Nitrogen agronomic efficiency under nitrogen fertilization does not change over time in the long term: Evidence from 477 global studies

Nitrogen agronomic efficiency (NAE) is crucial for global food security due to its important role in determining crop yields. However, we do not know whether NAE under nitrogen (N) application changes in the long term, and what controls these temporal dynamics. These knowledge gaps could result in high uncertainty in predicting the increase in global crop yields due to N fertilization in the future, threatening global food security. Here, we synthesized 477 N application studies with duration ≥ 5 years in croplands around the globe. Mean long term NAE was 20.8 kg kg-1, and N application rate, crop type, and N fertilizer type were the most important factors affecting NAE. Overall, NAE did not significantly change in the long term at the global scale. However, a third of studies showed evidence of a consistent directional change in NAE over time (increase: 21 %, decrease: 12 %). Mean annual precipitation (MAP) mostly explained the variation in the temporal trend of NAE. More specifically, NAE tended to decrease over time with the increasing of MAP. To achieve a high and sustained NAE, N should be applied as NH4NO3 or NH4 form along with both phosphorus and potassium fertilizers. Our meta-analysis study indicates that, although no significant temporal trend of NAE was found at the global scale, we may still over- or underestimate crop yields under certain types of management, resulting in global food insecurity in the future.