Objective  The paper aims to study the characteristics of phosphorus (P) forms in irrigation silting soils in the rotation of paddy rice and maize, and in order to provide the scientific basis for crop fertilization regulating on soil P availability in the Yellow River Irrigation Region in Ningxia.

  Method  In the paddy rice and maize rotation system, 7 fertilizer treatments were designed as: no nitrogen (N) application (T0), conventional fertilization (T1), optimized N application (T2), 70% optimized N application + 30% organic N application (T3), 80% optimized N application + 20% organic N application (T4), straw incorporation + 70% optimized N application + 30% organic N application (T5), and straw incorporation + 80% optimized N application + 20% organic N application (T6). A location field plot experiment was carried out to investigate the effects of different fertilization methods on total P, available P (Olsen-P) and inorganic P contents in irrigation silting soils.

  Result  During the rotation of paddy rice and maize, there were no significant effects on soil total P contents in each crop season and available P contents in dry land maize under different fertilization methods. However, topsoil available P contents were improved by 56.4 mg kg⁻¹ and 40.4 mg kg⁻¹ with straw incorporation and meanwhile combination application of chemical and organic fertilizers (treatments T5, T6) in paddy rice and maize, respectively. Transforming paddy field into dry land, soil available P contents were decreased by 7.6-18.1 mg kg⁻¹ under different fertilization methods. There were significantly influences on inorganic P (except Al-P) under different fertilization methods in paddy field. Al-P, Fe-P, and Obs-P contents were significantly affected by different fertilization methods in the dry land, but that no significant effects on Ex-P and Ca-P contents. Even in paddy field or dry land condition, Obs-P and Ca-P were as the main inorganic P forms in irrigation silting soils.

  Conclusion  Therefore, organic and chemical fertilizers combined with straw incorporation is the most effective method of regulating soil P availability in irrigation silting soils, and it is the significant regulation method in the paddy rice season in the flood and drought rotation system in the Yellow River Irrigation Region in Ningxia.