Nutrient dynamics of riparian ecotones: A case study from the Porijõgi River catchment, Estonia

The transformation of nitrogen and phosphorus was studied in various riparian ecotones of the moraine plain and moraine-hilly landscapes in the Porijõgi 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.