Plant and Soil 229: 83–96, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 83 Nitrogen cycling and proton fluxes in an acid forest soil E. D. R. Brierley 1,4 , P. J. A. Shaw 2 & M. Wood 3 1 Cranfield University, Institute of Water and Environment, Silsoe, Bedfordshire, MK45 4DT, UK. 2 Roehampton Institute London, Whitelands Cottage, West Hill, London, SW15 3SN, UK. 3 University of Reading, Department of Soil Science, Whitenights, Reading, RG6 6DW, UK. 4 Corresponding author ∗ Received 2 December 1999. Accepted in revised form 13 September 2000 Key words: acidification, nitrification, nitrogen budgets, proton fluxes Abstract 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. Differ- ences 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 CaCl 2 ). 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. Introduction The most intensive study of nitrogen cycling in a UK forest ecosystem in relation to production (and man- agement) remains that on Pinus nigra var maritima on the Culbin sands (Miller et al., 1979). More recent international investigations of forest nutrient cycling have been prompted by concerns over the relationship between atmospheric deposition and N cycling, such as those which comprised an Integrated Forest Study programme in the USA and Norway (Johnson et al., 1992). This UK based study was founded in both ap- proaches: N budgets were created for a forest ecosys- ∗ FAX No: 152586 3344. TEL No: 1525 86 3145. E-mail: e.d.r.Brierley@cranfield.ac.uk tem in its second and third years after establishment, based upon experimental plots of coniferous trees at Ironhill, near Liphook, Hampshire, UK. The pro- gramme was known as the Liphook forest fumigation project. The Liphook experiment was devised and im- plemented by the Central Electricity Research Labor- atories (CERL) of the Central Electricity Generating Board (CEGB) to investigate the impact of sulphur dioxide and ozone, at realistic levels of exposure, on the above-ground and below-ground processes of a forest ecosystem. The experimental design has been described by McLeod (1995) (and major research pro- grammes based at the experimental site are described in the same issue of Plant, Cell and Environment). Continuous fumigation with SO 2 and O 3 began on 25 May, 1987.