Aerobic rice: An adaptation strategy that also reduces methane emissions

May 30th, 2016

Dr. Dennis Wichelns, Stockholm Environment Institute, Asia Centre, Bangkok

Rice is the primary food crop for much of humanity, and rice production supports millions of livelihoods across Asia and Africa.1,2,3 Climate change will impact rice production through both direct and indirect effects. The rising temperatures and changes in rainfall accompanying climate change are likely to directly impair rice performance and reduce crop yields.4,5,6 Farmers wishing to sustain rice production will need to shift their planting schedules to accommodate changes in temperature and rainfall patterns. Some will need to select alternative rice varieties or discontinue rice production in the dry season, if irrigation water resources are reduced due to climate change.

The increasing atmospheric concentration of CO 2 will enhance plant growth in some areas, with positive implications for rice yields. However, the net impact will be negative where the yield impairment is substantial due to rising temperatures, drought conditions, or changing rainfall patterns. In regions as large and diverse as Asia and Africa, the impacts of climate change on rice production will vary with location and with differences in regional weather patterns and crop production settings.7

Climate change will impact rice production indirectly as well, through sea level rise, coastal erosion, and saline intrusion into coastal aquifers.8,9 Much of the rice production in South and Southeast Asia is found in the deltas formed by major rivers, such as the Mekong, Irrawaddy, and Ganges-Brahmaputra.9 Rice is well adapted to these deltaic regions, many of which are characterised by monsoonal climates. Rice plants can tolerate extended periods in which the paddy soils are flooded or partly submerged, yet they are susceptible to damage from complete submergence caused by short-term or extended flooding.10,11

The 2011 Southeast Asian flood caused water levels in Cambodia’s Tonle Sap to rise above normal for more than one month, destroying 12% of the area planted in rice in Battambang Province, with impacts on livelihoods and household food security.12 The frequency of such flooding is expected to increase with climate change.8,13 In sum, rice production is susceptible to yield impairment due to several aspects of climate change, including changes in rainfall patterns, higher temperatures, extended droughts, and an increase in the frequency and severity of storms and flooding events. Given the important role of rice production in rural economies across much of Asia, adaptation strategies are needed urgently to ensure that smallholder farmers can continue producing rice for domestic and international markets, while generating sufficient income and ensuring that household and national food security goals are achieved.

Adaptation strategies

Several authors have suggested that adaptation strategies in rice production should include investments in irrigation and rainwater harvesting, in areas where such strategies are feasible.14,15 Others have suggested increasing fertiliser applications, choosing shorter duration varieties, and altering the planting dates for rice, in response to changes in rainfall patterns and higher temperatures.5,15,16,17,18 Another strategy is that of switching from continuously flooded paddies to some form of aerobic rice production, particularly in irrigated areas, where farmers can control the volume and timing of water deliveries.19

Aerobic rice production enhances oxygen availability in the root zone, for at least some portion of the season. The oxygen enhances root development, which results in stronger, more resilient rice plants, with increased tolerance of drought, extended submergence, and pest infestations.20,21,22 Aerobic rice production can be implemented along a spectrum of water management regimes that include draining a flooded rice paddy just once at midseason, intermittent irrigation for much of the season, a programme of sustainable rice intensification (SRI), and the production of rice as an upland crop, for which irrigations are scheduled to replace soil water depletions.20 In a sense, any variation from the program of continuous flooding can be considered a form of aerobic rice production.

Reducing methane emissions

Flooded rice paddies have been known to be a major source of methane, an aggressive greenhouse gas, for many years.23,24,25,26 Methane is generated in the anaerobic conditions that prevail in flooded rice paddies. Rice production in flooded paddies generates higher methane emissions per hectare and per unit of yield than does the production of wheat or maize.27 Rice production in upland areas, in which the fields are maintained in aerobic conditions, generates much less methane per hectare.28,29,30

Methane emissions can be reduced by switching from continuously flooded paddies to a programme of intermittent irrigation and drainage, and by limiting the amount of plant residue incorporated in soils after harvest and before planting.31,32,33,34,35 Small reductions in the time that rice paddies are inundated can substantially reduce methane emissions. Switching from anaerobic to aerobic production can create conditions that increase nitrous oxide emissions.36 However, the degree to which nitrous oxide emissions increase ranges substantially and is influenced by soil characteristics and the history of soil and water management in a given location.32 Several authors have shown that methane emissions can be reduced substantially, while only slightly increasing nitrous oxide emissions.32,37

Farmers in Japan have been draining their rice paddies in midseason for many years, largely to increase crop yields, by enhancing oxygen in the root zone and minimising the excessive growth of ineffective tillers.38 Following the midseason drainage, which requires about seven to ten days, many farmers practice intermittent irrigation and drainage for the remainder of the season.39,40 This practice allows for continued root development, prevents roots from rotting, and reduces the volume of irrigation water from the volume required to maintain continuous flooding.38,41 The enhanced root development also reduces the likelihood of rice plants falling over (lodging) as harvest approaches. Midseason drainage and the intermittent irrigation and drainage practiced by Japanese rice farmers reduce methane emissions.38

In an experiment in Nanjing, China, Wang et al. (2012)32 compare methane emissions from continuously flooded fields (W0), with emissions from fields that were drained twice each season (W2): once for nine days at mid-season, and again for two weeks before harvest. The mean seasonal methane emissions from the W0 and W2 plots were 390 kg and 156 kg of methane per ha, respectively. Thus, modifying the irrigation strategy reduced seasonal methane emission by about 60%.

Summing up

In response to climate change, an adaptation strategy that includes switching from continuously flooded rice production to some form of aerobic production can generate at least three additional benefits: 1) smaller irrigation demands per hectare of rice, 2) a substantial reduction in methane emissions, and 3) an improvement in plant health and rice crop performance. Aerobic rice production might not be feasible in all settings or seasons. Farmers need assured access to water for irrigation before choosing to drain a field at midseason or to implement a programme of intermittent irrigation. Nonetheless, when considering regional investments in new irrigation infrastructure and other adaptation strategies, one might also consider promoting aerobic rice production as an alternative to the traditional, continuously flooded method.

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Dr. Wichelns is a Senior Research Fellow with the Stockholm Environment Institute, based in Bangkok, Thailand. He has served on the faculty of several colleges and universities, and he has conducted research in several countries in Asia and Africa. Dr. Wichelns has directed two research centers and he has served as Principal Economist with the International Water Management Institute. He is co-Editor-in-Chief of Agricultural Water Management and the Founding Editor-in-Chief of Water Resources & Rural Development.

The views expressed in this article belong to the individual author and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.