By Malini Williams
Rice is one of the most important crops in the world, representing an essential staple in the diets of billions of people around the world. It represents one-fifth of the calories consumed worldwide and is grown on more than 140 million hectares (Hawken, 2017; Wassman, Hosen, & Sumfleth, 2009). However, rice cultivation is a potent emitter of the greenhouse gas methane, responsible for around 10% of methane emissions from agriculture and between 9% and 19% of global methane emissions (Hawken, 2017). Methane is less long-lived in the atmosphere than carbon dioxide, yet has almost 34 times the warming potential (Hawken, 2017). Rice is grown in warm, flooded paddy fields that create optimal growing conditions for methane-producing microbes. Additionally, rice is mostly grown in warm climates, and higher temperatures increase the amount of methane emitted (Hawken, 2017). Thus, methane emissions from rice cultivation will continue to increase as the global temperature rises with climate change. As world food demand continues to grow, rice yields will need to increase by around 28% by 2050 to match demand, further increasing emissions (Jiang et al, 2019). To reduce emissions and keep up with the demands of a growing world population, rice cultivation will have to increase yields, whilst reducing methane emissions and maintaining nutrient quality.
In rice cultivation, methane is the product of anaerobic respiration by soil microbes, known as methanogens, that thrive in the wet soil of flooded rice paddies. Some methane is oxidized by aerobic soil bacteria known as methanotrophs, so the amount emitted depends on the balance between methanogens and methanotrophs in the soil (Yagi, Tsuruta & Minami, 1997). Methane is emitted from paddies through diffusion into the flood water, ebullition, and plant transport. Most strategies used around the world to reduce emissions focus on farming practices to reduce methane from the soil, while recent genetic engineering advancements have produced cultivars that limit emissions through plant transport.
An improved rice cultivation strategy known as the System of Rice Intensification (SRI) emerged in Madagascar in the 1980s. This technique increases rice yields while reducing the amount of seeds planted and also requires less water and chemicals (Styger et al, 2011). It has been adopted most widely by smallholder farms in Asia, where most of the world’s rice is grown. SRI involves planting young seedlings spaced apart in lines, rather than planting them in clumps by the handful (Hawken, 2017). Weeding is also done by hand, aerating the soil. Organic fertilizers are used, sequestering carbon and enhancing soil fertility, rather than using synthetic nitrogenous fertilizers, as high usage of nitrogenous fertilizers and subsequent increased nitrous oxide emissions could offset the benefits of reduction in methane emissions from using SRI (Hawken, 2017; Wassman, Hosen, & Sumfleth, 2009). Nitrous oxide is a greenhouse gas with a warming potential nearly 300 times that of carbon dioxide over a similar timespan, so fertilizers must not be applied in high amounts in order to limit nitrous oxide emissions (Liao et al, 2020).
Perhaps the most important aspect of SRI for the reduction of methane emissions from rice cultivation is midseason drainage of paddies. Most rice paddies are continuously flooded during the growing season, which allows methanogenic bacteria to thrive. Midseason drainage aerates the soil, which results in enhancement of methane oxidation by methanotrophs, whilst also disrupting conditions favorable for methanogens (Yagi, Tsuruta, & Minami, 1997). Midseason drainage alone has the potential to reduce emissions between 35% and 70% (Hawken, 2017). Intermittent flooding can also be used with the same effects, perhaps best used in regions where water supply is insufficient to keep rice fields flooded for long stretches of time (Styger et al, 2011). However, midseason drainage or alternate wetting and drying can also increase emissions of nitrous oxide into the atmosphere, as these practices create saturated soil conditions that promote nitrous oxide production (Wassman, Hosen, & Sumfleth, 2009). These emissions can be partially limited with appropriate use of organic nitrogenous fertilizers, although midseason drainage may still result in increased nitrous oxide emissions. However, the massive reduction in methane emissions offsets any increase in nitrous oxide emissions and the net global warming potential of rice paddies using midseason drainage or alternative wetting and drying will be negative.
A developing method to reduce rice farming emissions uses genetically modified rice plants that emit less methane. Researchers have reduced emissions from plant transport through the addition of transcription factor sugar signalling in barley 2 (SUBISA2) in rice plants, which also increases the starch content of rice grains (Su et al, 2015). SUBISA2 was cloned into a plasmid along with the promoter SBE11b before being introduced into rice seedlings via Agrobacterium-mediated transfer and then grown in open fields in southern China. The release of methane by plant transport is a result of association of methanogens around the roots of rice plants. SUBISA2 rice had significantly fewer methanogens around the roots, leading to lower emissions (Su et al, 2015). This is likely because SUBISA2, when highly expressed in leaves and stems, increases sink strength (the competitive ability of an organ to attract assimilates) and starch biosynthesis in these tissues. This leads to reduced availability of sugar in the roots and fewer nutrients for methanogens associated with rice roots. As a result, there were fewer methanogens in the soil and methane emissions decreased (Su et al, 2015). An additional benefit of SUBISA2 is increased starch content in the grains, which may address the growing demand for increased crop productivity. As genetic engineering technology and applicability continues to improve, it is likely that engineered rice cultivars with reduced emissions will become an important emission mitigation strategy alongside SRI.
It is estimated that improved rice cultivation, which involves SRI and better soil, nutrient, and water management, could reduce and sequester 11.3 gigatons of CO2 or its equivalent in methane if expanded to around 88 million hectares of rice paddies (Hawken, 2017). As the world population continues to grow and curbing methane emissions becomes an increasingly pressing issue, improved farming practices that reduce emissions and increase yield must be adopted widely. SRI and improved cultivars offer solutions to both these issues, curbing methane emissions from one of the most polluting sectors to mitigate climate change whilst addressing the demands of a growing population.
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