Nutrient dynamics of riparian ecotones: A case study from the Porijo˜gi River catchment, Estonia

Abstract

The transformation of nitrogen and phosphorus was studied in various riparian ecotones of the moraine plain and moraine-hilly landscapes in the Porijo˜gi River catchment area, southern Estonia. Soil water and ground water samples were collected once per month between 1992 and 1993 from piezometers installed between plant communities located along topo-edaphic gradients from moraine uplands to stream valleys at six sites. The production rate, as well as the N and P content in plant biomass from the 1 m2 plots located near the piezometers was analyzed.Results 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+NO2) in piezometer water was 3–40 mg N1−1, but in the grey alder forest (Alnus incana) total inorganic nitrogen never exceeded 1 mg N1−1. The most significant reduction was in the average concentration of NO3-N content which was reduced 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 stands, being 0.2-1.5 mg P1−1 in the arable land and less than 0.2 mg P 1−1 in the alder forest (less than 0. l mg P1−1 in loamy soils and 0.2 mg P1−1 in sandy soils). Wetland herb communities (Carex elata association and Filipendula ulmaria-Cirsium oleraceum-Aegopodium podagraria community) also had a significant influence on soil water quality. Plant biomass (sum of above- and below-ground biomass) of riparian and wetland communities accumulates up to 70 g N m−2 and up to 6 g P m−2during the growth season. Harvesting of riparian herbaceous communities may remove 20–30% of nutrient input. In the cultivated grasslands on sandy colluvial soils with a deep humus layer and sedge fens the content of NH4-N and total-P in groundwater toplayer significantly increased, rising to 3 mg and 0.5 mg−1, respectively. Also, it has been assumed that vertical penetration of deep groundwater (contaminated with nitrate and phosphate), may increase the nutrient load to surface water bodies, despite the physical presence of a wide buffer zone (e.g. sedge fen) along the river corridor. From the observations of this work, alder forests and/or willow bushes as buffer strips on and adjacent to the stream banks are recommended to control diffuse water quality.