Nitrogen from soy and maize agriculture influences nutrient export and stream productivity in headwater streams of the Amazon basin
Deforestation and agricultural intensification are rapidly changing the watersheds of southeastern Amazonia. With international pressure to reduce deforestation, many Brazilian farms have opted to intensify agriculture to a system of soybean-maize double cropping. As a result, this land use type has expanded rapidly in the last 10 years. Maize agriculture requires additional nitrogen (N) fertilizers, whose downstream fate is currently unknown. Typical fertilizer application rates range from 30-120 kg N ha-1, and have the potential to introduce large amounts of N to streams of the Amazon basin, which can alter stream biogeochemistry, nutrient transport, and productivity. Little data on nutrient export are available in the tropics in general, especially in this critical region of agricultural expansion on deep, very old soils. Therefore, we asked: 1) Has the annual export of inorganic N increased in watersheds dominated by soybean or soybean-maize agriculture? and 2) What are the implications for in-stream productivity? We worked in nine small watersheds on Tanguro Ranch draining forest, soybeans, and soybean-maize agriculture in the upper reaches of the Xingu Basin in central Mato Grosso, Brazil. We measured annual N and P export from streams draining forest, soybean and soybean-maize dominated watersheds and estimated in-stream productivity using measured changes in diel oxygen concentrations.
Annual export of N increased 4-fold in agricultural watersheds compared to forested watersheds. The increase in N export was due to a 4-fold increase in discharge. The export of P increased only 2-fold, suggesting that this land use shift may increase the uptake and retention of P in these streams. Early results suggest that both stream productivity and respiration increase in agriculturally dominated watersheds. The implications downstream are unknown but important to consider given the rapid expansion of soybean and maize agriculture in this region, and the reliance of many downstream Xingu communities on streams for drinking water and food. Ongoing work using nutrient pulse addition techniques is evaluating how far fertilizer N may travel once it reaches streams.