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

Abstract

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