The impact of nitrogen on forest soils and feedbacks on tree growth

The effects of pollutant nitrogen on forest soils and the potential feedbacks on tree growth are discussed using data from recent plot and catchment manipulation studies. Results indicate that N applied to the soil affects both soil N transformations and base cation status in some forest soils. Whilst reductions in tree growth are infrequently reported, a greater understanding of the effect of increased N deposition on soil N transformations, and associated changes in soil acidification and nutritional balance, is essential if sensitive stands are to identified. In particular, the factors controlling soil N accumulation rates and thus the onset of nitrate leaching are not clearly understood with increased nitrogen availability potentially increasing nitrogen accumulation due to abiotic fixation and lignolytic enzyme suppression, whilst the onset of nitrification as the C/N ratio of forest floor material declines, may reduce N retention efficiency. The switch from increased to decreased tree growth rate in response to N additions in some experiments highlights the need for long-term studies as a necessary component of future research.