Combined Effects of Water-Saving Irrigation and Biochar Application on Greenhouse Gas Emissions from Paddy Fields: A Review

Rice paddies are a major agricultural source of greenhouse gas emissions. However, balancing emission reduction with food security remains a core challenge for climate-smart rice production management. This review synthesizes current evidence on water-saving irrigation (WSI), biochar, and their combined effects on methane (CH4), nitrous oxide (N2O), global warming potential (GWP), and greenhouse gas intensity (GHGI) in paddy fields, emphasizing interaction mechanisms and environmental and management constraints. WSI generally suppresses CH4 emissions but can increases N2O emissions during drying–rewetting transitions. Mean reductions of approximately 43–52% in CH4 emissions and 37–47% in GWP have been reported in studies of alternate wetting and drying relative to flooded controls. Biochar can modify these responses through changes in soil aeration, pH, carbon availability, and nutrient retention. Under WSI, reported N2O decreases following biochar addition ranging from 19.3% to 62% relative to no-biochar controls under the same irrigation regime, although these decreases are not uniformly significant, and nonsignificant responses also occur. Additional CH4 mitigation is likewise context-dependent, with increases reported under some conditions. When CH4 reductions outweigh N2O increases in CO2-equivalent terms while yield remains stable, combined management can lower field-emission GWP and GHGI. A net benefit across the full production system additionally depends on carbon storage and upstream emissions. Excessive drying, poorly coordinated nitrogen inputs and unsuitable biochar–soil combinations can weaken these benefits. Uncertain aging effects, limited long-term multi-site evidence and inconsistent accounting boundaries constrain the assessment of persistence and transferability. Integrated WSI–biochar management therefore offers conditional mitigation potential, requiring locally evaluated coordination of water, nitrogen, and biochar inputs rather than uniform prescriptions.