Site-specific feasibility of alternate wetting and drying as a greenhouse gas mitigation option in irrigated rice fields in Southeast Asia: a synthesis

<p>This study comprises a comprehensive assessment, integration, and synthesis of data gathered from a 3-year field experiment conducted at four sites in Southeast Asia, namely Hue, Vietnam; Jakenan, Indonesia; Prachin Buri, Thailand; and Muñoz, Philippines, to assess the site-specific feasibility of alternate wetting and drying (AWD) as a greenhouse gas (GHG) mitigation option in irrigated rice fields. AWD effectively reduced water use compared to continuous flooding (CF) but did not significantly reduce rice grain yield and soil carbon content in all sites. Methane (CH₄) emissions varied significantly among sites and seasons as affected by soil properties and water management. AWD reduced CH₄ emissions relative to CF by 151 (25%), 166 (37%), 9 (31%), and 22 (32%) kg CH₄ ha⁻¹ season⁻¹ in Hue, Jakenan, Prachin Buri, and Muñoz, respectively. In Prachin Buri and Muñoz, AWD reduced CH₄ emissions only during the dry season. Site-specific CH₄ emission factors (EFs) ranged 0.13–4.50 and 0.08–4.88 kg CH₄ ha⁻¹ d⁻¹ under CF and AWD, respectively. The mean AWD scaling factors (SFs) was 0.69 (95% confidence interval: 0.61–0.77), which is slightly higher than the Intergovernmental Panel on Climate Change (IPCC)’ SF for multiple aeration of 0.52 (error range: 0.41–0.66). Significant reductions in the global warming potential (GWP) of CH₄+nitrous oxide (N₂O) by AWD were observed in Hue and Jakenan (27.8 and 36.1%, respectively), where the contributions of N₂O to the total GWP were only 0.8 and 3.5%, respectively. In Muñoz, however, CH₄ emission reduction through AWD was offset by the increase in N₂O emissions. The results indicate that the IPCC’s SF for multiple aeration may only be applied to irrigated rice fields where surface water level is controllable for a substantial period. This study underscores the importance of practical feasibility and appropriate timing of water management in successful GHG reductions by AWD.</p

Impacts of alternate wetting and drying on greenhouse gas emission from paddy field in Central Vietnam

<p>Vietnam is the world’s fifth largest rice producing country. Since methane (CH₄), a potent greenhouse gas (GHG), emission from the rice cultivation accounts for 14.6% of the national anthropogenic GHG emission, developing and disseminating mitigation options are the urgent need. Alternate wetting and drying (AWD) is the irrigation technique, in which a paddy field encompasses several soil-drying phases during the growth period, thereby reducing the CH₄ emission. However, field trials of the AWD’s feasibility in Central Vietnam are limited so far. We therefore carried out a 3-year experiment in a farmer’s field both in winter–spring season and summer–autumn season. CH₄ and nitrous oxide (N₂O) emissions were compared among the three treatments of water management: continuous flooding (CF), AWD, and site-specific AWD (AWDS) that changed the degree of soil drying depending on the growth stage. The total water use including irrigation and rainfall was significantly (<i>p</i> < 0.05) reduced by AWD (by 15%) and AWDS (by 14%) compared to CF, but rice grain yield did not differ among the three treatments. The seasonal cumulative CH₄ emission was significantly reduced by AWD (26%) and AWDS (26%) compared to CF, whereas the seasonal cumulative N₂O emission did not differ among treatments. The resultant global warming potentials (GWPs) of CH₄ and N₂O under AWD and AWDS were 26% and 29% smaller than that under CF, respectively. The GWP of N₂O was only 0.8% of the total GWP of CH₄ + N₂O. The yield-scaled GWP and water productivity (i.e., the ratio of grain yield to water use) were also improved by AWD and AWDS. No significant differences in the measured items between AWD and AWDS were attributed to similar variation patterns in the surface water level. The results confirm the AWD’s performance as a mitigation option for paddy GHG emission in Central Vietnam.</p

Actual dates encompassing the different growth stages in 2013DS, 2013WS, 2014DS, and 2014WS (DS–dry season; WS–wet season).

<p>Actual dates encompassing the different growth stages in 2013DS, 2013WS, 2014DS, and 2014WS (DS–dry season; WS–wet season).</p

Pearson correlation analysis between the diel fluxes of CH₄ and air temperature (<i>Ta</i>), floodwater temperature at 2.5 cm above the soil (<i>T</i>_<i>fw</i> <i>2</i>.<i>5 cm</i>), soil temperature at 2.5 cm depth (<i>T</i>_<i>s</i> <i>2</i>.<i>5 cm</i>), soil temperature at 5 cm depth (<i>T</i>_<i>s</i> <i>5 cm</i>), and solar radiation (<i>SR</i>) during the different growth stages of the rice plants (vegetative, reproductive, and ripening) in 2013 and 2014 wet seasons (WS).

