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Climate change is predicted to increase the frequency and severity of summer fires in the UK. However, the effects of fire on ecosystem processes are likely to depend on pre-burn vegetation characteristics, soil chemistry and microbiology, all of which are known to be affected by changes in nitrogen (N) deposition. The occurrence of a severe summer fire after eight years of experimental N additions at Thursley Common National Nature Reserve (UK) thus provided a unique opportunity to evaluate the impacts of a major perturbation on nutrient and microbial dynamics in soils of contrasting chemistry. Soil N and P concentrations were substantially reduced by fire, although significant pre-fire treatment effects on soil N were still apparent immediately after the fire. Soil nutrient availability, turnover rates and leaching losses were also increased by fire. Concentrations of extractable soil N and leaching losses of Ca 2+ , K + , Cl − and SO 4 2− immediately after the fire were significantly higher in plots which had previously received N additions, demonstrating effects of N loading on post-fire nutrient dynamics and economy. Effects of N addition on the activity of soil microbial populations were influenced by soil moisture levels, with significantly higher activity in +N plots compared to controls when soil moisture levels were relatively high (>15%). Microbial community composition varied considerably between sampling dates, although clear N treatment differences were apparent, particularly for bacterial communities, both immediately after and one year on from the fire. Persistent effects of wildfire and N deposition on the stocks and availability of N, and microbial community dynamics, are likely to have longer-term consequences for the functioning of heathlands and other similar semi-natural ecosystems.

The relationship between different features of lichen communities in Quercus robur canopies and environmental variables, including concentrations of NO 2 and NH 3 was investigated. NO 2 concentration was the most significant variable, it was positively correlated with the proportion of lichen cover comprising nitrophytes and negatively correlated with total lichen cover. None of the lichen community features were correlated with NH 3 concentrations, which were relatively low across the site. Since nitrophytes and nitrophobes are likely to react in opposite directions to nitrogenous compounds, total lichen cover is not a suitable indicator for these pollutants. It is, therefore, suggested that the proportion of lichen cover comprising nitrophytes may be a suitable simple indicator of air quality, particularly in locations where the pollution climate is dominated by oxides of nitrogen.

NO(2) and NH(3) concentrations were measured across a Special Area for Conservation in southern England, at varying distances from the local road network. Exceedances of the critical levels for these pollutants were recorded at nearly all roadside locations, extending up to 20 m away from roads at some sites. Further, paired measurements of NH(3) and NO(2) concentrations revealed differences between ground and tree canopy levels. At "background" sites, away from the direct influence of roads, concentrations were higher within tree canopies than at ground level; the reverse pattern was, however, seen at roadside locations. Calculations of pollutant deposition rates showed that nitrogen inputs are dominated by NH(3) at roadside sites. This study demonstrates that local traffic emissions contribute substantially to the exceedance of critical levels and critical loads, and suggests that on-site monitoring is needed for sites of nature conservation value which are in close proximity to local transport routes.