Agricultural nitrogen loss and downstream effects in the transboundary La Plata basin driven by soybean rotations

The issue of substantial fertilizer application in soybean fields in South America has led to a potential nitrogen (N) imbalance in croplands, posing a risk to downstream ecosystem stability. A critical strategy for mitigating this risk requires a detailed examination focusing on N surplus and downstream consequences in soybean-intensive watersheds. This study comprehensively assesses N dynamics and downstream effects in areas with soybean rotations across the transboundary La Plata basin in South America. We estimated N surplus and potential N loss through leaching and soil erosion in both established and recently converted soybean rotation fields and analyzed the impact of N surplus reduction scenarios on river N concentration variations. Results indicated that fertilizer N inputs increased by 38 % more than non-fertilizer N inputs from 2001 to 2016, despite biological N fixation contributing 45 % of total N inputs. N surplus increased by 19 % during this period, resulting in high-potential N loss across 31 % of soybean rotation fields. It was estimated that a 20 % reduction in N surplus could decrease total N concentration by 17 % ± 11 % and nitrate concentration by 16 % ± 10 % in soybean-intensive watersheds. Reducing N fertilizer inputs in soybean rotation fields, especially in Brazilian and Uruguayan La Plata, is a promising strategy for mitigating N pollution without significantly impacting soybean production. Our findings revealed widespread excessive fertilizer N inputs across the basin, contributing to strategy development for N pollution mitigation and underscoring the need for cross-national collaboration in N management to mitigate water pollution and ensure agricultural sustainability.