Review of soil dissolved organic nitrogen cycling: Implication for groundwater nitrogen contamination

Dissolved organic nitrogen (DON) in groundwater is derived from soil DON transformation and migration processes, which has been considered an emerging nitrogen (N) pollutant. However, due to the limitations of the analytical methods and the complexity of the involved transformation process, the role of DON in soil N cycling remains unclear. Therefore, this review aims to critically examine previous related studies on DON and highlight the knowledge gaps related to DON transformations and molecular characteristics in soils. In addition, the DON distributions and key transformation processes, as well as their influencing factors, were summarized. About 60% of DON components have not been determined due to the limited analytical techniques and methodologies. The depolymerization process of polymers into DON is the rate-limiting step of N mineralization. Furthermore, DON leaching amounts accounted for 7-1500% of soil nitrate (NO3- -N) amounts, becoming the dominate pathway of N loss. Further studies are required to provide accurate information on DON compositions and transformation mechanisms, as well as their influencing factors, in soils. The suggested studies can provide further insights into the role of DON in soil N cycling, thereby controlling effectively groundwater N contamination.