Poly-γ-glutamic acid (γ-PGA) is regarded as a promising fertilizer synergist in agricultural system and plays an important role in soil nitrogen (N) cycle. Amide N fertilizer such as CO(NH2)2and ammonium N fertilizer such as (NH4)2SO4have different N turnover in soil. However, how the fate of CO(NH2)2and (NH4)2SO4in plant-soil systems would be affected by γ-PGA is unclear. We conducted a pot study using 15N-labeled CO(NH2)2and (NH4)2SO4to investigate the fate of these two fertilizers in soil planted with Brassica chinensiswith and without addition of γ-PGA. Addition of γ-PGA significantly increased plant dry weight at harvest when applied with both CO(NH2)2and (NH4)2SO4. γ-PGA addition increased plant uptake of CO(NH2)2-N and (NH4)2SO4-N, thus resulted in higher N use efficiency of both fertilizers. γ-PGA addition increased the amount of CO(NH2)2-N found in the NH4+-N pool and decreased the amount of (NH4)2SO4-N found in the NH4+-N and NO3−-N pool, and promoted greater incorporation of fertilizers-N into the soil microbial biomass N pool. γ-PGA addition decreased the amount of (NH4)2SO4-N remaining in soil, while it had no effect on the amount of CO(NH2)2-N remaining in soil. γ-PGA addition reduced the overall loss of CO(NH2)2-N and (NH4)2SO4-N by 6.93% and 3.54% of applied N, respectively, and reduced the soil N loss by 2.65% and 1.63% of applied CO(NH2)2-N and (NH4)2SO4-N, respectively. Applying γ-PGA with fertilizer has many N management benefits, for example, notably a reduction in loss of both fertilizer and soil N. Thus, it can be recommended for further consideration and used in agricultural practice management.