Opportunities for water quality improvements in a Mississippi River Basin watershed: Hotspots for agricultural conservation practices.

Excess nutrients in aquatic systems can cause hypoxia, eutrophication, and algal blooms resulting in substantial modification and losses of ecosystem services. Given limited resources, approaches are needed to address water quality issues efficiently and effectively. Prioritized conservation efforts aim to concentrate resources on areas with the greatest impact and have been shown to be effective in mitigating agricultural non-point source nutrients reaching receiving waters. Here we propose a method leveraging hydrologic and biogeochemical modeling paired with soil and terrain analysis to characterize and select the most suitable fields and conservation practices to acheive nitrogen load mitigation. Our approach prioritizes agricultural fields by taking advantage of the spatial distribution of agricultural nitrogen use efficiency and surplus nitrogen to identify croplands that are likely contributing disproportionately high nitrate loads to river water. We assessed cover cropping, conservation tillage, and cropland-to-grassland restoration as within-field conservation practices to enhance nitrogen retention, targeting areas where the greatest gains toward water quality restoration goals could be achieved. Fields under cover crop and cropland-to-grassland conversion generally increased their nitrogen use efficiency, whereas implementation of conservation tillage showed more mixed responses in different fields and parts of the subbasin. Implementing cover crops on the 35.5 % of cropland (30.1 % of total land area) prioritized for the practice within the watershed reduced nitrate load at the subbasins outlet by 15.5 % (11,399 tons nitrate). Cropland-to-grassland restoration (prioritized on 6.3 % of cropland; 5.3 % coverage of the subbasins) reduced 5.2 % of the total nitrate load. Implementing conservation tillage on 18.7 % of cropland (15.8 % of subbasin) produced the lowest improvement in water quality with 1.8 % reduction in the nitrate load. The observed shifts were largely attributed to changes in the nitrate loss pathways. Cover crop and cropland-to-grassland implementation also influenced subbasins' water flow volume by decreasing the outlet daily discharge, by 28.6 % and 3.7 % respectively. Conservation tillage, however, increased the mean daily discharge by 9.5 %. This analysis effectively combines estimates loads and surpluses across the watershed to pinpoint areas where nitrogen management is most and least efficient, providing land management, policy, and investment decision makers with a clear platform to identify potential water quality issues and sites for effective intervention.