<p>Pearson correlation analysis between the diel fluxes of CH₄ and air temperature (<i>Ta</i>), floodwater temperature at 2.5 cm above the soil (<i>T</i>_<i>fw</i> <i>2</i>.<i>5 cm</i>), soil temperature at 2.5 cm depth (<i>T</i>_<i>s</i> <i>2</i>.<i>5 cm</i>), soil temperature at 5 cm depth (<i>T</i>_<i>s</i> <i>5 cm</i>), and solar radiation (<i>SR</i>) during the different growth stages of the rice plants (vegetative, reproductive, and ripening) in 2013 and 2014 wet seasons (WS).</p

Incremental changes in CH₄ concentrations and derived flux rates measured under low background level of CH₄ fluxes; concentration increments below LOD_gc and LOD_flux are marked in red cells.

<p>Incremental changes in CH₄ concentrations and derived flux rates measured under low background level of CH₄ fluxes; concentration increments below LOD_gc and LOD_flux are marked in red cells.</p

Increasing sensitivity of methane emission measurements in rice through deployment of ‘closed chambers’ at nighttime - Fig 1

<p>a,b,c,d,e,f. Mean diel patterns of CH₄ flux during the different growth periods of the rice plant (vegetative, reproductive, and ripening) in the dry seasons (DS) of 2013 (a,b,c) and 2014 (d,e,f). The solid line represents the mean of all days at a given time of day (in 30 min intervals) and the shaded area represents the standard deviation.</p

Coefficient of variation (%) of diurnal and nocturnal CH₄ fluxes during the different growth stages of the rice plants (vegetative, reproductive, and ripening) in 2013 and 2014 dry (DS) and wet seasons (WS).

<p>Coefficient of variation (%) of diurnal and nocturnal CH₄ fluxes during the different growth stages of the rice plants (vegetative, reproductive, and ripening) in 2013 and 2014 dry (DS) and wet seasons (WS).</p

Emission ratios depicting the relative share of nocturnal emissions derived from 30-min recording intervals over 4 seasons; emission records of a given day (diel period) were divided into nocturnal period (00:30–06:00 and 18:30–24:00) and diurnal period (06:30–18:00).

<p>Emission ratios depicting the relative share of nocturnal emissions derived from 30-min recording intervals over 4 seasons; emission records of a given day (diel period) were divided into nocturnal period (00:30–06:00 and 18:30–24:00) and diurnal period (06:30–18:00).</p

Methane emission from rice cultivation in different agro-ecological zones of the Mekong river delta: seasonal patterns and emission factors for baseline water management

<p>This study comprises a set of methane emission measurements in rice fields located in the four agro-ecological zones of the Mekong River Delta (MRD), namely the zones with (i) alluvial soils, (ii) salinity intrusion, (iii) deep flood, and (iv) acid sulfate soils. These zones have very distinct bio-physical conditions and cropping cycles that will affect methane emissions in various forms. Our study includes comprehensive mapping of these zones as well as an overview of rice statistics (activity data) at provincial level for each cropping season. Emission data were obtained by the closed chamber method. The available data set comprises 7 sites with 15 cropping seasons. Mean emission rates showed large variations ranging from 0.31 to 9.14 kg CH₄ ha⁻¹ d⁻¹. Statistical analysis resulted in weighted means for all zones that we use as zone-specific CH₄ emission factors (EF_z) in the context of the IPCC Tier 2 approach. The lowest EF_z was computed for the saline accounting for 1.14 kg CH₄ ha⁻¹ d⁻¹ (confidence interval: 0.60–2.14). The EF_z values of the alluvial and acid sulfate zones were 2.39 kg CH₄ ha⁻¹ d⁻¹ (2.19–4.13) and 2.78 kg CH₄ ha⁻¹ d⁻¹ (2.65–3.76), respectively, which indicated that they were not different from each other derived from their confidence intervals. The deep flood zone, however, required a season-specific, assessment of EF_z because emission in the autumn–winter cropping season, corresponding to the wet period, was significantly higher (9.14 kg CH₄ ha⁻¹ d⁻¹ (7.08–11.2)) than the other seasons (2.24 kg CH₄ ha⁻¹ d⁻¹ (1.59–3.47)). Although these emission factors correspond to baseline water management and do not capture the diversity of farmers’ practices, we see the availability of zone-specific data as an important step for a more detailed assessment of Business as Usual emissions as well as possible mitigation potentials in one of the most important rice growing regions of the world.</p