Nitrogen cycling and proton fluxes in an acid forest soil

Ironhill, near Liphook, UK, was the site of a forest fumigation experiment. Nitrogen cycling within the humoferric podzol soil was a component of the study into the impacts of sulphur dioxide and ozone on coniferous trees. Variation in total soil N and N mineralisation was too great to determine impacts from the fumigant gases. Differences in the nitrogen mineralisation potential of the soils were unrelated to the initial levels of mineral or total N, or to pH. Mineralisation potential was affected by temperature and a Q(10) of approximately 3 was demonstrated. Mineralisation potential was reduced in very dry soils, but the wetting of these dry soils did not result in enhanced mineralisation, relative to fresh samples of equivalent moisture content. Nitrification potential was detected in this forest soil of pH 3 (in 0.01 m CaCl2). The soil N data and those from the analysis of N within vegetation were used to prepare N budgets for the second and third seasons' growth of a mixed conifer forest; by the third year, N appeared to limit tree growth. The relative magnitude of proton fluxes from plant growth, nitrification and atmospheric inputs was estimated. Acidity generated from the balance of cations and anions in plant uptake, and soil N transformations was estimated to be comparable to that from 'acid rain'. This comparison was based on only parts of the N cycle because they may occur remotely, in time or space, from other transformations of N. The comparison is valid, therefore, at the scale of individual trees or small-scale experimental plots, but at forest scale, wet and dry deposition were predicted to be the more significant for ecosystem acidification.