Field Crop 2024, Vol.7, No.5, 243-251 http://cropscipublisher.com/index.php/fc 244 N2O emissions, assesses their potential to balance yield and emissions reduction, and identifies knowledge gaps and future research needs, aiming to provide decision-makers and stakeholders with information on best practices for sustainable rice cultivation in the context of climate change. 2 Mechanisms of Greenhouse Gas Emissions from Rice Paddies 2.1 Methane emissions: sources and pathways Methane CH4 emissions from rice paddies primarily result from microbial methanogenesis, which occurs under anaerobic conditions in waterlogged soils. Methanogenesis is the final step in the anaerobic degradation of organic matter, where methanogens utilize substrates such as acetate (aceticlastic methanogenesis) or hydrogen plus carbon dioxide (hydrogenotrophic methanogenesis) (Conrad, 2020). Additionally, methane can be oxidized anaerobically in the presence of alternative electron acceptors like nitrate, ferric iron, or sulfate, which can mitigate CH4 emissions (Fan et al., 2020). The presence of rice plants also influences methane emissions by providing pathways for methane transport through aerenchyma and by supplying substrates for methanogenesis, although they can also suppress emissions by delivering oxygen to the rhizosphere, which enhances methane oxidation (Oda and Chiếm, 2019). 2.2 Nitrous oxide emissions: mechanisms of production Nitrous oxide N2O emissions from rice paddies are primarily produced through microbial processes such as nitrification and denitrification. Nitrification occurs under aerobic conditions where ammonia is oxidized to nitrate, while denitrification happens under anaerobic conditions where nitrate is reduced to N2O and nitrogen gas. The intermittent wetting and drying cycles in rice paddies create fluctuating aerobic and anaerobic conditions that favor these processes (Gupta et al., 2021). The application of nitrogen fertilizers can significantly influence N2O emissions by providing substrates for nitrification and denitrification (Liu et al., 2016). 2.3 Factors influencing emission levels Several factors influence the levels of greenhouse gas emissions from rice paddies, including water management, soil type, and agricultural practices. Water management practices, such as intermittent flooding, can reduce methane emissions by promoting aerobic conditions that inhibit methanogenesis and enhance methane oxidation (Malyan et al., 2016). Soil type also plays a crucial role, as soils with higher organic matter content tend to produce more methane due to the availability of substrates for methanogens (Sun et al., 2018). Additionally, the use of fertilizers and amendments can impact emissions; for instance, nitrate-based fertilizers can enhance anaerobic methane oxidation, thereby reducing methane emissions. The presence of specific microbial communities, such as sulfate-reducing bacteria and methane-oxidizing bacteria, can also mitigate methane production through competitive and mutualistic interactions. Furthermore, the conversion of rice paddies to other land uses, such as aquaculture, has been shown to reduce both methane and nitrous oxide emissions significantly. 3 Strategies for Reduction of Greenhouse Gas Emissions 3.1 Water management techniques Alternate wetting and drying (AWD) is a water management technique that involves the periodic drying and re-flooding of rice paddies (Figure 1). This method has been shown to significantly reduce methane (CH₄) emissions, which are a major contributor to greenhouse gas emissions from rice fields. Studies have demonstrated that AWD can reduce CH₄ emissions by up to 95% compared to continuous flooding (CF) systems, while also conserving water by 25%~70% (Runkle et al., 2018). Additionally, AWD has been found to maintain or even improve rice yields under certain conditions, making it a viable alternative to traditional irrigation methods (Sriphirom et al., 2019; Malumpong et al., 2020). Synchronous irrigation involves coordinating the irrigation schedules of multiple fields to optimize water use and reduce greenhouse gas emissions. This technique can be particularly effective when combined with AWD, as it allows for more efficient water distribution and reduces the overall water footprint of rice cultivation. Studies have shown that synchronous irrigation can further enhance the benefits of AWD by improving water productivity and reducing the global warming potential (GWP) of rice paddies.
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