Half-Century History of Crop Nitrogen Budget in the Conterminous United States: Variations Over Time, Space and Crop Types

Abstract

Spatiotemporal patterns of crop nitrogen (N) budget have important implications for agricultural N management and environmental policy. Previous studies examined crop N budget in different countries but often overlooked cross-crop differences at sub-national scales. In this study, we synthesize multiple databases to examine the N budget of eight major crops in the United States at the county scale during 1970–2019. Our analyses show that national crop N use efficiency (NUE) increased from 0.55 kg N kg−1 N in the 1970s to 0.65 kg N kg−1 N in the 2010s. Four out of eight crops such as corn, rice, cotton, and sorghum demonstrated an increasing NUE trend during the study period, whereas the other crops overall presented a declining NUE trend. Nationwide, about 41% of the total N input was not used by these crops (i.e., N surplus) over the study period, of which temporal variation was mainly driven by corn due to its large planting area and high N input. The national N surplus first increased in the 1970s and remained relatively stable till the 2000s. Since the early 2010s, however, N surplus began to decline and approached the levels in the early 1970s—an encouraging development that may lead to decreased N pollution to the environment. The hotspots of national N surplus coincided with corn- and rice-producing counties. The sub-national variations and temporal dynamics in crop N budget revealed in this study highlight the urgent need to understand the farm-level crop N balance and the dominant factors controlling crop NUE for mitigating N pollution.