Download Aerobic rice: An adaptation strategy that also reduces methane

Aerobic rice: An adaptation strategy that also reduces methane
emissions
Dr. Dennis Wichelns, Stockholm Environment Institute, Asia Centre, Bangkok
Rice farmer in paddy, Vietnam. Source:
GWF Flickr Resource
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 CO2 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
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Aerobic rice: An adaptation strategy that also reduces methane
emissions
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
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Aerobic rice: An adaptation strategy that also reduces methane
emissions
Flooded rice paddies, Southeast Asia.
Source: GWF Flickr Resource
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
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Aerobic rice: An adaptation strategy that also reduces methane
emissions
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
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Aerobic rice: An adaptation strategy that also reduces methane
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