In the tropics,frequent nitrogen(N)fertilization of grazing areas can potentially increase nitrous oxide(N2O)emissions.The application of nitrification inhibitors has been reported as an effective management practice for potentially reducing N loss from the soil-plant system and improving N use efficiency(NUE).The aim of this study was to determine the effect of the co-application of nitrapyrin(a nitrification inhibitor,NI)and urea in a tropical Andosol on the behavior of N and the emissions of N2O from autotrophic and heterotrophic nitrification.A greenhouse experiment was performed using a soil(pH 5.9,organic matter content 78 g kg-1,and N 5.6 g kg-1)sown with Cynodon nlemfuensis at 60％ water-filled pore space to quantify total N2O emissions,N2O derived from fertilizer,soil ammonium(NH4+)and nitrate(NO3),and NUE.The study included treatments that received deionized water only(control,CK)and two doses of 15N-enriched urea(65(UR)and 129 mg N kg-1(UD))without or with 350 g nitrapyrin for each 100 kg N(UR+NI and UD+NI).No significant differences were observed in soil NH4+ content between the UR and UR+NI treatments,probably because of soil mineralization and immobilization(influenced by high soil organic matter content).Nitrapyrin application failed to maintain a stable pool of labeled NO3 due to the additional NO3- produced by heterotrophic nitrification,which is not effectively inhibited by nitrapyrin.After 56 d,N2O emissions in UR(0.51±0.12 mg N2O-N kg-1)and UR+NI(0.45±0.13 mg N2O-N kg-1)were not significantly different;by contrast,emissions were 36.3％ lower in UD+NI than in UD.It was concluded that the soil organic N mineralization and heterotrophic nitrification are the main processes of NH4+ and NO3- production.Additionally,it was found that N2O emissions were partially a consequence of the direct oxidation of the soil's organic N via heterotrophic nitrification coupled to denitrification.Finally,the results suggest that nitrapyrin would likely exert significant mitigation on N2O emissions only if a substantial N surplus exists in soils with high organic matter content.