An analysis of BMPs and their combined effectiveness at reducing nitrate-nitrogen export to the Black Hawk Lake Watershed, Iowa

Subsurface tile drainage systems have transformed Iowa’s prairie pothole ecosystem, enabling the United States Corn Belt to become one of the most agriculturally productive areas in the world; however, an unintended consequence has been increased nitrogen losses to surface waters. The literature contains numerous field and plot studies focusing on a single practice, but catchment-scale studies of multi-process best management practices (BMPs) are lacking, which mirrors the rarity of these alternative management practices on the landscape. Nitrate losses were monitored at five sites, two surface, two tile outlets, and a grass waterway, in three subwatersheds of Iowa’s Black Hawk Lake Watershed over a 3 yr. period. The objectives were to quantify NO3-N export patterns at the subwatershed scale and discuss relationships between precipitation, flow, BMP implementation, and NO3-N losses to better inform water quality improvement goals. Subwatersheds ranged in size (221.2 - 882.5 ha) and have varying levels of BMP implementation (22.5 - 87.5 % aerial extent). BMPs include grass waterways, native perennial CRP vegetation, nutrient management plans, reduced tillage, terraces and cover crops. Subwatersheds with low BMP implementation had NO3-N load estimates of 52-170 kg ha-1 yr-1 while sites with high BMP implementation experienced unit-area NO3-N load estimates of and 26-128 kg ha-1 yr-1. Overall, water yields were in range within Iowa and the upper Midwest, but associated NO3-N concentrations and losses were among the highest reported, in part due to an extremely high precipitation year in 2015. Over the study period, high BMP sites observed 64% less NO3-N export compared to low BMP subwatersheds, and experienced lower mean NO3-N concentrations for all years of the study. A consistent result for all subwatersheds was high losses early in the year. With variable spring weather patterns which favor more frequent and more intense storms early in the growing season, management approaches need to be designed to address early season NO3-N losses associated with higher flows