30-Year effects of wood ash and nitrogen fertilization on soil chemical properties, soil microbial processes and stand growth in a Scots pine stand

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 5Mgashha⁻¹ (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 185kgha⁻¹. 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.