3,4-Dimethylpyrazole phosphate (DMPP) was validated as an effective nitrification inhibitor to reduce nitrate leaching. Its effects on ammonia (NH(3)) volatilization were not clear, especially on farmland scale with crop rotations. In this study, on-farm experiments at the Jiaxing (JX) and Yuhang (YH) sites in Taihu Lake Basin, China were conducted to evaluate the effect of DMPP application on mineral nitrogen (N) (NH(4)-N and NO(3)-N) leaching and NH(3) volatilization losses in a rice-rapeseed cropping system. Treatments included urea alone (UA), urea + 1% DMPP (UD), and no fertilizer (CK). The results show that DMPP reduced NO(3)-N leaching fluxes by 44.9 to 59.9% and increased NH(4)-N leaching fluxes by 13.0 to 33.3% at two sites during rice and rape seasons compared with urea alone. Reductions in mineral-N leaching fluxes by DMPP in two seasons at the JX and YH sites were 9.5 and 14.3 kg N ha(-1), respectively, compared with UA treatment. The application of DMPP had no significant effects on NH(3) volatilization loss fluxes at either site. The rice and rapeseed yields were 5.3 to 7.4% higher in UD plots than in UA plots at two sites. These results that indicate DMPP could reduce leaching losses of mineral-N from crop fields and promote grain yields by conserving more applied N in soil in rice-rapeseed rotation systems.