Abstract: In an agricultural ecosystem,nitrogen is one of three essential nutrients required by crops.Soluble organic nitrogen(SON),as a form of soil nitrogen,plays an important role in the nitrogen cycle in an agri-ecosystem.This paper summarizes the recent research results on SON worldwide,as well as the sources,components,extraction methods,contents, factors, seasonal variations, and ecological functions of SON in the ecosystems.Recommendations for future studies and theoretical considerations relating to SON are presented.