Nitrogen critical loads for spruce plantations in Wales: Is there too much nitrogen?

We have used the mass balance approach for calculating nitrogen critical loads (CL(N)) to avoid eutrophication for Sitka spruce plantation forestry in Wales. The various approaches for assigning values to the parameters in the mass balance equation are discussed with particular reference to the soil nitrogen immobilization value. A CL(N) value of 11 kgN ha(-1) a(-1) was calculated for an intensively studied site in Wales of Yield Class 14 on a freely draining acid soil. If this site is assumed to represent a typical spruce stand, application of the CL(N) value means that 97 per cent of the area of coniferous forest in Wales, which is predominantly Sitka spruce, is currently receiving nitrogen deposition in excess of the CL(N). The area of coniferous forest at risk is reduced to 72 per cent if the proposed empirical CL(N) for managed acidic coniferous forests to prevent ecological changes (10-20 kgN ha(-1) a(-1)) is applied and to 45 per cent if the empirical CL(N) to prevent nitrogen saturation, nitrate leaching and depletion of soil base cations is applied (10-25 kgN ha(-1) a(-1)). Irrespective of the choice of CL(N) values, the implications of critical load exceedance need urgent investigation. Available information at present indicates that the main known consequence of chronic atmospheric nitrogen deposition to coniferous forest ecosystems is enhanced nitrate and associated aluminium leaching to freshwaters. There is insufficient information regarding the potential adverse effects of eutrophication of soils and waters and of impacts on tree health and production.