Mitigation of N2O emissions in water-saving paddy fields: Evaluating organic fertilizer substitution and microbial mechanisms

Water-saving irrigation strategies can successfully alleviate methane emissions from rice fields, but significantly stimulate nitrous oxide (N2O) emissions because of variations in soil oxygen level and redox potential. However, the relationship linking soil N2O emissions to nitrogen (N) functional genes during various fertilization treatments in water-saving paddy fields has rarely been investigated. Furthermore, the mitigation potential of organic fertilizer substitution on N2O emissions and the microbial mechanism in rice fields must be further elucidated. Our study examined how soil N2O emissions were affected by related functional microorganisms [ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nirS, nirK and nosZ] to various fertilization treatments in a rice field in southeast China over two years. In this study, three fertilization regimes were applied to rice cultivation: a no nitrogen (N) (Control), an inorganic N (Ni), and an inorganic N with partial N substitution with organic manure (Ni+No). Over two rice-growing seasons, cumulative N2O emissions averaged 0.47, 4.62 and 4.08 kg ha−1 for the Control, Ni and Ni+No treatments, respectively. In comparison to the Ni treatment, the Ni+No fertilization regime considerably reduced soil N2O emissions by 11.6% while maintaining rice yield, with a lower N2O emission factor from fertilizer N (EF) of 0.95%. Nitrogen fertilization considerably raised the AOB, nirS, nirK and nosZ gene abundances, in comparison to the Control treatment. Moreover, the substitution of organic manure for inorganic N fertilizer significantly decreased AOB and nirS gene abundances and increased nosZ gene abundance. The AOB responded to N fertilization more sensitively than the AOA. Total N2O emissions significantly correlated positively with AOB and nirS gene abundances while having a negative correlation with nosZ gene abundance and the nosZ/nirS ratio across N-fertilized plots. In summary, we conclude that organic manure substitution for inorganic N fertilizer decreased soil N2O emissions primarily by changing the soil NO3−-N, pH and DOC levels, thus inhibiting the activities of ammonia oxidation in nitrification and nitrite reduction in denitrification, and strengthening N2O reduction in denitrification from water-saving rice paddies.