NUTRIENT DYNAMICS OF RIPARIAN ECOTONES - A CASE-STUDY FROM THE PORIJOGI RIVER CATCHMENT, ESTONIA

The transformation of nitrogen and phosphorus was studied in various r iparian ecotones of the moraine plain and moraine-hilly landscapes in the Porijogi River catchment area, southern Estonia. Soil water and gr ound water samples were collected once per month between 1992 and 1993 from piezometers installed between plant communities located along to po-edaphic gradients from moraine uplands to stream valleys at six sit es. The production rate, as well as the N and P content in plant bioma ss from the 1 m2 plots located near the piezometers was analyzed. Resu lts show that various riparian ecotones had a significant influence on soil water quality. In the arable land, the mean annual concentration of total inorganic nitrogen (NH4 + NO2 + NO3) in piezometer water was 3-40 mg N 1(-1), but in the grey alder forest (Alnus incana) total in organic nitrogen never exceeded 1 mg N 1(-1). The most significant red uction was in the average concentration of NO3-N content which was red uced from 25 mg 1(-1) under the arable land to 0.5 mg 1(-1) within the alder forest. The average total phosphorus (orthophosphate phosphorus + organic phosphorus) concentration also decreased under the alder st ands, being 0.2-1.5 mg P 1(-1) in the arable land and less than 0.2 mg P 1(-1) in the alder forest (less than 0.1 mg P 1(-1) in loamy soils and 0.2 mg P 1(-1) in sandy soils). Wetland herb communities (Carex el ata association and Filipendula ulmaria-Cirsium oleraceum-Aegopodium p odagraria community) also had a significant influence on soil water qu ality. Plant biomass (sum of above- and below-ground biomass) of ripar ian and wetland communities accumulates up to 70 g N m-2 and up to 6 g P m-2 during the growth season. Harvesting of riparian herbaceous com munities may remove 20-30% of nutrient input. In the cultivated grassl ands on sandy colluvial soils with a deep humus layer and sedge fens t he content of NH4-N and total-P in groundwater toplayer significantly increased, rising to 3 mg and 0.5 mg 1(-1), respectively. Also, it has been assumed that vertical penetration of deep groundwater (contamina ted with nitrate and phosphate), may increase the nutrient load to sur face water bodies, despite the physical presence of a wide buffer zone (e.g. sedge fen) along the river corridor. From the observations of t his work, alder forests and/or willow bushes as buffer strips on and a djacent to the stream banks are recommended to control diffuse water q uality.