The effects of pollutant nitrogen on forest soils and the potential feedbac
ks on tree growth are discussed using data from recent plot and catchment m
anipulation studies. Results indicate that N applied to the soil affects bo
th soil N transformations and base cation status in some forest soils. Whil
st reductions in tree growth are infrequently reported, a greater understan
ding of the effect of increased N deposition on soil N transformations, and
associated changes in soil acidification and nutritional balance, is essen
tial if sensitive stands are to identified. In particular, the factors cont
rolling soil N accumulation rates and thus the onset of nitrate leaching ar
e not clearly understood with increased nitrogen availability potentially i
ncreasing nitrogen accumulation due to abiotic fixation and lignolytic enzy
me suppression, whilst the onset of nitrification as the C/N ratio of fores
t floor material declines, may reduce N retention efficiency. The switch fr
om increased to decreased tree growth rate in response to N additions in so
me experiments highlights the need for long-term studies as a necessary com
ponent of future research.