The missing nitrogen pieces: A critical review on the distribution, transformation, and budget of nitrogen in the vadose zone-groundwater system.

Intensive agriculture and urbanization have led to the excessive and repeated input of nitrogen (N) into soil and further increased the amount of nitrate (NO) leaching into groundwater, which has become an environmental problem of widespread concern. This review critically examines both the recent advances and remaining knowledge gaps with respect to the N cycle in the vadose zone-groundwater system. The key aspects regarding the N distribution, transformation, and budget in this system are summarized.