Climate impact and food benefit of nitrogen fertilizer use in China

Background/Question/Methods

To meet increasing food demands for feeding 22% of world’s population, China has long used nitrogen (N) fertilizer as one of the most important agronomic practices to stimulate crop production. During 2007-2008, China ranked as the top consumer of nitrogen fertilizer, accounting for 32% of global N consumption. Per-hectare anthropogenic N additions in many fields of China, especially for typical double-, triple-cropping systems, surpassed those in the United States and Northern Europe. The concurrent effects of N fertilizer application on crop yield, soil C sequestration and biogenic N₂O emissions across the entire nation are not well studied. Here we use a carbon-nitrogen (C-N) coupled, processes-based ecosystem model, driven by spatially-explicit data on N fertilizer uses and other environmental factors including climate, atmospheric CO₂, tropospheric O₃, N deposition, land use and land cover change to quantify the impacts of agronomic N addition on crop yield, soil carbon (C) sequestration, and nitrous oxide (N₂O) emission in China’s cropland during 1949-2008.

Results/Conclusions

Our simulations showed that N fertilizer-induced crop production and soil C sequestration reached their peaks in the 1990s, implying the failure of surplus N in further stimulating crop production and C storage in cropland’s soils. Nevertheless, N₂O emission was found to continue rising as N fertilizer application increased. Over the recent 9 years (2000-2008), adverse effect of excessive N fertilizer uses on global climate warming through N₂O emission was estimated to more than outweigh their climate benefit in increasing CO₂ uptake. Our study reveals that net warming effect of N fertilizer uses mainly centered in the Northern China Plain and in the middle and lower reaches of Yangtze River Basin, with N₂O emission completely counteracting or even exceeding, by more than a factor of 2, the CO₂ sink during the recent 9 years. Future efforts must curb the N₂O emission from agronomic practices for climate stability and put more emphasis on enhancing N use efficiency, rather than increasing N input into crop fields.