Organic P fractions in mineral soil
Tree species did not affect the contents of POlab and POmod in the uppermost layers of the studied soils (Figure 1). However, the tree species effect was observed for the remaining organic P fractions. The lowest contents of Pfulv, Phum and POreswere measured in soils under Scots pine (31.4 µg g-1, 19.8 µg g-1 and 57.4 µg g-1, respectively) (Figure 1). The soils under the remaining tree species did not differ in terms of Pfulv (39.5 µg g-1 – 40.5 µg g-1), Phum (30.7 µg g-1– 41.5 µg g-1) and POres (76.9 µg g-1– 81.6 µg g-1) contents. When the contents of particular organic P fractions were expressed as percentage of Porg no significant differences between the tree species were observed (Figure S1).
Concentrations of all organic P fractions (except POlab) depended on the substrate type (Figure 1). The highest contents of POmod were measured in Ashes (38.7 µg g-1), followed by Sands (20.7 µg g-1) and Clays (13.6 µg g-1). Sands were characterized by significantly lower Porgconcentrations (106.5 µg g-1; Table 4) and significantly lower contents of Pfulv, Phum and POres fractions (20.8 µg g-1, 19.1 µg g-1 and 21.5 µg g-1, respectively) than two other substrate types (Figure 1). Clays had the highest Porg concentration (272.1 µg g-1; Table 4) resulting from the highest concentration of POres (143.1 µg g-1). Ashes contained significantly less of this fraction (56.8 µg g-1) than Clays but the contents of Pfulv and Phum were similar for both substrate types (46.0 µg g-1 – 46.6 µg g-1 for Pfulv and 37.4 µg g-1 – 40.0 µg g-1Phum) (Figure 1). The Corg-to-Porg ratio depended on substrate type with Clays exhibiting the highest (271), Ashes intermediate (225) and Sands the lowest (166) value (Table 4). The effect of substrate type was evident also when the contents of particular organic P fractions were expressed as percentage of Porg. Sands had the largest share of POlab,, Ashes of Pfulvwhile Clays of POres (Figure S1).