Impacts and fate of experimentally enhanced nitrogen deposition on a British lowland heath

Abstract

Between 1989 and 1996, nitrogen, in the form of (NH 4) 2SO 4 (7.7 or 15.4 kg ha −1 year −1), was added to experimental plots of Calluna at a lowland dry heath in the south of England. Background deposition at this site was estimated at between 13–18 kg N ha −1 year −1, with experimental additions therefore taking total deposition slightly above the proposed critical nitrogen load for lowland heaths. A destructive harvest, carried out in December 1996, revealed large increases in above-ground growth and litter production in response to nitrogen treatments. Plant, litter and soil nitrogen concentrations were also significantly increased. Higher levels of soil decomposer activity, together with faster turnover of litter in nitrogen-treated plots suggest an effect of treatment additions on rates of internal nitrogen cycling. A nitrogen budget for the site revealed that only a relatively small proportion (14–18%) of the nitrogen additions had been accumulated in plant biomass. A similarly small proportion (10–15%) of the extra nitrogen was stored in the litter layer. Since leaching and denitrification losses were minimal, the bulk of the experimental additions were therefore accumulated within the soil compartment. Although no changes in species composition were seen during this experiment, effects on internal nutrient cycling and soil nitrogen status could be expected to have an impact on the outcome of competition between regenerating Calluna and grass seedlings. The results indicate that use of the standard management practice at this site, involving cutting and removal of only above-ground plant material, whilst essential for the maintenance of a mixed-age Calluna sward, will have only a relatively small impact on nitrogen accumulation within this system. The importance of management in modifying the impact of enhanced nitrogen deposition is discussed in the context of critical loads for healthland systems in the UK.