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Highlights

Corn yields increased from 1989 to 2021 in Iowa experimental plots at nitrogen fertilizer rates consistently just above current recommendations.

Abstract. Aquatic problems from the export of nutrients, especially nitrate, from row crops are recalcitrant in the Mississippi-Atchafalaya River Basin and globally severe. Previous studies have proposed to reduce these problems in part by improving crop yields, particularly corn, leaving less nitrate surplus to export. Simultaneous increases in fertilizer application rates and grain yields in recent decades have made testing this notion with large-scale agricultural statistics difficult. This experiment in Iowa featured a corn-soybean rotation with corn fertilized with nitrogen at a nearly consistent rate from 1989 to 2021. Corn yields increased at a rate not statistically distinguishable from the surrounding county‘s (144 vs. 148 kg ha-1 yr-1), but drainage nitrate concentration and loading remained flat overall, oscillating with precipitation. Results suggest that increasing corn yield, and thereby partial factor productivity, with standard shifts in cultivars over time, cannot alone solve the U.S. Corn Belt‘s nitrate surplus problem, supporting previous recommendations for active and multi-layered conservation efforts. Five- to ten-year positive and negative sub-trends in nitrate export within the longer dataset reaffirm the importance of truly long-term experiments and monitoring to accurately assess the impacts of management.