Nitrogen (N) fertilizer and crop residue amendments are important agricultural practices that could increase soil health, fertility, and crop yield. Such practices may also change soil denitrification processes where contradictory observations have been reported on soil N
2O emissions with fewer studies on N
2 emissions due to its large atmospheric background concentrations limiting its soil-borne measurement. This study aims to investigate N
2O production and reduction of N
2 emissions under a conducive denitrifying environment (like anaerobic microsites, 80% WFPS, available N and C) after rice straw amendment and KNO
3 application to three different soil types (fluvo-aquic, black, and paddy soils). In this regard, three treatments for three different soil types were set consisting of (a) a non-amended treatment (control), (b) a KNO
3 treatment (KNO
3, 20 mM KNO
3), and (c) a straw plus KNO
3 treatment (2.5 g rice straw kg
−1 dry soil and 20 mM KNO
3), which were incubated under 80% WFPS. Moreover, direct N
2O and N
2 fluxes were measured over 17 days in the current incubation experiment with a robotized incubation system using a helium atmosphere. Results showed that rice straw amendment combined with N fertilizer increased both N
2O and N
2 fluxes compared with control or KNO
3 treatments in all three soil types. Overall, compared with the black and paddy soils, the N
2O and N
2 fluxes were higher in the fluvo-aquic soil, with a maximum of 234.2 ± 6.3 and 590.1 ± 27.3 g N ha
−1 from F_SK treatment, respectively, during the incubation period. The general trends in three soil types of both N
2O and N
2 emissions were control < KNO
3 < rice straw plus KNO
3 treatments. Straw amendment in combination with KNO
3 can stimulate a high denitrification rate (less N
2O and higher N
2), whereas their effect on stoichiometric ratios of N
2O/(N
2O + N
2) highly depends on soil nitrate concentration, oxygen level, soil moisture content, and labile C. The current study underscores that the rice straw amendment in combination with N fertilizer can trigger denitrification with less increment on soil N
2O but higher N
2 emissions under conditions favoring denitrification.
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