Land use effects on soil N, P, C and pH persist over 40–80 years of forest growth on agricultural soils

Previous cultivation may have long-lasting effects on oak forests, e.g., nitrogen (N) and carbon (C) depletion, phosphorus (P) enrichment and lowered soil acidity, which may influence soil processes, wood production and plant diversity. We used a large database on land use to sample 18 sites where oak had been planted 40–80 years ago on continuously forested land and formerly cultivated fields that had similar soil properties before the different land uses. We estimated the ‘total’ pools of N, P and C and ‘available’ fractions of N and P at three soil depths. The soil data were compared with element concentrations in leaves of four plant species. Soil solution concentration of nitrate at 50–60 cm was used as an estimate of nitrate leaching. Total N and mineralisable N in the soil were higher in the continuously forested land than in the formerly cultivated fields and this was reflected in the N concentration in Dryopteris filix-mas. Total P and available P tended to be lower in the continuously forested land than in the formerly cultivated fields which were reflected in the P concentrations in Dryopteris and Quercus robur. Soil C was higher and pH was lower in the continuously forested land than in the formerly cultivated fields. Leaching of NO3−-N started to appear in higher concentrations below pH 4.5 and the leaching is interpreted more as a pH than a land use effect. Aluminium (Al) concentrations were higher in Dryopteris and Quercus and the iron (Fe) concentration in Milium effusum, while calcium (Ca) was lower in all plant species. Potassium (K) was lower in Stellaria holostea and no difference was found for the magnesium (Mg) concentration in plants. As previous land use seems to affect soil nutrients and potentially toxic elements it is important to identify the limiting factors for the different ecosystem components to be able to manage the forests correctly. It is probable that areas with significant N deposition are not N-limited even in the N-depleted formerly cultivated fields and that a higher P content and soil pH there may result in higher biomass production and changes in plant diversity. The risk of nitrate leaching in the most acidic, continuously forested land must also be taken into consideration in the management of forests.