Nutrient retention in the vegetation of an overland flow treatment system in northern Finland

The importance of vegetation for nutrient retention was estimated in an overland flow area (OFA) in northern Finland, where peat mining water is purified by conducting it across a virgin mire. In the OFA the phytomass of herbs, sedges, graminoids and below-ground parts of plants was greater, and that of shrubs and bryophytes smaller, than in a reference area (RA). Overland flow treatment favoured plant species, such as Menyanthes trifoliata and Carex lasiocarpa, and discriminated against Sphagnum spp. in particular. The differences in the nutrient contents of plants in the OFA resulting from nutrient enrichment by peat mining water were compared with the actual decrease in amounts of nutrients 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 concentration decreased from 0.26 to 0.22% of dry weight. These changes in total phytomass and nutrient concentrations resulted in a 40% increase 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 years of usage accounted for only about 4% of the observed total retention 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 decreases in both P and N in the OFA in 1987–1991 were 55%. Thus, the net retention of N and P from the peat mining water was mainly the result of other processes rather than retention in the plant cover.