Twenty years of afforestation of former agricultural lands with silver birch plantations affects vertical distribution of SOC and macronutrients in the topsoil layer

Purpose Fast-growing tree plantations on abandoned agricultural soils is a promising management system to sequester atmospheric CO2. However, the effects of fast-growing trees on the nutritional and organic carbon (SOC) status of soils degraded by agriculture, are poorly understood.Methods We sampled the soil after 20 years in 10 silver birch plantations on former agricultural soils in hemiboreal Estonia to assess changes in soil chemical properties (SOC, N, C:N ratio, pH(KCl), P, and K) in 10-cm vertical mineral soil layers to a depth of 30 cm and to determine the potential environmental drivers of plant-soil interactions.Results We observed no depletion of SOC or macro-nutrients in the upper 0-30-cm soil layer, but found some vertical shifts among the sublayers. The SOC concentration increased by 22% in the upper 0-10-cm soil layer, especially in sites with higher above-ground productivity. Simultaneously, SOC concentration decreased by 17% in the 20-30-cm soil sublayer, which indicating trees' ability to alter decomposition activity in deeper vertical soil layers. In the 20-30-cm sublayer, SOC mineralization was supported by an 11% decrease in the C:N ratio. Similarly, the total N concentration increased in the 0-10-cm soil layer by 13%. The concentration of plant-available P increased by similar to 30% in the 20-30-cm sublayer.Conclusion Two decades of afforestation of former agricultural soils caused vertical stratification of SOC in the upper mineral soil layer (0-30 cm) where trees can access deeper nutrient pools for active cycling, but caused no loss of SOC or nutrients.