Abstract: The effects of exogenous nitrogen additions on soil fixed ammonium content and amount of maximum ammonium fixation were needed to clarify in protected vegetable fields with different planting years. Four protected vegetable fields with different planting years (0, 2, 13 and 21 years) were selected for a 36-day incubation experiment. Four fertilization treatments (CK: control without any N fertilizer applied; CF: Conventional N application, i.e. N 374 mg kg⁻¹ dry soil, provided by urea. RCF: reduced N application (a 46% reduction), i.e., N 200 mg kg⁻¹ dry soil, provided by urea; RCF + OM: 25% of the urea N was replaced by chicken manure on the basis of reduced N application) were set for each planting year to measure soil fixed ammonium content and to calculate the amount of maximum ammonium fixation. Fixed ammonium content in soil increased gradually with the increasing planting years of protected vegetables, but the amount of maximum ammonium fixation decreased. Under the same planting year, soil fixed ammonium content under different fertilization treatments ranged as CK < RCF + OM < RCF < CF, indicating that fertilization had an increasing effect on soil fixed ammonium, but the effect was influenced by the amount and type of N fertilization. Soil fixed ammonium content under CF treatment was the largest (98.61 mg kg⁻¹), significantly higher than that in RCF and RCF + OM treatments. The amount of ammonium fixation in RCF treatment (84.76 mg kg⁻¹) was larger than that in RCF + OM treatment (77.34 mg kg⁻¹). Soil fixed ammonium content increased with increasing N fertilization rate in protected vegetable fields, and the effect of chemical fertilizer is better than that of organic fertilizer.