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

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.

Automated summary

The nitrogen use efficiency (NUE) of eight major crops in the United States has increased from the 1970s to the 2010s, with a significant increase in NUE for crops like corn, rice, cotton, and sorghum. However, there has been an overall decline in NUE for the other crops. The national nitrogen surplus, which was mainly driven by corn, has shown a decreasing trend since the early 2010s, potentially leading to reduced nitrogen pollution.