Effects of calcium biphosphate(MAP)and potassium chloride(KCl)on transformation of ammonium sulfate (AS)in the paddy soil were investigated through laboratory incubation.The results show that MAP can only affect transformation of AS to a limited extent.The increase in nitrification in the soil without ammonium applied and decrease in nitrification in the soil with application of MAP might be associated with change in soil pH.The application of KCl significantly affected transformation of AS in the paddy soil,mainly in the following aspects:(1)The increase in water soluble ammonium(WSA)and decrease in exchangeable ammonium(EA)as a result of the addition of KCl at the initial stage of incubation were attributed to replacement of K⁺ and NH₄⁺ in the soil,but the increase of both WSA and EA at a later stage was related to the inhibitive effect of K on nitrification.(2)The presence of potassium significantly suppressed release of fixed ammonium during incubation or extraction of the soil with NaCl.The different effects of K on WSA in the fresh soil and ai-rdried soil were also attributed to K inhibit ing ammonium release during air-drying course.(3)When potassium and ammonium were applied together into the soil,potassium increased ammonium fixation in the soil.(4)Nitrification of ammonium in the paddy soil was significantly suppressed by KCl, which might not result directly from K inhibiting release of ammonium under the controlled condition.Application of potassium alone might decrease availability of inorganic nitrogen in the paddy soil without N application.Thus it can be inferred that combined application of potassium and ammonium will not only reduce leaching and denitrification of nitrite,but also increase buffer capacity of soil ammonium.The results indicate that the interactions between NH₄⁺ and K⁺ in the soil may be one of the reasons why combined application of potassium with nitrogen fert ilizers could increase efficiency of nitrogen fertilizer in production.