Organic P in temperate forest mineral soils as affected by humus form and mineralogical characteristics and its relationship to the foliar P content of European beech

Phosphorus (P) deficiency of beech (Fagus sylvatica L.) is widespread in Europe. However, it is difficult to determine at which sites trees are prone to P deficiency, since foliar analysis is the only tool widely accepted for assessing the P nutritional status of beech. Our central objective was to identify the main influencing factors on organic P in forest soils and to test its usefulness as a predictor of the foliar P content of beech. Our hypotheses were: (1) stocks of total organic P (TOP) and microbial biomass P (MBP) in the mineral soil are higher at mull than at moder/mor sites, (2) the TOP content and the soil organic carbon (SOC):TOP ratio are affected by soil mineralogical characteristics, such as oxalate-extractable iron and aluminum (Feox, Alox) and clay, (3) multiple regression models that include the TOP stock, soil mineralogical parameters and the humus form as predictors are useful to explain the variability of the foliar P content of beech. We sampled the mineral soils of 9 mull and 11 moder/mor sites at 0–50 cm depth and took foliar samples of beech trees in three subsequent years. In contrast to our hypothesis (1), we did not find significantly higher TOP or MBP stocks, but determined lower SOC:TOP ratios at mull compared to moder/mor sites. In line with our hypothesis (2), the TOP content was positively related to Feox, Alox and clay contents, while the SOC:TOP ratio was negatively related to Feox, Alox and clay contents, suggesting strong effects of clay, Al and Fe on the accumulation of TOP. In partial agreement with our hypothesis (3), the TOP stock was included as a positive and the clay content as a negative predictor in a multiple regression model of the foliar P content, suggesting decreasing bioavailability of TOP with increasing clay content. Across all sites, the model consisting of these two parameters better explained the foliar P content (R² = 0.60) than the stocks of Bray-II- (R² = 0.37) or Olsen-extractable P (R² = 0.53) as single predictors. Overall, our results suggest that the accumulation of TOP in forest mineral soils and its potential availability to beech are strongly affected by soil mineralogical parameters, which may be included in statistical models to improve the predictability of P availability to trees.