Are we getting better in using nitrogen?: Variations in nitrogen use efficiency of two cereal crops across the United States

Authors: LU, CHAOQUN - Iowa State University; ZHANG, JIEN - Iowa State University; CAO, PEIYU - Iowa State University; Hatfield, Jerry

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2019
Publication Date: 7/25/2019
Citation: Lu, C., Zhang, J., Cao, P., Hatfield, J.L. 2019. Are we getting better in using nitrogen?: Variations in nitrogen use efficiency of two cereal crops across the United States. Proceedings of the National Academy of Sciences. https://doi.org/10.1029/2019EF001155.
DOI: https://doi.org/10.1029/2019EF001155

Interpretive Summary: Crops are dependent upon fertilizer inputs to obtain maximum productivity; however, excess nutrient application results in environmental damage. A study was undertaken to evaluate if there has been a change in the efficiency of nitrogen use by corn and wheat across the United States. As the fertilizer rate increases above 150 kg/ha, the efficiency in the use of nitrogen decreases and shows more variation among years and locations. This effect was evident for both corn and wheat with the fertilizer rate being 50 kg/ha. To achieve greater nutrient use efficiency will require more efficient management of our nutrient resources to supply these when the crop is able to effectively use the nutrients. This research will help scientists, agricultural consultants, and fertilizer dealers understand where improvements can be made in nutrient use in corn and wheat.

Technical Abstract: Spatial variation and temporal trajectory of crop nitrogen use efficiency (NUE) has important implications for targeted nitrogen management and environmental conservation. Previous studies have examined cross-nation divergences in crop NUE but often overlooked its spatial heterogeneity and cross-crop differences at sub-national scales. We examined the relationship between state-level NUE and nitrogen fertilizer uses for two major fertilizer-consuming crops, corn and winter wheat, which account for over half of national fertilizer uses in the United States. Since 1970, as N fertilizer rates change, the responses of crop yield and NUE exhibit large variations, both spatially and temporally. It is evident that NUE of corn begins to decline when N fertilizer application rate exceeds ~150 kg N ha-1, and that yield response of winter wheat slows down with annual N fertilizer input above ~50 kg N ha-1. State-level NUE in both crops has been raised in recent decades, which potentially reduces N loss from agricultural production. Across the U.S., some major corn-producing states demonstrate a shift from an increasing trend of NUE during the period 1970 to 1999 to a decreasing trend after 2000, whereas winter wheat-producing states present an opposite pattern. Furthermore, this study indicates that annual dynamics of N surplus in corn are closely tied with grain yields, while that in winter wheat significantly correlates with N fertilizer input. A larger proportion of N loss would be anticipated if no further increase in corn yield was obtained or fertilizer use kept rising in winter wheat.