Nitrous oxide emissions from a peatbog after 13 years of experimental nitrogen deposition

Abstract. Nitrogen deposition was experimentally increased on a Scottish peatbog over a period of 13 years (2002–2015). Nitrogen was applied in three forms, NH₃ gas, NH₄Cl solution, and NaNO₃ solution, at rates ranging from 8 (ambient) to 64 kg N ha⁻¹ yr⁻¹, and higher near the NH₃ fumigation source. An automated system was used to apply the nitrogen, such that the deposition was realistic in terms of rates and high frequency of deposition events. We measured the response of nitrous oxide (N₂O) flux to the increased nitrogen input. Prior expectations, based on the IPCC default emission factor, were that 1 % of the added nitrogen would be emitted as N₂O. In the plots treated with NH₄⁺ and NO₃⁻ solution, no response was seen, and there was a tendency for N₂O fluxes to be reduced by additional nitrogen, though this was not significant. Areas subjected to high NH₃ emitted more N₂O than expected, up to 8.5 % of the added nitrogen. Differences in the response are related to the impact of the nitrogen treatments on the vegetation. In the NH₄⁺ and NO₃⁻ treatments, all the additional nitrogen is effectively immobilised in the vegetation and top 10 cm of peat. In the NH₃ treatment, much of the vegetation was killed off by high doses of NH₃, and the nitrogen was presumably more available to denitrifying bacteria. The design of the wet and dry experimental treatments meant that they differed in statistical power, and we are less likely to detect an effect of the NH₄⁺ and NO₃⁻ treatments, though they avoid issues of pseudo-replication.