Mitigation of yield-scaled greenhouse gas emissions in subtropical paddy rice under alternative irrigation systems

Flood irrigated rice accounts for 20 % of all agricultural methane (CH₄) emissions in Southern Brazil. In this study, the potential of intermittent irrigation regimes [viz., intermittent irrigation (II), sparse intermittent irrigation (SII) and frequent intermittent irrigation (FII)] for mitigating yield-scaled CH₄ and nitrous oxide (N₂O) emissions was evaluated in relation to continuous flood irrigation (CF) in the subtropical region of Brazil. Irrigation regimes were evaluated for two growing seasons (GS’s), where II was evaluated in GS-1 and SII and FII in GS-2. In relation to CF, the intermittent irrigation reduced season CH₄ emissions by 47 % in GS-1 and 85 and 71 % for SII and FII in GS-2, respectively. Although the intermittent irrigation significantly increased N₂O emissions, they reduced the partial global warming potential (pGWP = CH₄ × 25 + N₂O × 298) by 37 % (GS-1) and 70 % (GS-2) with respect to CF. For each day without a standing water layer on the soil surface, pGWP was reduced linearly by 249 kg CO₂-eq ha⁻¹. Considering the water regimes did not affect rice yield (mean of 11,910 kg ha⁻¹ for GS-1 and 10,638 kg ha⁻¹ for GS-2), the intermittent irrigation regimes led to significantly lower yield-scaled pGWP values (0.19–0.57 kg CO₂-eq kg⁻¹ grain) compared to CF (0.8 kg CO₂-eq kg⁻¹ grain on average with GS-1 and GS-2). Our results show that intermittent irrigation provides an effective strategy for mitigating greenhouse gas emissions from irrigated subtropical Brazilian rice systems, without any negative impact on rice productivity.