Fixed Ammonium in Major Types of Paddy Soils in Hunan Province, China

The contents, affecting factors, seasonal changes and availability of fixed ammonium in major types of paddy soils derived from different parent materials in Hunan Province, China, were studied using the Silva-Bremner method by laboratory and pot experiments. Results showed that the content of fixed ammonium in the plough horizons ranged from 88.3 mg kg^-1 to 388.1 mg kg^-1, with 273.2 4±77.7 mg kg^-1 on average,accounting for 11.2% of total soil N on average. Content of fixed ammonium decreased in the order of newly lacustrine clayey paddy soil > alluvial sandy paddy soil > purple clayey paddy soft > newly alluvial sandy paddy soil > yellow clayey paddy soil > reddish-yellow clayey paddy soil > granitic sandy paddy soil. There were four distribution patterns of fixed Rmmonium in the profiles to 1-m depth, i.e., increase with the depth,decrease with increasing depth, no distinct change with the depth, and abrupt increase or decrease in some horizon. Percentage of fixed Rmmonium in total N increased with the depth in most of the soils. Fixation of NH4^+ by soil was higher at 30℃ than at 20℃ and 40℃, and continuous submergence benefited the fixation of NH4^+ in newly alluvial sandy paddy soil, purple clayey paddy soil and alluvial sandy paddy soil,while alternating wetting and drying contributed to the fixation of NH4^+ in yellow clayey paddy soil mostly.Fixed Rmmonium content in the test paddy soils was significantly correlated with < 0.01 mm clay content(P < 0.05), but not with < 0.001 mm clay content, total N, organic N and orgvaiic matter. Fixed ammonium content varied with rice growth stages. Application of N fertilizer promoted fixation of NH4^+ by soil, and N uptake by rice plant promoted release of fixed Ammonium from the soil. Recently fixed Ammoniuin in paddy soil after N fertilizer application was nearly 100% awailable to rice plant, while native fixed ammonium was only partly available, varying with the soil type and rice type.