Effectiveness of grass filter strips for runoff nutrient and sediment reduction in dairy sludge-amended pastures

Abstract

An experiment was conducted to test the hypothesis that grass filter strips are effective in reducing nutrient and sediment concentrations in runoff from grazed pasture amended with dairy manure sludge. The experiment was carried out under recommended practices in two fields of a dairy farm in San Sebastián municipality, Puerto Rico. Runoff generated following a precipitation event was diverted into runoff-collection devices placed at 0, 10, and 20 m within a grass filter barrier. Samples were analyzed for suspended solids (SS), total Kjeldahl nitrogen (TKN), dissolved phosphorus (DP), and total phosphorus (TP). Suspended solid concentrations in runoff entering the filter strips were minimal, which is indicative that SS losses are not numerically significant from pasture fields exhibiting high vegetative coverage. Elevated TP and TKN concentrations were observed in runoff events occurring within 10 days after manure application. This finding indicates that farmers must avoid scheduling manure applications at times when significant rains are expected, because direct runoff will result in excessive off-field nutrient losses if no filter strip is present. In both fields, DP concentrations in runoff were significantly reduced with a filter strip 20 m wide, whereas TP concentrations were significantly reduced only from the field exhibiting the highest concentration in runoff, i.e., Toronjo field. A 27% decrease in TKN concentration was observed in the Toronjo field as a result of the 20-m filter strip (relative to the entrance), but such reduction was nonsignificant. Although the 20-m grass filter strip was effective in reducing nutrient concentrations in runoff from manure-amended fields, the implementation of other best management practices is needed to reduce the impact of nutrient losses to levels that do not pose a threat to the integrity of the receiving waters.