Abstract: In order to understand the mechanism of nitrification in limy soil for different exogenous nitrogen sources, the study was conducted to investigate the effects of different additive amounts and nitrogen sources on the nitrification in fresh Cal-Ustic Isohumasols by using the plot experiment, and the corresponding model was obtained. The results show that the curve between the ammonium nitrate consumption rate and nitrogen nitrate increasing rate can be described as an S shape, and the ammonium nitrate consumption rate is higher than the nitrogen nitrate increasing rate. The ammonium nitrate consumption rate and nitrogen nitrate increasing rate are positively correlated with the nitrogen addition. Except the nitrifying bacteria, other organisms absorptions of ammonium nitrate and nitrogen nitrate are positively related to the addition of nitrogen nitrate. Applications of different nitrogen amounts have great influences on the nitrification in soils. When the nitrogen addition is 75 mg N/kg soil, the nitrification is more completely. SO42- can hasten the nitrification rate and can also improve the bio-availabilities of ammonium nitrate and nitrogen nitrate for other organisms.