Mitigating nitrous oxide emissions through iron amendments in water-saving irrigated paddy fields: A review

Rice cultivation is a major contributor to agricultural nitrous oxide (N2O) emissions, a greenhouse gas with a global warming potential approximately 300 times greater than carbon dioxide (CO2) and an atmospheric lifetime of ~121 years. Although water-saving irrigation practices, including Alternate Wetting and Drying (AWD) and Mid-Season Drainage (MD), effectively reduce methane (CH4) emissions by up to 27.6% and decrease irrigation water use by 15–30%, they often intensify soil aeration and stimulate microbial nitrification-denitrification, leading to substantial increases in N2O emissions. Reported increases range from 28.8% to more than 16-fold, with specific studies showing rises from 0.02 to 0.51 kg N2O-N ha−1 under AWD and up to 242% under MD. These trade-offs threaten the long-term sustainability of water-saving rice systems. Iron-based soil amendments (IA) have emerged as a promising mitigation strategy to counteract these elevated N2O emissions. For instance, iron (Fe) powder enhances the activity of Fe-reducing bacteria, such as Geobacter and Anaeromyxobacter, generating Fe2+ and lowering the soil's redox potential, which promotes the complete reduction of N2O to N2. Furthermore, other Fe amendments, including Fe-modified biochar and soluble ferrous iron (Fe2+), help mitigate N2O emissions by immobilizing NH4+, reducing the populations of ammonia-oxidizing bacteria, and supplying surplus electrons that enable denitrifiers to fully reduce N2O to N2. Empirical studies show that Fe-based amendments can reduce N₂O emissions by ~40% (iron-slag silicate fertilizer) and lower nitrification rates from 9.38 to 5.43 μg N g−1 d−1 when applied as Fe-modified biochar. Iron powder also enhances atmospheric N fixation, reducing reliance on synthetic nitrogen fertilizers. Integrating IA with AWD and/or MD, therefore, offers a synergistic pathway to sustain the benefits of water-saving irrigation while minimizing unintended increases in N2O emissions. Field-scale, multi-season studies are still needed to validate long-term impacts and assess residual Fe behavior, but current evidence demonstrates strong potential for these combined strategies to support climate-resilient, low-emission rice production aligned with global mitigation goals.