Periphytic biofilms are commonly presented at the water-soil interface in paddy fields.Different fertilization methods can affect the concentration and distribution of nutrients in paddy fields and thus affect the development of periphytic biofilms.In this study,the roles of periphytic biofilms in nitrogen (N)cycling in paddy systems and how they are affected by different fertilization methods were studied using microcosm experiments.Microcosms were prepared using soil samples from a paddy field and treated with surface and deep fertilization under light and dark conditions.Surface fertilization under light condition promoted the development of periphytic biofilms,while deep fertilization under dark condition inhibited their development.The development of periphytic biofilms increased the pH and dissolved oxygen levels in the overlying water.Surface fertilization resulted in high N concentrations in the overlying water and the topsoil layers,which enhanced NH3 volatilization and nitrification-denitrification but inhibited N fixation.The development of periphytic biofilms reduced NH3 volatilization loss but increased nitrification-denitrification loss and the overall N loss in the paddy system.The results from this work suggest that the presence of periphytic biofilms in paddy fields could increase N loss by 3.10％-7.11％.Deep fertilization is an effective method to retard the development of periphytic biofilms in the paddy system and can potentially increase the overall N use efficiency.