The aim of this paper was to determine the soil phosphorus release risk from drained and re-wetted peatlands under different landcovers. The study was carried out at 10 soil profiles (organic soils, frequently with mursh in the topsoil, developed from low peat with various degrees of decomposition, on mineral subsoil or gyttja) located south-west of Lake Kośno nature reserve in northern-central Europe (north-eastern Poland). The soil samples were collected from the depths of 0-30 cm, 50-80 cm and 100-130 cm during spring, summer and autumn (in April, July, and October). In the laboratory, the following soil properties were determined: ash content, soil reaction, content of total organic carbon, total nitrogen, total and plant-available phosphorus. Gyttja formations and hemic peats had the highest total P content (6.77 g∙kg-1 and 2.29 g∙kg-1, respectively). Murshes contained twice less P (1.18 g∙kg-1). In spring, more plant-available phosphorus (AP sp) was released in murshes (0.29 g∙kg-1) than in peats (0.07-0.14 g∙kg-1). In the summer, higher amounts of plant-available P were stated in fiber peats (0.37 g∙kg-1) than in murshes (0.26 g∙kg-1), whereas in autumn, the amounts of P in murshes (0.22 g∙kg-1) and peats (0.21-0.23 g∙kg-1) did not vary. Generally, average contents of AP were the highest in murshes (0.26 g∙kg-1) and fiber peats (0.25 g∙kg-1), and the lowest in sapric peats (0.15 g∙kg-1). The release of phosphorus from organic compounds or binding of P by organic matter (immobilization) depends on the C:P ratio in plant residues and soil. If it is wide (>300), P is bound, if it is narrower (150–200), P is released. In the tested soil formations, the C:P ratio was frequently above 300. Only in some murshes and gyttjas the C:P ratio was below 150, which may indicate a risk of phosphorus release to groundwaters.
