Field-scale evaluation of effects of nitrogen deposition on the functioning of heathland ecosystems

Summary 1. Plant communities that are adapted to low levels of nutrient availability are particularly sensitive to the effects of elevated nitrogen (N) deposition. Although site-level manipulation experiments have been used widely to examine the effects of N deposition on (semi-)natural ecosystems under controlled conditions, relatively few studies have investigated field-scale impacts of elevated ambient N deposition on native plant communities and ecosystems. 2. This study investigates the relationship between atmospheric N deposition, plant and soil nutrient status and microbial enzyme activities at 32 lowland heathland sites across the UK. Sites were chosen to cover a range of N deposition rates (13.3–30.8 kg N ha )1 year )1 ), geographical areas and different geologies. 3. Significant relationships were found between rates of N deposition (total, reduced and oxidized) and concentrations of N and phosphorus (P) in Calluna shoots, litter and soil; relationships were generally stronger with total and reduced forms of N, compared with oxidized N. Litter and humus layers were deeper at sites receiving higher atmospheric N inputs, suggesting increased rates of soil carbon and N accumulation, despite higher levels of phenol-oxidase activity (implying faster rates of organic matter decomposition) at these sites. 4. The combination of elevated plant and soil P concentrations at sites receiving greater N inputs suggests strongly that N is increasing the availability and uptake rates of P, in addition to N. Furthermore, significant interactions between temperature and N deposition on indices relating to productivity and the turnover and uptake of nutrients highlight the influence of climate on ecosystem response to N deposition. 5. Synthesis: Field-scale evidence of changes in rates of nutrient cycling, organic matter accumulation and plant biochemistry suggests that ambient levels of N deposition are affecting the functioning of many heathland ecosystems and that the magnitude of these effects is also influenced by climate. Since such changes are known to be associated with reduced resistance to environmental stress and loss of plant diversity, current (and predicted future) levels of N deposition are likely to have important implications for the conservation and long-term sustainability of nutrient-poor ecosystems, particularly in the face of climate change.