Soil fertility of afforested arable land compared to continuously

In Finland, over 220,000 ha of arable land has been afforested in recent decades. To meet the goals of forest management on afforested fields, information on the effects of former agricultural land use on soil fertility is needed. In this study, we examine the soil fertility of 12 former arable fields afforested either 10 or 60-70 years ago with Norway spruce (Picea abies (L.) Karst.) and adjacent sites that have been forested continuously. Volumetric soil samples were collected from the organic soil layer and from mineral soil to a depth of 40 cm. Soil samples were analyzed for pH, bulk density, organic matter content and amounts of nutrients (Kjeldahl N, extractable P, K, Ca, Mg, Zn and B). On afforested fields, amounts of nutrients in the mineral soil, especially in 10-year-old afforestations, were higher than on continuously forested sites. In the organic layer plus the 0-40 cm soil layer, the 10-year-old afforestations had 68% more N, 41% more P, 83% more K, 252% more Ca, 6% more Mg, 61% more Zn and 33% more B than the continuously forested sites at a comparable soil depth. In the 60-70-year-old afforestations, the differences were significant only for N, Ca and Zn (20% more N, 121% more Ca and 115% more Zn than on the continuously forested sites). The effects of agriculture on amounts of nutrients were most clearly detected in the former plough layer (0-20 cm) of the 10-year-old afforestations and in the top layer (0-10 cm) of the older afforestations. Amounts of nutrients in the organic layer of the afforested sites were lower, but their concentrations were higher than in the continuously forested sites. On the 10-year-old afforestations, the bulk density of the mineral soil tended to be lower and the organic matter content higher than on the continuously forested sites. On both young and old afforestations, soil pH was higher than on the continuously forest sites. According to these results, changes in soil properties caused by agriculture have increased the soil fertility and therefore probably also the site index. The results also suggest that changes in soil properties due to agricultural land use are quite long lasting.