Changes in the properties of grassland soils as a result of afforestation

The effects of afforestation on physical, physico-chemical, and biological properties of grassland soils were investigated in three sites (215-230 m a.s.l.) located within the urban area of Krakow (southern Poland) after 45-60 years since the introduction of forest tree species. We compared the contents of nutrients and the composition of humus between forest and adjacent grassland soils, as well as the quality of the forest soils in relation to the introduced tree species (alder, pine, oak, birch, maple, elm). We hypothesized that afforestation of grassland soils results in the increase of acidity and nutrient contents. Studied soils belong to Dystric Gleysols (forest) and Eutric Gleysols (grassland). Overall, 168 soil samples were taken from two layers (0-10 cm and 10-30 cm) both from forest and grassland soils. The results showed an increase of organic carbon (up to 150%), total nitrogen (up to 70%) and total acidity (up to 18 times), as well as a decrease of pH (up to 40%) and activity of dehydrogenase (up to 60%) in forest soils in relation to the respective grassland soils. The most intensive changes were observed in the topsoil layers (0-10 cm). We also calculated the Soil Quality index (SQI) based on PCA in which only soil parameters with high load factors were taken into consideration. SQI ranged from 0.39 to 0.41 in grassland soils and from 0.33 to 0.37 in forest soils. Among forest soils, the highest value of SQI was obtained for stands dominated by black alder, indicating that such species is the most suitable for afforestation purposes under the study conditions. The results may be helpful in the realization of afforestation plans of humid grassland soils.