Tracking the fate of nitrogen fertilizers in intensively cropped watersheds in southeastern Amazonia
The surface waters of the southeastern Brazilian Amazon face unprecedented pressures from the expansion and intensification of croplands. Agricultural production in the region has changed dramatically, shifting from cattle production for domestic consumption to intensive commodities production for export to global markets. That shift has been accompanied by several waves of land-use change. Large-scale deforestation in the region began in the 1980s, driven by cattle ranching and land speculation, and spiked again as global demand for soybeans grew in the early 2000s. As pressure to reduce deforestation has mounted, farmers in the region have intensified production on already-cleared land, producing more cattle per hectare and multiple crops per growing season. Intensification of this scale could trigger large changes to the nitrogen (N) cycle, as observed in agricultural regions in the temperate zone. Increased fertilizer use and biological N fixation could increase leaching of N from farm fields to surface waters and change solute chemistry. However, differences in fertilizer regimes in the tropics – together with the deep, well-drained soils that occur in the region– could slow land-water nutrient transport and cause different responses than in temperate regions. We compare solute chemistry (soilwater, groundwater, and streamwater) in watersheds draining forests, single-cropped soybeans (no N fertilizer), and double-cropped soybean-maize (N fertilizer). Data were collected in nine first and second order streams over two years at Fazenda Tanguro, a large (80,000 ha) farm in Mato Grosso, Brazil.
Headwater streams in single-cropped soybean watersheds showed no increase in N concentrations compared to forested streams of similar size. Groundwater samples indicated some increase (though not significant) in soybean watersheds compared with forest watersheds. Preliminary results from double-cropped soybean-maize watersheds indicate that dissolved N is significantly higher in groundwater samples in double-cropped watersheds, but not in streamwater. The delta 18O composition of potential water sources to streamflow indicate that these streams are almost exclusively dominated by groundwater inputs. This suggests that modification of solute concentrations during long-distance transport through groundwater is the primary control on land-water solute delivery, and that N retention in deep soil and groundwater flowpaths is critical to understanding solute export and response to agricultural fertilizer inputs at the watershed scale. Ongoing work is focused on quantifying these pathways through measuring soilwater solute concentrations in deep soil pits, as well as dissolved N2 concentrations in groundwater and streamwater.