Nitrogen use efficiencies in Chinese agricultural systems and implications for food security and environmental protection

Nitrogen (N) use efficiency (NUE) is an important indicator to evaluate the performance of N use in agricultural systems. Understanding changes in NUE is crucial to improve N management and reduce harmful environmental impacts. Here we analyzed the historical changes in NUE in three agricultural subsystems in China: cropland, confined livestock feedlot and grassland with grazing animals, as well as their impacts on food security and the environments using the mass balance approach. The NUE in croplands increased from 40% in 1980 to 45% in 1984 and then decreased to 36% in 2003, after which it rebound to 39% in 2010, explained by the interaction of socioeconomic development and natural factors. The livestock and grazing/grassland subsystems saw continuous improvement of NUE from 6 to 16 and 1 to 5%, respectively, associated with the increase in average livestock farm size and related technological innovations and livestock specie changes between 1980 and 2010. The low agricultural NUE has resulted in the loss of 18.4-44.8 Tg N year(-1) to the atmosphere or hydrosphere, or accumulation in the soil as reactive N in Chinese agriculture systems between 1980 and 2010. Despite the large amount of N used and lost during food production, the overall protein self-sufficiency ratio decreased from over 90% in the 1980s to 79% in 2010. Future agricultural development in China faces multiple challenges on increasing the productivity and reducing the N loss, for which increasing the NUE in all three agricultural subsystems should be a priority.