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Why focus on paddy rice?

Paddy rice is the staple crop for most of the world’s population. In 2012 rice was grown on more than 164 million ha worldwide in more than 100 countries. Asia, with a total of some 650 million metric tons (MT), accounts for about 90% of rice production, followed by Latin America (25 million MT) and sub-Saharan Africa (21 million MT). Globally, irrigated lowland rice occurs on about 80 million ha and provides 75% of the world’s rice production. Irrigated rice is the most important rice production system for food security, particularly in Asia. Women’s labor plays a significant role in rice production—anywhere from 50% in Indonesia, Thailand, and the Philippines to as much as 80% in India and Bangladesh (CGIAR n.d.).

Flooded rice systems, comprising irrigated, rainfed, and deepwater rice, emit significant amounts of methane. Although estimates vary and have high uncertainty, flooded rice produces approximately 20–40 Tg CH4/year,  or 500,000–1,000,000 Gg CO2e/year, of methane emissions (Yan et al. 2009), about 10% of anthropogenic emissions in the agriculture sector globally (Fig. 1).  Methane in wet or “paddy” rice soils is produced anaerobically after the flooding of rice fields. Emissions are sustained only under flooded conditions. 

Huge mitigation potential in paddy rice

Emissions from irrigated rice can be reduced by a number of practices that usually involve management of water and organic inputs (Smith et al. 2007; Yagi et al. 1997; Wassmann et al. 2000; Aulakh et al. 2000). At present, Alternate Wetting and Drying (AWD) is widely accepted as the most promising practice for its high methane reductions and multiple benefits. AWD is practiced by draining rice fields two to three weeks before the rice plant’s panicle, or head, starts to form in the base of the shoots and then intermittently irrigating the field, allowing the water to dry below the soil surface before re-flooding (Rothenberg et al. 2011). AWD can be combined with other mitigation measures such as direct seeding; efficient nitrogen use; soil amendments such as calcium carbide, calcium silicate, phosphogypsum, and biochar with urea fertilizer; cover crop management; reduced burning of rice straw; and composting.

AWD typically results in a 30–40% reduction of methane emissions (Wassmann et al. 2010) and conserves about 30% of irrigation water without compromising rice yields (Fig. 3).  A review of 24 studies conducted from 1989 to 2011 in eight countries  found that continuously flooded fields produced significantly higher methane emissions (p < .05%) compared to the noncontinuous water management regimes of rainfed rice, intermittent flooding and nonflooding irrigated rice (Sanchis et al. 2012). Methane emissions from the noncontinuous regimes did not significantly differ among each other. 

To maintain yields comparable to or higher than flooded conditions, AWD should maintain water savings levels below approximately 50% or what can be referred to as “safe” AWD (Sander, personal communication). When adequate water is maintained, studies show no statistically significant difference in production per hectare (Bouman et al. 2007, Cabongan et al. 2004, Lampayan et al. 2005). 

AWD also optimizes water consumption and thereby reduces fuel costs, making it attractive to farmers who pump irrigation water. For farmers using gravity-driven canal irrigation, the most important incentive is that AWD allows the plant to develop stronger roots, which reduces the chance of the plant tipping over during storms. Incentives can also be created by irrigation agencies by how and when they allocate water to farmers.