Changes in Terrestrial N Budgets and Riverine Nitrate-N Yields from Mississippi River Basin Watersheds 2000 to 2020

The Mississippi–Atchafalaya River Basin (MARB) drains approximately 40% of the continental United States and discharges into the Northern Gulf of Mexico, where a large area of benthic hypoxia forms in late summer, largely in response to N loads from the MARB. Trend analysis revealed changes in nitrate-N (NO 3 -N) loads in MARB subbasins from 2000 to 2020, although causes are uncertain. Net anthropogenic N inputs (NANI), the net of estimated N inputs and outputs from human activities, has been correlated with riverine N loads in many regions worldwide. Here, we incorporate reported changes in crop N content and biological N fixation into NANI estimates and investigate correlations with flow-normalized (FN) riverine N yields from 217 watersheds in the MARB. Reductions in crop N content in corn and soybean influenced values of NANI but had little influence on NANI correlations with riverine FN NO 3 -N, which generally ranged from 0.5 to 0.7. In the late 1990s and early 2000s, NANI values and FN NO 3 -N yields were generally high for watersheds with a high percentage of cultivated cropland and tile drainage; by 2020, both NANI and FN NO 3 -N yields from these watersheds had declined. The magnitude of the decline in FN NO 3 -N yields from watersheds with greater than 20% tile drainage averaged 552 kg N km −2 year −1 . In these watersheds, the largest reductions in NANI were due to greater N harvest in maize and wheat with little change in N fertilizer, and a reduction in atmospheric N deposition.