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Discharges of water and nutrients by the Vistula... – M. Pastuszak, Z. Witek
stable isotope pairs can often give a unique characterization of nitrate from different sources. The proportion of nitrate originating from agricultural runoff, which was calculated using two isotope mixing models, showed higher values of δ15N-NO3-(‰) in the Oder River than in the Vistula River in 2000-2002. Also the load weighted monthly δ15N-NO3- values showed different annual patterns, with the Vistula pattern being less pronounced (Voss et al., 2006). The more flattened annual pattern showed also NO3-N concentrations in the Vistula River (Fig. 9.6). As to point sources, it has been evidenced that the investment in mitigation of N and P from this sources has been spatially diversified, with much more dynamic and efficient actions taking place in western Poland (Dubicki, 1999, 2000; Anonymous, 2000; Program…, 2006). Between 1998 and 2008, the reduction in N loads, discharged from municipal WWTPs into surface waters or soil in the Vistula and Oder basin, reached 4,540 tons and 10,400 tons, respectively. That explains a more pronounced decline in N emission from WWTPs in the Oder basin (by 40%) than in the Vistula basin (by 15%) between 1995-2002 and 2003-2008. The most spectacular decrease in P emission was observed in the case of WWTPs pathway and the decline reached 48% in the Vistula and 61% in the Oder basin between the two sub-periods i.e. 1995-2002 and 2003-2008 (Kowalkowski et al., 2012; Chapter 8). Natural factors constitute the third group of factors affecting spatially diversified nutrient emission in Poland. The high percentage of unconsolidated bedrock in the Vistula (59.3% of the area) and Oder (70.2% of the area) basins explains the high percentage contribution of groundwater pathway to nitrogen emission into the Vistula (40%) and the Oder basin (24%). However, the higher percentage of high porosity bedrock in the Vistula than in the Oder basin seems to be responsible for much higher percentage contribution of emission via groundwater in the Vistula catchment (Kowalkowski et al., 2012; Chapter 8). In the case of deep soils and permeable bedrock, percolation to groundwater rather than channeling of flow laterally occur (Heathwaite et al., 2005). That pathway considerably prologs the transit time of water and that is of high importance with respect to enhanced N retention (Nixon et al., 1996). Therefore, the enhanced N and P leaching driven by anthropogenic factors can be even strengthened in the Oder basin by these natural factors.
9.5. Nitrogen and phosphorus outflow in the light of agricultural practices in various European countries In many Eastern and Central European countries huge changes in agricultural practices have taken place over the last two decades. These changes encompassed a decrease in animal stocking, a decline in application of mineral fertilizers, and investment in efficient sewage treatment. There are few river basins for which impact of reduced pressures, imposed by farming and point sources (waste water treatment plants – WWTPs), on riverine nutrient discharges, has been specified (Dannowski et al., 2002; Iital et al., 2003, 2005; Stålnacke et al., 2003, 2004; Hussian et al., 2004;