The stationary expenment was conducted for 12 years at Hunan Agricultural University to investigate the effects of rice-based
cropping system,
organic manure and grotmdwater level on
phosphate sorption by paddy soils developed on red earth,in the treatments of 3 types of rice-based cropping systems, 3 application rates of organic manure and 2 levels of groundwater table. Results showed that the phosphate adsorption isotherms for soils in all the treatments are quite similar in shape, belonging to curve in the shape of "L" with high affinity. The adsorption data all fit in well with the three adsorption equations at a very significant level of 1.0 % ,but the One-surface Langmuir equation is better than Freundlich and Temkin equations in this respect. In comparing the three rice-based cropping systems, the soil treated with "early rice-late rice-flooded fallow in winter" has a much larger phosphate sorption capacity than soils treated with "early rice-late rice-green manure crop in winter" or"early rice-late rice-rapeseed in winter". Both applying organic manure and raising the groundwater level significantly increase the phosphate sorption capacity of the soils. After removing free Fe oxides and amorphous Fe oxides from the soil， the phosphate sorption capacity of the soil decreases markedly. The content of amorphous Fe oxides in soil is positively correlated with the decrease in the phosphate sorption capacity of the soil after amorphous Fe oxides are removed and with the Qm value in One-surface Langmuir equation, at a significance level of5.0％ and 1.0％, respectively. The effect of rice-based cropping system, organic manure and groundwater level on phosphate sorption by the tested soils is brought about mainly by influencing the activity of free iron oxides in soils. The Qm value in One-surface Langmuir equation is negatively correlated with the content of Olsen-available P, at a significance level of 1.0％.