RT Dissertation/Thesis
T1 Greenhouse gas emissions from rice production in the Vietnamese Mekong River Delta as affected by varietal selection and water management
A1 Vo,Thi Bach Thuong
WP 2023/08/16
AB The topic of this dissertation deals with rice production, the predominant source of daily nourishment for more than half of the worlds population. Rice production is directly affected by global climate change through aggravating climatic conditions, but is also one of the major sources of greenhouse gases (GHG) in the agricultural sector. The latter aspect is investigated in 4 publications by assessing the factors contributing to emissions, the quantification of GHG emissions across different scales, and possible mitigation of GHG emissions. In totality, these studies aim at bridging the gap between field measurements to national extrapolations in view of both GHG inventories and future mitigation programs. In terms of methodologies, the publications compiled in the following chapters represent a broad spectrum ranging from field measurements to meta-analysis, but they all deal with the emission of methane (CH4) which is generated in rice fields due to the unique feature of ‘semi-aquatic’ soils. The publications based on newly conducted field measurements also a nitrous oxide (N2O) which is a potent GHG emitted typically emitted from rice fields in low quantities. 
Chapter 2 (Vo et al. 2018) compiles field measurements from the Vietnamese Mekong River Delta (MRD) which accounts for more than 50% of the country’s rice production.  Emission factors (EFs) are used to estimate total emissions associated with the area of rice production. The Intergovernmental Panel on Climate Change (IPCC) has given the default EFs that are based on global averages as Tier 1 approach. However, the IPCC guidelines encourage national reporting institutions to conduct field measurements of GHG emissions and to determine country-specific EFs as the basis of the Tier 2 approach. Tier 2 further accounts for the fact that emissions may also be highly variable within a given country by requesting for disaggregation of EF at a sub-national scale. Therefore, the most recent GHG inventories for Vietnam are based on region-specific EFs under the IPCC Tier 2 approach, which is implemented using national activity data (i.e., national average cultivation period of rice and harvested area). In Chapter 2, we developed the specific EFs for different hydrological sub-zones and growing seasons in the MRD to achieve disaggregated EFs that could be used for the National Communications submitted to the United Nations Framework Convention on Climate Change (UNFCCC). Due to the distinct bio-physical condition and cropping cycle, the results show the lowest emissions in the saline sub-zone. While alluvial, acid sulfate soils had intermediate levels, the highest emissions were found in the deep flood sub-zone.
In Chapter 3 (Vo e al. 2018), we expanded the geographical scope of the GHG assessment to the entire country. This meta-analysis of CH4 data covers 73 cropping seasons at 36 field sites across the rice-growing areas of Vietnam under the IPCC’s baseline conditions (i.e., continuously flooded, no organic amendments) in the three main cropping seasons. As an output of this study, a structured database contained the location and season of each measurement as well as site-specific bio-physical factors and crop management at the site scale. In the next step, we developed disaggregated EFs for different zones and cropping seasons across the country that can be used for future reporting commitments of Vietnam as part of a more accurate Tier 2 assessment. The calculated EFs were generally higher than the IPCC defaults and the values used for Vietnam’s 3rd National Communications for the North, Central, and South Vietnam.
Chapter 4 (Vo et al. 2023) has to be seen in the context of Vietnam’s climate change policy that aims at reducing GHG emissions from rice production. Mitigation in rice production will be crucial for Vietnam because CH4 from rice accounts for about 15 % of the national GHG which is more than the entire transport sector even without considering CO2 and N2O emissions along the rice value chain. Previous studies have assessed the potential practices by changes in farming practices, namely water, nutrient, and straw management, and almost uniformly concluded that Alternate Wetting and Drying (AWD) is the most promising strategy for achieving a sizable mitigation of GHG emissions. Given the intense rainy season in southeast Asia, however, the precipitation is often too high to implement this water regime and will not provide any economic benefit from water saving. In turn, it is important to consider other mitigation strategies such as the selection of low-emitting cultivars. We conducted a field screening of 20 rice varieties that was expanded by assessing the interactive effect of variety selection and AWD. An experimental layout with 120 plots (based on 3 replicates) was required to assess this interaction of variety and water management in the field using the closed chamber method to collect air samples followed by lab analysis (using a gas chromatograph) to quantify the CH4 and N2O concentrations. The results of this study confirmed that GHG emissions from rice fields are dominated by CH4 emissions whereas N2O emissions were negligible. Compared with IPCC default values, the data set from two dry seasons yielded higher emissions under a baseline of continuous flooding (EF = 2.96 kg CH4 ha-1 d-1) and lower Scaling Factors (SF) of AWD (SF = 0.4). 
Chapter 5 (Asch et al. 2023) deals with the agronomic aspects of both AWD and variety selection and their implications on the economic viability of future mitigation efforts. While AWD is more efficient in reducing CH4 emissions than variety selection, this water management practice resulted in a slight yield decrease in our field study. Given the limited applicability of AWD, the selection of varieties is a much more adaptable approach and is also beneficial in terms of farmers’ adoption because it does not require any crop management changes. However, this strategy could also impact profits since the lowest-emitting variety may not have the highest rice yields.
In the context of future mitigation programs in the MRD, the dry season allows good control of the water table, so AWD should be the core of any mitigation effort. Variety selection on the other hand should be targeted in those seasons and locations that do not allow draining the fields. In turn, low-emitting varieties should become an integral part of future mitigation programs to supplement AWD within a systematic out scaling. In terms of economic trade-offs for the farmers, we assumed a scenario with compensation derived from the still premature carbon markets. The potential profit increments are very low and not attractive if distributed to farmers directly, but may collectively be used for investments in rural development by government agencies for benefitting farmers indirectly, e.g. by improving the irrigation infrastructure.

K1 Reis
K1 Methan
PP Hohenheim
PB Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
UL http://opus.uni-hohenheim.de/volltexte/2023/2212