Effects of simulated acid mist on a sitka spruce forest approaching canopyclosure: Significance of acidified versus non-acidified nitrogen inputs

Effects of enhanced N, S and NS Acid additions, up to approximately 100 kg N and S ha(-1) y(-1), are described for a 15-year-old Sitka spruce forest g rowing on an acid peat in Scotland. Groups of 10 trees, replicated over 4 b locks, have been treated at canopy height on approximately 50 or 100 occasi ons with 2 mm precipitation equivalent, between April and November, since 1 996. Relative stem volume increment (RSVI) has been consistently higher in the NS Acid treated trees compared with control or N treated trees since th e second year of treatment, although no dose response was found. Litterfall was also considerably increased in the NS Acid treatments and showed a cle ar dose effect but was not correlated with RSVI. Base cation concentrations in soil waters, collected using zero tension lysimeters reflected the pres ence or absence of the spray treatments and showed a dose related increase in response to NS Acid inputs. Treatment with 2NS Acid increased NH4-N and NO3-N by an order of magnitude. Results after 4 years of treatment showed a positive growth and litterfall response to NS Acid inputs but no effect of N alone. Enhanced stemwood growth may be linked to the higher base cation and phosphate concentrations measured in the soil water in the presence of NS Acid and S treatments.