Objective  To investigate the effects of ridge irrigation with different ridge widths and reduced nitrogen fertilizer at different growth stages on methane (CH₄) emissions, soil organic acid content and expression of enzyme encoding genes related to CH₄ formation and transformation (including methyl coenzyme M reductase gene mcrA in methanogenic archaea, and methane monooxygenase gene sMMO in methane oxidizing bacteria) in paddy fields, and reveal the effects of soil organic acid content and the expression of mcrA, sMMO on CH₄ fluxes in paddy fields.

Method A field experiment was conducted with three irrigation modes (flooding irrigation, ridge irrigation with ridge widths of 80, 100 cm) and three nitrogen treatments (conventional nitrogen application: 135 kg·hm⁻², including seedling fertilizer 47.25 kg hm⁻², tillering fertilizer 54.00 kg hm⁻² and booting fertilizer 33.75 kg hm⁻²; Nitrogen reduction at seedling stage: 110 kg·hm⁻², including seedling fertilizer 22.25 kg hm⁻², tillering fertilizer 54.00 kg hm⁻² and booting fertilizer 33.75 kg hm⁻²; Nitrogen reduction at booting stage: 110 kg·hm⁻² , including seedling fertilizer 47.25 kg hm⁻², tillering fertilizer 54.00 kg hm⁻² and booting fertilizer 8.75 kg hm⁻²). The paddy field CH₄ fluxes, soil organic acid content and mcrA, sMMO expression levels in different treatments were measured, and the relationships among them were analyzed.

Result Under the same nitrogen treatment, ridge irrigation significantly reduced CH₄ emission from paddy fields compared with flooding irrigation. Under the same irrigation mode, nitrogen reduction at seedling stage significantly reduced CH₄ emission from paddy field compared with conventional nitrogen application. The total organic acid content of soil in ridge irrigation with ridge width of 80 cm + nitrogen reduction at seedling stage treatment was 71.7% higher than that before fertilization on the 10th day after fertilization, and 28.8% higher in flooding irrigation + conventional nitrogen application treatment. Under the condition of nitrogen reduction at seedling stage, the expression of mcrA in ridge irrigation with ridge width of 80 cm soil was overall lower than that in flooding irrigation soil, and the expression of sMMO was higher than that in flooding irrigation except 25 days after transplanting. CH₄ fluxes were significantly correlated with mcrA expression and total organic acid content in soils (P<0.01), with correlation coefficients of 0.644 and −0.348, respectively. There were significant correlations between soil total organic acid content and the expressions of mcrA and sMMO (P < 0.05), with correlation coefficients of −0.240 and 0.197, respectively.

Conclusion The ridge irrigation with the ridge width of 80 cm + nitrogen reduction at seedling stage treatment can reduce CH₄ flux from paddy fields. Soil total organic acid content and mcrA expression significantly affect paddy field CH₄ flux, while soil sMMO expression may indirectly affect CH₄ flux.