The second-largest zone of coastal hypoxia in the world is found on the northern Gulf of Mexico continental shelf adjacent to the outflows of the Mississippi and Atchafalaya Rivers. Studies performed in the late 90s indicated that hypoxic conditions likely began to appear around the turn of the last century and became more severe since the 1950s as the nitrate flux from the Mississippi River to the Gulf of Mexico tripled. Although the hypoxia formation in the Gulf of Mexico is predominantly driven by increased riverine nitrogen (N) export from the Mississippi-Atchafalaya River basin, it remains unclear how hydroclimate extremes affect downstream N loads.
We analyzed the flow-weighted nitrate-N concentration (FWNC; mg NO3--N L-1), load (kg NO3--N y-1), and yields (kg NO3--N ha-1 maize and soybeans cropland y-1) for 44 watersheds in Iowa, USA over the last two decades and estimated the probability of measuring non-weather related 41% reductions in nitrate losses. Our objectives were: 1) to identify the magnitude and spatial variability of three nitrate levels, across 44 watersheds during the period from 2001 to 2018; 2) measure the probability of measuring a 41% real reduction; and 3) explain which main factors are explaining these reductions, and the time-periods needed to achieve these goals.
We found that reductions in FWNC over 15 years resulting from changes in land use and management exceeded 59% in all watersheds. In contrast, over the same timeframe, the mean probability across all watersheds of measuring 41% reductions in load and yield from changes in land use and management were only 49%, and 47%. Weather, land-uses, and soils explained 90% of the cross-watershed variability in FWNC. Overall, this study brings new data and analysis to assist decision-making in monitoring nitrogen in Iowa
