title: Inertia in an ombrotrophic bog ecosystem in response to 9 years' realistic perturbation by wet deposition of nitrogen, separated by form.
abstract: Wet deposition of nitrogen (N) occurs in oxidized (nitrate) and reduced (ammonium) forms. Whether one form drives vegetation change more than the other is widely debated, as field evidence has been lacking. We are manipulating N form in wet deposition to an ombrotrophic bog, Whim (Scottish Borders), and here report nine years of results. Ammonium and nitrate were provided in rainwater spray as NH4 Cl or NaNO3 at 8, 24 or 56 kg N ha(-1)  yr(-1) , plus a rainwater only control, via an automated system coupled to site meteorology. Detrimental N effects were observed in sensitive nonvascular plant species, with higher cumulative N loads leading to more damage at lower annual doses. Cover responses to N addition, both in relation to form and dose, were species specific and mostly dependent on N dose. Some species were generally indifferent to N form and dose, while others were dose sensitive. Calluna vulgaris showed a preference for higher N doses as ammonium N and Hypnum jutlandicum for nitrate N. However, after 9 years, the magnitude of change from wet deposited N on overall species cover is small, indicating only a slow decline in key species. Nitrogen treatment effects on soil N availability were likewise small and rarely correlated with species cover. Ammonium caused most N accumulation and damage to sensitive species at lower N loads, but toxic effects also occurred with nitrate. However, because different species respond differently to N form, setting of ecosystem level critical loads by N form is challenging. We recommend implementing the lowest value of the critical load range where communities include sensitive nonvascular plants and where ammonium dominates wet deposition chemistry. In the context of parallel assessment at the same site, N treatments for wet deposition showed overall much smaller effects than corresponding inputs of dry deposition as ammonia.
authors.affiliation: Centre for Ecology & Hydrology Edinburgh, Bush Estate, Penicuik, EH26 0QB, UK.
authors.forename: Lucy J
authors.initials: LJ
authors.lastname: Sheppard
authors.forename: Ian D
authors.initials: ID
authors.lastname: Leith
authors.forename: Toshie
authors.initials: T
authors.lastname: Mizunuma
authors.forename: Sarah
authors.initials: S
authors.lastname: Leeson
authors.forename: Sanna
authors.initials: S
authors.lastname: Kivimaki
authors.forename: J
authors.initials: J
authors.lastname: Neil Cape
authors.forename: Netty
authors.initials: N
authors.lastname: van Dijk
authors.forename: David
authors.initials: D
authors.lastname: Leaver
authors.forename: Mark A
authors.initials: MA
authors.lastname: Sutton
authors.forename: David
authors.initials: D
authors.lastname: Fowler
authors.forename: Leon J L
authors.initials: LJ
authors.lastname: Van den Berg
authors.forename: Alan
authors.initials: A
authors.lastname: Crossley
authors.forename: Chris
authors.initials: C
authors.lastname: Field
authors.forename: Simon
authors.initials: S
authors.lastname: Smart
mesh_terms: D000393:Air Pollutants / Q000378:metabolism*; D044822:Biodiversity; D017753:Ecosystem; D006863:Hydrogen-Ion Concentration; D009584:Nitrogen / Q000378:metabolism*; D012606:Scotland; D012621:Seasons; D012987:Soil / Q000737:chemistry*; D053833:Wetlands
chemical_list: D000393:Air Pollutants; D012987:Soil; D009584:Nitrogen
keywords: Calluna; Sphagnum capillifolium; ammonium; chronology; cover; critical loads; growth; nitrate; peatland