The effects of different doses of wood ash, given together with urea-N fertilizer, on chemical properties of
the organic layer, soil microbial processes related to C and N cycling, and stand growth were studied in a
Scots pine (Pinus sylvestris) stand on mineral soil 30 growing seasons after fertilization. The treatments
were an unfertilized control, a nitrogen treatment (N) and a combined treatment with N and three different
doses of wood ash: 1, 2.5 and 5 Mg ash ha1 (WA1 + N, WA2.5 + N and WA5 + N). There were six
replications in the control and N treatment and two replications in each ash + N treatment. The amount
of N applied was 185 kg ha1. Compared to the control, the WA5 + N treatment significantly increased the
pH (0.5 pH units) and decreased the exchangeable acidity in soil (45%). The concentrations of exchangeable
Ca, K and Mg in the soil in the WA2.5 + N and WA5 + N treatments and those of extractable P in all
the wood ash + N treatments were significantly higher than in the control and N-alone treatment. The Ca
concentrations in the WA2.5 + N were two times and in the WA5 + N treatment three times that in the
control treatment. The mean increment in basal area of the tree stand was significantly higher in the
WA1 + N and WA2.5 + N treatments than in the control or N-alone treatment during the 30-year study
period. Although application of wood ash increased the growth response to N, at the end of the study period
the response had virtually stopped. To compare the statistical significance of the differences in microbial
biomass and activities between different treatments, all six WA + N plots were combined, regardless
of the different ash doses. Amounts of C and N in the microbial biomass, the rate of C mineralization (CO2
production) and concentration of K2SO4-extractable dissolved organic carbon were all higher in the
WA + N treatment than in the N-alone treatment. The rate of net N mineralization, and the ratios between
net N mineralization and C mineralization and between net N mineralization and microbial biomass N
were all highest in the WA + N treatment, indicating better availability of N. In all treatments, however,
both the rate of net nitrification and the NO3–N concentrations were negligible. In conclusion, wood ash
application together with N seems to have very long-term effects on soil chemical properties and microbial
processes in C and N cycling which give at least some explanations for the response in tree growth to
this treatment.