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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 NH4Cl 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. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen (N) deposition has increased in the last few decades with implications for the functioning of Sphagnum mosses, the main peat forming genus in peatlands. However, there are few in situ measurements of the carbon balance, especially where the N additions have been realistically manipulated in the field, and none with respect to the effect of N form. The aim of this study was to look at the effects of experimental N additions as oxidized or reduced N, with and without phosphorus (P) and potassium (K), on CO2 fluxes from Sphagnum capillifolium hummocks in a long-term N addition experiment, Whim bog, in the Scottish Borders. In situ static chamber measurements were made during 2008 on 20 plots (control, and N treatments receiving 56kgNha−1 y−1 of either nitrate (NO3 −) or ammonium (NH4 +) added, with and without PK) to assess N effects on CO2 exchange. Almost all the measured fluxes were negative, i.e. Sphagnum hummocks lost CO2 to the atmosphere, irrespective of the treatments applied. N treatment did not have a significant effect on ecosystem respiration (ER) or net ecosystem productivity (NEP) but adding PK with N increased gross photosynthesis (PG) significantly, compared to the other treatments. Summed monthly averages of NEP for each treatment indicated that increasing N deposition increased CO2 loss from the system. The form of N affected the response: compared to the control, adding nitrate increased the CO2 loss more than ammonium, both with and without PK. Nitrogen (both forms) increased the ecosystem respiration fluxes at a certain temperature, adding PK with N further enhanced the response. The positive (increasing) temperature response of ecosystem respiration with N suggests that in high N deposition areas the potential increase in ecosystem respiration, CO2 loss, will be exacerbated with climate change. [Copyright &y& Elsevier]