Wood ash in boreal, low-productive pine stands on upland and peatland sites: Long-term effects on stand growth and soil properties

•On an N-poor peatland site, ash increased Scots pine growth by 17% during 25years.•On an N-poor upland site, ash increased Scots pine growth by 11% during 20years.•On an N-poor upland site, ash increased duration of stand growth response to N.•On a peatland site, ash stimulated C mineralization and cellulose decomposition.•On an N-poor upland site, Ash+N stimulated processes in both C and N cycling. The effect of wood ash on growth of Scots pine was studied in 64- to 75-year-old stands on three upland sites (Exps. 402, 407 and 408) for 20years and in a 30-year-old Scots pine stand on an oligotrophic peatland site (Exp. 251) for 25years. In Experiments 407 and 251 the responses of soil chemical properties and soil microbiological processes related to C and N cycling were also studied. The upland experiments included a control and a treatment with 3Mgha−1 of wood ash. In Exp. 407, 120kg Nha−1 was applied together with ash; this experiment also included a treatment with N alone. The peatland experiment included a control and a treatment with 4.8Mgha−1 of wood ash. All experiments had 3 replications. Wood ash significantly decreased soil acidity on all sites. On the upland site, after 20years, the concentration of K2SO4-extractable DOC and the rates of C mineralization (CO2–C production) and net N mineralization were all higher in the Ash+N treatment than in the control or N treatments. However, the treatments did not significantly affect the amounts of C or N in the microbial biomass or the concentration of NH4–N. On the peatland site, after 27years, ash stimulated C mineralization and cellulose decomposition, but microbial biomass C or N, net N mineralization or the concentration of N were not affected significantly. On both the upland and peatland site, net nitrification was very low in all treatments. In Exp. 408, the volume growth in the control and Ash treatment was during the 20-year study period 60 and 64 m3ha−1, respectively, and in Exp. 402 108 and 120m3ha−1, respectively, the latter difference being significant. In Exp. 407, the volume growth in the Ash+N treatment was during the 20-year study period significantly higher (92m3ha−1) than in the N and control treatments (76 and 73m3ha−1, respectively). On the peatland site during the 25-year study period the growth was 145 and 169m3ha−1, in the control and Ash treatments, respectively. In conclusion, the long-term positive response of stem growth to wood ash on peatlands and N fertilized upland sites