Findlay
authors.forename: Stanley V
authors.initials: SV
authors.lastname: Gregory
authors.forename: Nancy B
authors.initials: NB
authors.lastname: Grimm
authors.forename: Sherri L
authors.initials: SL
authors.lastname: Johnson
authors.forename: William H
authors.initials: WH
authors.lastname: McDowell
authors.forename: Judy L
authors.initials: JL
authors.lastname: Meyer
authors.forename: H Maurice
authors.initials: HM
authors.lastname: Valett
authors.forename: Jackson R
authors.initials: JR
authors.lastname: Webster
authors.forename: Clay P
authors.initials: CP
authors.lastname: Arango
authors.forename: Jake J
authors.initials: JJ
authors.lastname: Beaulieu
authors.forename: Melody J
authors.initials: MJ
authors.lastname: Bernot
authors.forename: Amy J
authors.initials: AJ
authors.lastname: Burgin
authors.forename: Chelsea L
authors.initials: CL
authors.lastname: Crenshaw
authors.forename: Laura T
authors.initials: LT
authors.lastname: Johnson
authors.forename: B R
authors.initials: BR
authors.lastname: Niederlehner
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: O'Brien
authors.forename: Jody D
authors.initials: JD
authors.lastname: Potter
authors.forename: Richard W
authors.initials: RW
authors.lastname: Sheibley
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Sobota
authors.forename: Suzanne M
authors.initials: SM
authors.lastname: Thomas
mesh_terms: D000383:Agriculture; D001419:Bacteria / Q000378:metabolism; D003198:Computer Simulation; D017753:Ecosystem; D005843:Geography; D006802:Human Activities; D009566:Nitrates / Q000032:analysis* / Q000378:metabolism*; D009573:Nitrites / Q000032:analysis* / Q000378:metabolism*; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism; D009587:Nitrogen Isotopes; D010944:Plants / Q000378:metabolism; D045483:Rivers / Q000737:chemistry*; D014507:Urbanization
chemical_list: D009566:Nitrates; D009573:Nitrites; D009587:Nitrogen Isotopes; D009584:Nitrogen