Abstract: Abstract: Nitrification inhibitor has been proved to be effective in retarding nitrification process of nitrogen in the soil. Application of nitrification inhibitors to the field is considered to be a major method in controlling nonpoint pollution induced by nitrogen fertilizer in agricultural production. Dicyandiamide and thiosulfate are two types of nitrification inhibitors widely used throughout the world. A large number of studies have been carried out on dicyandiamide and thiosulfate focusing on their inhibitory effects. Till now, very limited information is available on the comparative effects of dicyandiamide and thiosulfate on transformations of nitrogen in the soil. The aim of this experiment was to investigate the effects of nitrification inhibitor dicyandiamide, potassium thiosulfate and their combination on the changes of mineral nitrogen, nitrification process and nitrogen recovery in the soil. A forty-day incubation experiment was conducted under the laboratory conditions (20 ℃). The experiment included unfertilized control, soil appended with urea nitrogen of 200 mg/kg alone, soil appended with urea nitrogen and dicyandiamide of 40 mg/kg, soil appended with urea nitrogen and potassium thiosulfate of 64 mg/kg, and soil appended with urea nitrogen and two inhibitors. Soil moisture in different treatments remained 60 percent of field water capacity. During the experimental period, soil samples were collected dynamically and the contents of mineral nitrogen such as nitrate nitrogen, ammonium nitrogen and nitrite nitrogen were analyzed. The result showed that an active nitrification process was found in the soil appended with nitrogen without nitrification inhibitors, where 77.7 percent of NH4+-N in applied nitrogen disappeared from the mineral nitrogen pool during the whole incubation period of 40 d. It was estimated that 56.6 percent of applied nitrogen was accumulated as NO3--N. Addition of nitrification inhibitors dicyandiamide and potassium thiosulfate resulted in a decrease in the extent of NH4+-N disappearance by 74.1 percent and 16.6 percent, respectively. However, addition of dicyandiamide and potassium thiosulfate caused the increase in soil NH4+-N concentration. The nitrification process of nitrogen was inhibited in the soil appended with dicyandiamide and dicyandiamide combined with potassium thiosulfate, and the inhibition rates from 35.5 percent to 98.7 percent and from 82.2 percent to 103.5 percent were recorded, respectively. The delay of nitrification process was more than 20 days in two treatments. The nitrification rate in the soil appended with potassium thiosulfate was from 1.6 percent to 62.6 percent and the delay of nitrification process was 10 days. It was indicated that dicyandiamide induced a more pronounced inhibition on nitrogen nitrification as compared to potassium thiosulfate. Furthermore, a more efficient inhibition was observed for addition of dicyandiamide combined with potassium thiosulfate than that of dicyandiamide or potassium thiosulfate alone. It was also observed that potassium thiosulfate increased the accumulation of NO2--N in the soil while dicyandiamide suppressed the production of NO2--N. Nitrification inhibitor dicyandiamide or dicyandiamide combined with potassium thiosulfate increased the concentration of mineral nitrogen significantly while decreased the concentration of nitrogen unaccounted. Application of dicyandiamide, dicyandiamide combined with potassium thiosulfate increased the nitrogen recovery by 14.7 percent and 12 percent, respectively, compared with that in the treatment receiving nitrogen fertilizer alone. In summary, dicyandiamide proved superior to potassium thiosulfate in maintaining NH4+-N, inhibiting nitrification process and enhancing nitrogen recovery. A synergistic effect was also observed for the combined application of dicyandiamide and potassium thiosulfate. Results in this study are expected to provide scientific basis for practical application of two nitrification inhibitors. The inhibitory effect of dicyandiamide and potassium thiosulfate depends on soil temperature, moisture, microbial activity and other factors, and field experiment needs to be conducted to test the inhibitive characteristics of dicyandiamide and potassium thiosulfate on the nitrification.