This study investigated the effects of organic amendments on gaseous nitrogen losses from inland saline-alkali soil in northwestern China and their potential mechanisms. A laboratory incubation experiment was conducted using a typical saline-alkali soil from Xinjiang to compare the effects of wheat straw, biochar, and humic acid at different application rates, as well as combined biochar and humic acid application, on nitrous oxide (N2O) emissions and ammonia (NH3) volatilization. Soil physicochemical properties, nitrogen-cycling functional gene abundances, and partial least squares structural equation modeling (PLS-SEM) were used to examine the associated factors. Compared with urea application alone, all organic amendment treatments reduced cumulative N2O emissions by 27.3%–85.7%, while individual amendments reduced cumulative NH3 volatilization by 27.6%–82.4%. Wheat straw achieved the greatest simultaneous reductions, decreasing N2O emissions and NH? volatilization by 82.4%–85.7% and 77.8%–82.4%, respectively. Increasing application rates did not significantly affect cumulative N2O emissions for any individual amendment but significantly increased cumulative NH3 volatilization under biochar and humic acid treatments. Combined biochar and humic acid application did not significantly improve N2O mitigation compared with the corresponding individual amendments at low application rates and increased cumulative NH3 volatilization by 32.4% relative to urea alone. Wheat straw significantly decreased soil urease activity, which was significantly positively correlated with the cumulative losses of both gases. PLS-SEM indicated that soil nitrogen pools had significant positive indirect associations with both gas losses through nitrogen availability and transformation, whereas the total effects of pH and nitrogen-cycling microorganisms were not significant. In conclusion, under the experimental conditions, wheat straw application simultaneously reduced N2O emissions and NH3 volatilization from inland saline-alkali soil in northwestern China. This mitigation effect may be associated with changes in urea hydrolysis and subsequent nitrogen transformations. The use of biochar and humic acid requires consideration of application rates and amendment combinations, together with a joint assessment of their effects on N2O emissions and NH3 volatilization. These findings provide a reference for the appropriate use of organic amendments in inland saline-alkali soils in northwestern China, although their effectiveness under field conditions requires further verification.