Abstract: In soil,nitrogen mostly exists in the form of organic polymers,which can transform into soluble organic nitrogen(SON)through a series of complex biological and non-biological processes.In this paper,SON refers to the organic nitrogen extracted by water or salt solution,rather than the dissolved organic nitrogen(DON)collected in situ.Currently,the research on SON largely focuses on forest-,grassland-and/or agro-ecosystems.There are differences in the content and composition of the soil SON in different ecosystems that vary in soil type,land use,vegetation,soil management and/or environmental factors.A great deal of the information obtained thus far indicates a correlation between SON content and the quantities and components of the microorganisms in soil,the activities of extracellular enzymes,etc.The transformation process may include breakdown of the polymeric nitrogen by bacteria,fungi and/or extracellular depolymerases secreted by microbes in soilor plants into soluble subunits that can be utilized by microorganisms or plants.The soluble subunits may also be further degraded into ammonium(NH+4)and nitrate(NO-3),which are the preferred N source for bacteria,fungi and plants.At present,few study targets the microbes that are capable of degrading organic nitrogen into subunits,while only a few reported the degradation and absorption of organic nitrogen by mycorrhizal fungi.Thus,future studies ought to be directed toward the understanding of the regulatory mechanisms of soil organic nitrogen transformation.