Acidification and increase of phosphorus levels in Pampean streams after 12 years of agricultural intensification

Abstract

Abstract Agricultural intensification is a process that is still undergoing in many emergent economies. In the Pampas (central Argentina), the former low-external input farming was replaced by a model based on genetically modified crops and the intensive use of pesticides (mainly glyphosate) in the last decades. Here, we analyzed changes in water chemistry (pH, conductivity, dissolved oxygen, nutrients, and carbonates) in 41 streams of Buenos Aires province between 2003/04 and 2015/16, and the impact of geology, soil type and land use change on water chemistry. We also used the SPARROW model to further analyze possible drivers of stream phosphorus loads. We observed modifications at reach scale in several streams, including changes in channel morphology, riparian vegetation and land use, and a moderate expansion of agriculture in most basins. Mean nitrate concentration did not significantly change between 2003/04 and 2015/16, and it was related to cropland cover in the basins. Dissolved phosphorus concentration increased in the streams, especially in the southern fluvial regions, but contrary to our expectations, phosphorus levels were not associated to land use but to pH. The SPARROW model also supported the link between water acidification and phosphorus concentration, and indicated that the whole basin acts as a phosphorus source. Possible explanations of fluvial acidification may be related to current agricultural practices, including higher inputs of labile carbon from croplands, soil acidification by nitrogenous fertilizers, and the generalized use of glyphosate. This highlights the necessity of adopting new agricultural paradigms to reduce the use of agrochemicals in Pampean basins.