Field experiments on dynamics and availability of soil fixed ammonium in rice-duck ecosystems and its influencing factors were conducted from May to September in 2007. The results showed that during rice growth fixed ammonium changed constantly in paddy soils, and application of fertilizer could promote fixation of ammonium while rice growth boosted the release of fixed ammonium. Further, during rice growth, recently fixed ammonium was almost completely released, while native fixed ammonium was not released. Compared to conventional rice field (CK), rice-duck complex ecosystem (RD) could significantly increase fixed ammonium content; moreover, the mean content of fixed ammonium in RD was (103.25±19.15) mg/kg, 7% higher than that in CK. Close correlations existed between soil exchangeable NH+4, pH and fixed ammonium at the significant differences of 1% level. Soil temperature (at 5cm depth) was not associated with soil fixed ammonium. Soil fixed ammonium was positively related to >0.2mm sand at the significant differences of 5% and negatively related to <0002mm clay at the significant differences of 1% level, and not associated with 0.02-0.2mm and 0.002-0.02mm particles.