How important is N 2O production in removing atmospherically deposited nitrogen from UK moorland catchments?

Abstract

Nitrate (NO 3 −) leaching due to anthropogenic nitrogen (N) deposition is an environmental problem in many parts of the UK uplands, associated with surface water acidification and affecting lake nutrient balances. It is often assumed that gaseous return of deposited N to the atmosphere as N 2O through denitrification may provide an important sink for N. This assumption was tested for four moorland catchments (Allt a’Mharcaidh in the Cairngorms, Afon Gwy in mid-Wales, Scoat Tarn in the English Lake District and River Etherow in the southern Pennines), covering gradients of atmospheric N deposition and surface water NO 3 − leaching, through a combination of field and laboratory experiments. Field measurements of N 2O fluxes from static chambers with and without additions of NH 4NO 3 solution were carried out every 4 weeks over 1 yr. Wetted soil cores from the same field plots were used in experimental laboratory incubations at 5 and 15 °C with and without additions of NH 4NO 3 solution, followed by measurement of N 2O fluxes. Field measurements showed that significant N 2O fluxes occurred in only a very small number of plots with most showing zero values for much of the year. The maximum fluxes were 0.24 kg-N/ha/yr from unamended plots at the River Etherow and 0.49 kg-N/ha/yr from plots with NH 4NO 3 additions at the Allt a’Mharcaidh. Laboratory incubation experiments demonstrated that large N 2O fluxes could be induced by warming and NH 4NO 3 additions, with the top 5 cm of soil cores responsible for the largest fluxes, reaching 11.8 kg-N/ha/yr from a podsol at Scoat Tarn. Acetylene block experiments showed that while N 2 was not likely to be a significant denitrification product in these soils, reduced N 2O fluxes indicated that nitrification was an important source of N 2O in many cases. A simple model of denitrification suggesting that 10–80% of net N inputs may be denitrified from non-agricultural soils was found to greatly over-estimate fluxes in the UK uplands. The proportion of deposition denitrified was found to be much closer to the IPCC suggested value of 1% with an upper limit of 10%. Interception of N deposition by vegetation may greatly reduce the net supply of N from this source, while soil acidification or other factors limiting carbon supply to soil microbes may prevent large denitrification fluxes even where NO 3 − supply is not limiting.