Wood ash and N fertilization in the Canadian boreal forest: Soil properties and response of jack pine and black spruce

•We studied ash and N fertilization of a jack pine boreal stand of Eastern Canada.•Ash application improved forest floor and mineral soil acid–base status over 8years.•Ash increased forest floor net mineralization rate and decreased its C content.•Jack pine five-year relative growth rate responded to N fertilization only.•Deleterious effects of ash on black spruce growth need further investigation. Wood ash fertilization has yet to be investigated in Canadian boreal forests. Ash often improves soil acid–base status, but without N addition it seldom increases tree growth on poor mineral soils. We report results of a large scale experiment conducted in a boreal jack pine (Pinus banksiana Lamb) stand growing on sandy acidic soil in Northeastern Canada. The experiment was completely random with four replications (1ha each) of five treatments (0, 1, 2, 4 and 8 dry Mgha−1) of loose fly ash with and without urea (280kgNha−1). Soils were sampled in the 0, 2, and 8Mgha−1 treatment up to eight years after application. Foliar nutrition and stand growth were assessed in all treatments one and two years and five years after treatment, respectively. Even under low ash loading, forest floor exchangeable base cations, pH, and base saturation increased within a year of application. Ash application also resulted in a swift decrease in forest floor organic C and an increase in N potential net mineralization rate. The initial dominant pattern of upper mineral soil properties in relation to ash loading was a curvilinear relationship with the highest values observed in the 2Mgha−1 treatment. Eight years after ash application, significant linear relationships were found between ash loading and base cations and base saturation in the forest floor and mineral soil (0–10cm, 10–20cm). Contrary to N fertilization, ash had no effect on jack pine foliar nutrition and on its five-year growth. However, a decrease of 30% in relative growth rate was observed between the control and the 8Mgha−1 ash treatment for large (⩾10cm DBH) black spruces (Picea mariana (Mill.) BSP). Black spruce is the dominant commercial species of Canadian eastern boreal forests and thus, additional studies are needed to validate the deleterious effect of ash on spruce growth and to elucidate the mechanisms involved.