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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. [ABSTRACT FROM AUTHOR]

In order to test the short-term effects of straw amendment on soil organic C (SOC) stabilization, SOC protection mechanisms, and soil bacterial community, we examined which bacterial taxonomic groups were associated with protected C fractions via different soil depths. We conducted a 5-year field experiment including a total of four treatments: S0 (no straw amendment), S1 (0–20 cm straw-amended soil), S2 (0–40 cm straw-amended soil) and S3 (0–60 cm straw-amended soil). The core method was used for soil sampling, and 180 soil samples was collected. Our results showed that straw amendment significantly increased bulk soil C content, enhanced the constituents of physically separated fractions and their OC contents, and changed the soil bacterial community composition at different soil depths. SOC was more accelerated in macroaggregate-derived unprotected and microaggregate-derived physically protected fractions at soil depths of 0–20 cm. Physically protected and physico-biochemically protected fractions were the major C protection mechanisms at soil depths of 20–40 cm and 40–60 cm soil depths. Our study also provides evidence that straw amendment significantly increases the bacterial phyla abundance of Proteobacteria and Bacteroidetes at each soil depth. Moreover, straw amendment enhanced the relative abundances of Gemmatimonadetes and Nitrospirae at soil depths of 40–60 cm and have a positive correlation with physically and physico-biochemically protected C pools. These results indicate that straw amendment can regulate C sequestration processes by enhancing SOC physical protection and modulating bacterial community, especially in the deep soil. In addition, straw amendment in subsoil (0–40 cm or 0–60 cm) is more beneficial for C storage and stabilization. [ABSTRACT FROM AUTHOR]