Nitrogen Balance in a Highly Fertilized Rice–Wheat Double‐Cropping System in Southern China

During the past two decades, large amounts of chemical N fertilizers have been applied in rice ( Oryza sativa L.)–wheat ( Triticum aestivum L.) double‐cropping systems in the Taihu Lake region of southern China to achieve high yield. To understand the N balance and environmental impact, a 3‐yr field experiment was conducted to determine the integrated N input and output in a rice–wheat rotation system in this region. The results indicated little N surplus in the soil despite different patterns of N input and output in rice and wheat seasons. Although total N input was higher for rice than wheat, the output was also proportionately higher so that the balance was similar between the two crops. Crop harvest removed 48% of N input for both rice and wheat seasons. Denitrification and NH 3 volatilization were the primary paths for N loss (22% each of total output) in the rice season, while runoff and leaching (22%) and denitrification (21%) were the main paths for N loss in the wheat season. Although the total annual N input reached up to 606 kg N ha −1 , the annual N output was as high as 599 kg N ha −1 , of which 52% was lost into the environment, leaving little N in the soil. These results demonstrated that the overwhelming majority of N input did not remain in the soil but rather was quickly emitted. Proper practices are imperative to optimize the N balance and minimize N loss.