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

• Published in: Water research (Oxford) 2019-11, Vol.165, p.114977, Article 114977
• Main Authors: Xin, Jia, Liu, Yang, Chen, Fei, Duan, Yijun, Wei, Guanli, Zheng, Xilai, Li, Miao
• Format: Article
• Language: English
• Subjects: Dissolved organic nitrogen; Groundwater; Nitrate leaching; Nitrogen; Nonagricultural source; Vadose zone
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2019.114977

Intensive agriculture and urbanization have led to the excessive and repeated input of nitrogen (N) into soil and further increased the amount of nitrate (NO3−) 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. Three major missing N pieces (N in dissolved organic form, N in the deep vadose zone, and N in the nonagricultural system), which are crucial for closing the N cycle yet has been previously assumed to be insignificant, are put forward and discussed. More work is anticipated to obtain accurate information on the chemical composition, transformation mechanism, and leaching flux of these missing N pieces in the vadose zone-groundwater system. These are essential to support the assessment of global N stocks and management of N contamination risks. [Display omitted] •The review focuses on N cycle in the vadose zone-groundwater system.•N distribution levels, transformation activity and leaching flux are collected.•Methodologies of N analysis, N transformation and N budget analysis are summarized.•Three missing N pieces and future research needs are highlighted.