Afforestation of former agricultural land with Salicaceae species – Initial effects on soil organic carbon, mineral nutrients, C:N and pH

Abstract Different species of Salicaceae are of interest for bioenergy production in Sweden. Surplus agricultural land is available and can be used for bioenergy plantations. Documentation of the effects of afforestation on soil properties is of importance for future recommendations on the choice of tree species and management measures. Hybrid aspen ( Populus tremula  ×  Populus tremuloides ), poplar ( Populus spp.) and willow ( Salix spp.) were planted on former arable land at four sites in Sweden. Planting densities were 1500 plants ha −1 for hybrid aspen and poplar, and 14,800 plants ha −1 for short rotation coppice (SRC) willow. The present study comprised five years following plantation with the aim to quantify the afforestation effects on mineral soil variables, i.e. soil organic carbon (SOC), nitrogen (N), phosphorus (P), base cations (K, Mg, Ca), C:N ratios and pH. The soil variables were estimated by repeated sampling, i.e. prior to planting and at the end of the fifth season. A litter layer was present in the fifth year and included in the C estimates. The SOC pools were generally unchanged compared to pre-planting conditions. Total N pools had increased in SRC willow plantations and P pools had decreased in hybrid aspen and poplar plantations. Plant available K and Mg concentrations increased in the upper 0–10 cm soil and decreased in the lower 10–30 cm for all species, indicating redistribution by plant uptake and recycling through litter decay. Concentrations and pools of Ca were unchanged. The C:N ratio and pH were reduced with ca 10% and 0.1 units, respectively, for all species. No specific effect of species was observed in the mineral soil, but SRC willow had lower C:N ratio in aboveground litter compared to the other species. Thus, five years growth of Salicaceae on former agricultural land resulted mainly in general soil effects associated with the cessation of annual management measures, enhanced litter production from trees and ground vegetation, and probably also with altered soil physical conditions, for example humidity and temperature, in the growing plantations.