Abstract: Three paddy soils with higher content of fixed ammonium in Hunan Province were used as test soils in pot experiment to study the seasonal variation of their fixed ammonium content and its bioavailability during rice growth period.The results showed that the fixed ammonium content in the paddy soils changed constantly during rice growth.Applying Nfertilizer and organic manure increased the fixed ammonium content, which was decreased through the Nuptake by rice plants.The recently fixed ammonium was almost fully available to rice, while the native fixed ammonium was partly available.The release of fixed ammonium during rice growth was newly alluvial sandy soil > purple clayey soil> alluvial sandy soil, and was greater at early growth stage than at later growth stage.