Abstract: ObjectiveThe effects of long-term potassium(K)fertilizer application on yields in paddy-upland rotation system,distribution of soil aggregate,and available K content were studied to ascertain the distribution rules of K in the soil aggregate components in paddy-upland rotation system,and then to provide theoretical basis for soil structure im-provement,potassium pool management and reasonable application of K fertilizer.MethodRice-winter oil rapeseed ro-tation experiments were carried out in Jianghan Plain,Hubei Province from 2011 to 2017. Two treatments namely,nitro-gen phosphorus fertilizer(application without K fertilizer,-K)and nitrogen,phosphorus and potassium fertilizer(applica-tion of K fertilizer,+K)were selected in this study to make comparative analysis on the effects of K fertilizer application on the yields in rice-winter oil rapeseed rotation system,available K content and soil aggregate distribution at different soil depths.ResultThe output in the crop rotation system was affected by both year and fertilization. Compared with the-K treatment,after application of+K treatment,the average annual yield increases of rice and winter oil rapeseed were 0.51 and 0.33 t/ha respectively,with the growing rates of 7.5% and 14.1%. The long-term application of K fertilizer(7 years)had no obvious effects on the vertical distribution of soil aggregates,but it could increase the water-soluble K con-tent and available K content at soil layers of 20-30 and 30-40 cm,and the available K content in each size fraction aggre-gates. In addition,the amplitude of available K balance in the 10-20 and 30-40 cm soil layers were the most obvious through calculating the surplus-deficit of available K in soil aggregates at different soil layers.ConclusionCompared with the initial available K content of farmland in 2011,the application rate of K fertilizer in 2017 will lead to a sustained and slow decline in the soil K content of the farmland. Therefore,it is suggested to pay attention to straw returning and the supplement of organic fertilizer to maintain the productivity of rotation system and K balance in farmland.