Phosphorus retention properties of forest humus layer in buffer zones and clear-cut areas in southern Finland

Hydrological losses of phosphorus (P) from forested areas have been reported to increase after harvesting operations. An efficient method of decreasing P losses has been to leave unmanaged buffer zones along forest brooks, streams and lakes. However, the processes controlling P retention by buffer zones are still not fully understood. To assess the importance of forest humus layer in P retention a small-scale survey was carried out at three buffer zones and adjoining clear-cut areas in southern Finland. In general, the humus layer in the buffer zones adsorbed more P than in clear-cut areas and the P adsorption was strongly related to the contents of oxalate extractable iron and aluminium. However, there were indications that the P retention capacity of the humus may decrease with the length of time the clear-cut area or buffer zone has been under the impacts of clear-cutting. There may be significant seasonal changes in humus P retention, but further studies are needed for a critical evaluation of the changes in humus P adsorption between seasons and years. Nevertheless, when compared with the increase in P release after harvest in a boreal forest, the maximum P retention capacity of the humus layer was small. Therefore, the role of the forest humus layer in P retention in clear-cut areas and buffer zones may be insignificant.