Land use effect on nutrient concentrations in Pampas streams (Buenos Aires, Argentina)

Abstract

The objective of this study was to assess the influence of land use on nutrient concentration in first-order Pampas streams. The concentration of nutrients in the base flow and flood stream conditions, as well as in the runoff water in a stream flowing through a cultivated field was compared over a three-year period. The surrounding plots were cultivated with the three principal crops grown in the region: wheat, corn, and soybeans. The concentrations of nutrients in base flow conditions were compared with that of the nearby stream that originated in a livestock plot located in an unfertilized grassland. The concentration of nitrate in runoff was found to be lower when the adjacent plot was cropped with soybeans and not fertilized with urea, in comparison to the plots cropped with either wheat or corn, both of which were fertilized with urea. The nitrate concentrations in runoff from plots cropped with corn and wheat were not significantly different, with both received receiving approximately the same amount of nitrogen fertilizers. The occurrence of exceptionally high nitrate concentrations in runoff water in coincidence with the first rain event following fertilization indicates that the elevated nutrient concentration is attributable to the fertilizer input. The stream draining the livestock plot exhibited lower soluble reactive phosphorus (SRP) and nitrite concentrations than the stream adjacent to fertilized crops. It can be concluded that the ongoing agricultural practices increase nutrient concentrations in adjacent streams by increasing runoff loads following crop fertilization. The establishment of livestock on natural, unfertilized pastures along the stream borders would result in a decrease in stream nutrient concentrations.