Effects of Straw Amendment in Combination with Synthetic N Fertilizer Addition on N₂O, N₂, and Their Stoichiometric Ratios in Three Different Agro-Ecosystems

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 N₂O emissions with fewer studies on N₂ emissions due to its large atmospheric background concentrations limiting its soil-borne measurement. This study aims to investigate N₂O production and reduction of N₂ emissions under a conducive denitrifying environment (like anaerobic microsites, 80% WFPS, available N and C) after rice straw amendment and KNO₃ 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 KNO₃ treatment (KNO₃, 20 mM KNO₃), and (c) a straw plus KNO₃ treatment (2.5 g rice straw kg⁻¹ dry soil and 20 mM KNO₃), which were incubated under 80% WFPS. Moreover, direct N₂O and N₂ 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 N₂O and N₂ fluxes compared with control or KNO₃ treatments in all three soil types. Overall, compared with the black and paddy soils, the N₂O and N₂ 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 N₂O and N₂ emissions were control < KNO₃ < rice straw plus KNO₃ treatments. Straw amendment in combination with KNO₃ can stimulate a high denitrification rate (less N₂O and higher N₂), whereas their effect on stoichiometric ratios of N₂O/(N₂O + N₂) 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 N₂O but higher N₂ emissions under conditions favoring denitrification.