Correlating phosphorus extracted by simple soil extraction methods with foliar phosphorus concentrations of Picea abies (L.) H. Karst. and Fagus sylvatica (L.)

Phosphorus (P) concentrations in needles and leaves of forest trees are declining in the last years in Europe. For a sustainable forest management the knowledge of site specific P nutrition/availability in forest soils is vital, but we are lacking verified simple methods for the estimation of plant available P. Within this study, four soil P extraction methods [water (P-H2O), double-lactate (P lac), citric acid (P-cit), and sodium bicarbonate (P-HCO3)], as well as total P content of the soil (P tot) were tested to investigate which method is best correlated with foliar P concentrations of spruce [Picea abies (L.) H. Karst.] and beech [Fagus sylvatica (L.)]. Mineral soil samples from 5 depth levels of 48 forest sites of the Bavarian sample set of the second National Forest Soil Inventory (BZE II) were stratified according to tree species (spruce and beech) and soil pH (pH < 6.2 and > 6.2), covering the whole range of P nutrition. The extractable amount of P per mass unit of soil increased in the order P-H2O << P-lac < P-HCO3 < P-cit, decreased with soil depth, and was higher in soils with pH < 6.2. Citric acid extracted up to 10% of P-tot in acidic soils. Whereas P-cit delivers adequate regression models for P nutrition in the case of spruce (R-2 up to 0.53) and beech (R-2 up to 0.58) for acidic soils, P-HCO3 shows good results for spruce growing on acidic soils (R-2 up to 0.66) and for beech on soils with pH > 6.2 (R-2 up to 0.57). P lac produces adequate models only for beech on high pH soils (R-2 up to 0.64), while PH2O did not produce acceptable regression models. P tot seems suitable to explain the P nutrition status of beech on acidic (R-2 up to 0.62) and alkaline soils (R-2 up to 0.61). Highest R(2)s are obtained mostly in soil depths down to 40 cm. As PHCO3 and P-cit showed good results for both investigated tree species, they should be considered preferentially in future studies.