title: Salt marsh ecosystem biogeochemical responses to nutrient enrichment: a paired 15N tracer study.
abstract: We compared processing and fate of dissolved NO3- in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of approximately 1-4 micromol NO3-/ L and the other receiving experimentally fertilized flood tides containing approximately 70-100 micromol NO3-/ L. We conducted simultaneous 15NO3- (isotope) tracer additions from 23 to 28 July 2005 in the reference (8.4 ha) and fertilized (12.4 ha) systems to compare N dynamics and fate. Two full tidal cycles were intensively studied during the paired tracer additions. Resulting mass balances showed that essentially 100% (0.48-0.61 mol NO3-N.ha(-1).h(-1)) of incoming NO3- was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO3- concentrations were 1.3-1.8 micromol/L. In contrast, only 50-60% of incoming NO3- was processed in the fertilized system when NO3- concentrations were 84-96 micromol/L; the remainder was exported in ebb tidewater. Gross NO3- processing was approximately 40 times higher in the fertilized system at 19.34-24.67 mol NO3-N.ha(-1).h(-1). Dissimilatory nitrate reduction to ammonium was evident in both systems during the first 48 h of the tracer additions but <1% of incoming 15NO3- was exported as 15NH4+. Nitrification rates calculated by 15NO3- dilution were 6.05 and 4.46 mol.ha(-1).h(-1) in the fertilized system but could not be accurately calculated in the reference system due to rapid (<4 h) NO3- turnover. Over the five-day paired tracer addition, sediments sequestered a small fraction of incoming NO3-, although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 mol.ha(-1).d(-1), respectively. Macrophyte NO3- uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO3- by plants was lower in the reference system (1.75 mol NO3-.ha(-1).d(-1)) than the fertilized system (approximately 10 mol NO3-.ha(-1).d(-1)). Nitrogen processing efficiency decreased with NO3- load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.
authors.affiliation: The Ecosystems Center Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts 02543, USA. Deanne.Drake@wits.ac.za
authors.forename: D C
authors.initials: DC
authors.lastname: Drake
authors.forename: Bruce J
authors.initials: BJ
authors.lastname: Peterson
authors.forename: Kari A
authors.initials: KA
authors.lastname: Galván
authors.forename: Linda A
authors.initials: LA
authors.lastname: Deegan
authors.forename: Charles
authors.initials: C
authors.lastname: Hopkinson
authors.forename: J Michael
authors.initials: JM
authors.lastname: Johnson
authors.forename: K
authors.initials: K
authors.lastname: Koop-Jakobsen
authors.forename: Lynsey E
authors.initials: LE
authors.lastname: Lemay
authors.forename: Christian
authors.initials: C
authors.lastname: Picard
mesh_terms: D000818:Animals; D005308:Fertilizers; D005618:Fresh Water; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen; D009587:Nitrogen Isotopes; D012623:Seawater; D055871:Tidal Waves; D014872:Water Movements; D053833:Wetlands
chemical_list: D005308:Fertilizers; D009566:Nitrates; D009587:Nitrogen Isotopes; D009584:Nitrogen