NUTRIENT RETENTION IN THE VEGETATION OF AN OVERLAND-FLOW TREATMENT SYSTEM IN NORTHERN FINLAND

The importance of vegetation for nutrient retention was estimated in a n overland flow area (OFA) in northern Finland, where peat mining wate r is purified by conducting it across a virgin mire. In the OFA the ph ytomass of herbs, sedges, graminoids and below-ground parts of plants was greater, and that of shrubs and bryophytes smaller, than in a refe rence area (RA). Overland flow treatment favoured plant species, such as Menyanthes trifoliata and Carer lasiocarpa, and discriminated again st Sphagnum spp. in particular. The differences in the nutrient conten ts of plants in the OFA resulting from nutrient enrichment by peat min ing water were compared with the actual decrease in amounts of nutrien ts in the area. Total phytomass in the OFA increased by 20%, and its N concentration increased from 1.00 to 1.24% of dry weight, but the P c oncentration decreased from 0.26 to 0.22% of dry weight. These changes in total phytomass and nutrient concentrations resulted in a 40% incr ease in the N bound by plants in the area, but a 5% decrease in P. The increase in N bound by the total phytomass in the OFA during the 6 ye ars of usage accounted for only about 4% of the observed total retenti on of inorganic N. In contrast, the vegetation in the area served as a source of P rather than a sink with the decrease in P accounting for nearly 20% of the measured PO4-P retention. The average total decrease s in both P and N in the OFA in 1987-91 were 55%. Thus, the net retent ion of N and P from the peat mining water was mainly the result of oth er processes rather than retention in the plant cover.