In recent years,nitric oxide(NO) has been receiving more attention due to its important roles in biogeochemical cycling,atmospheric chemistry and global warming.Forest soils are among the major sources for atmospheric NO nitrification,denitrification,nitrifier denitrification and chemodenitrification are the four dominant processes for the production of NO from forest soils.However,the relative contributions of these sources to NO production,as well as the relationship between its production and the biotic(e.g.functional genes) and abiotic factors are still not well understood.Here,we reviewed recent literatures to fully understand each of the main processes producing NO in forest soils and the methods for measuring the contribution of each source to the NO production as well as the biotic(e.g.functional genes) and abiotic factors influencing its emission to the atmosphere.Moreover,the weaknesses of the present studies are pointed out,and the future research directions are provided.Overall,nitrification and denitrification are the major processes for NO production,but chemodenitrification in acidic forest soil cannot be ignored.The application of dual isotopic approaches(~(15)N and ~(18)O) may provide a key to better quantify the contributions of these processes to the emission of NO in forest soils.The production of NO involves complex biotic and abiotic processes.Up to date,few studies have focused on the relationship between the NO derived from different sources and the functional genes involved in nitrogen cycling.More attention has been paid to the effects of individual environmental factor on the production of NO via nitrification and denitrification,whereas studies about the effects of environmental factors on the NO production via nitrifier denitrification and chemodenitrification are rare.Moreover,little information is avaibale about the comprehensive effects of environmental factors on NO production.