Extensive grasslands as an effective measure for nitrate and phosphate reduction from highly polluted subsurface flow – Case studies from Central Poland

Multifunctional ecosystem of narrow grasslands located between croplands and streams slowly disappears from the agricultural landscape in Poland despite its importance to reduce the impact of land-based nutrients on freshwater ecosystems. We studied the effects of five extensive grasslands on nitrate and phosphate reduction in the catchment scale. During our 4-year monitoring, we did not observe the saturation effect in case of the two buffer zones that were receiving high nitrate load via subsurface flow. Highest nitrate concentration exceeded twice the level of 50 mg NO3/L, which is considered a threshold level of water pollution status by the EU Nitrates Directive. Concentrations above 100 mg NO3/L were reduced by 68% and 99% passing through the 25 m and 45 m of grassland, respectively. The efficiency of buffer zone to mitigate phosphate losses varied depending on the input load. The results obtained for high concentrations (above 1.5 mg PO4/L) showed 81% and 76% effectiveness of 45 m and 47 m grassland, respectively. However, the release of phosphates was reported as well and occurred at the buffer zones characterized by low inflow P concentrations when assimilation-decomposition processes dominated ecotone P dynamics. The analysis of nutrient retention in vegetation showed that harvesting of grassland removed 131 kg N/ha/yr and 19.4 kg P/ha/yr. Furthermore, the amount of nitrate and phosphate removed by buffer zones were statistically and positively correlated (r = 0.62, p < 0.05 and r = 0.52, p < 0.05, for NO3 and PO4 respectively) with the biodiversity (expressed as Shannon index), which underlines the importance of marginal parts of buffer zones.