Climate-Based Suitability Assessment for Methane Mitigation by Water Saving Technology in Paddy Fields of the Central Plain of Thailand

The alternate wetting and drying (AWD) water management technique has been identified as one of the most promising options for mitigating methane (CH$_{4}$) emissions from rice cultivation. By its nature, however, this option is limited only to paddy fields where farmers have sustained access to irrigation water. In addition, large amounts of rainfall often make it difficult to drain water from paddy fields. Therefore, it is necessary to understand the specific conditions and suitability of an area in which AWD is foreseen to be applied before its CH$_{4}$ mitigation potential can be assessed in view of planning regional and national mitigation actions. In this study, we applied a methodology developed for assessing the climatic suitability of AWD to paddy fields in the central plain of Thailand in order to determine the potential spatial and temporal boundaries given by climatic and soil parameters that could impact on the applicability of AWD. Related to this, we also assessed the CH$_{4}$ mitigation potential in the target provinces. Results showed that the entire area of the six target provinces was climatically suitable for AWD in both the major (wet) and second (dry) rice seasons. A sensitivity analysis accounting for uncertainties in soil percolation and suitability classification indicated that these settings did not affect the results of the suitability assessment, although they changed to some extent the distribution of moderate and high climatic suitability areas in the major rice season. Following the methodologies of the Intergovernmental Panel on Climate Change Guidelines, we estimated that the AWD scenario could reduce annual CH$_{4}$ emissions by 32% compared with the emissions in the baseline (continuously flooded) scenario. The potential of AWD for annual CH$_{4}$ emission reduction was estimated to be 57,600 t CH$_{4}$ year$^{-1}$, equivalent to 1.61 Mt CO$_{2}$-eq year$^{-1}$, in the target provinces. However, we recognize the possibility that other parameters not included in our current approach may significantly influence the suitability of AWD and thus propose areas for further improvement derived from these limitations. All in all, our results will be instrumental in guiding practitioners at all levels involved in water management for rice cultivation

Carbon and hydrogen isotope fractionation by moderately thermophilic methanogens

A series of laboratory studies were conducted to increase understanding of stable carbon (13C/12C) and hydrogen (D/H) isotope fractionation arising from methanogenesis by moderately thermophilic acetate- and hydrogen-consuming methanogens. Studies of the aceticlastic reaction were conducted with two closely related strains of Methanosaeta thermophila. Results demonstrate a carbon isotope fractionation of only 7‰ (α= 1.007) between the methyl position of acetate and the resulting methane. Methane formed by this process is enriched in 13C when compared with other natural sources of methane; the magnitude of this isotope effect raises the possibility that methane produced at elevated temperature by the aceticlastic reaction could be mistaken for thermogenic methane based on carbon isotopic content. Studies of H2/CO2 methanogenesis were conducted with Methanothermobacter marburgensis . The fractionation of carbon isotopes between CO2 and CH4 was found to range from 22 to 58‰ (1.023 ≤ α ≤ 1.064). Greater fractionation was associated with low levels of molecular hydrogen and steady-state metabolism. The fractionation of hydrogen isotopes between source H2O and CH4 was found to range from 127 to 275‰ (1.16 ≤ α ≤ 1.43). Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation factor was 1.43, independent of the δD-H2 supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in both hydrogen and carbon isotope fractionation during methanogenesis from H2/CO2. The relatively small fractionation associated with deuterium during H2/CO2 methanogenesis provides an explanation for the relatively enriched deuterium content of biogenic natural gas originating from a variety of thermal environments. Results from these experiments are used to develop a hypothesis that differential reversibility in the enzymatic steps of the H2/CO2 pathway gives rise to variability in the observed carbon isotope fractionation. Results are further used to constrain the overall efficiency of electron consumption by way of the hydrogenase system in M. marburgensis, which is calculated to be less than 55%. © 2004 Elsevier Ltd

Hydrogen isotope fractionation during H_2/CO_2 acetogenesis: hydrogen utilization efficiency and the origin of lipid-bound hydrogen

Hydrogen metabolism was studied in the anaerobic bacterium, Sporomusa sp. strain DMG 58, by measuring natural abundance levels of deuterium in H_2, H_(2)O, and individual fatty acids during acetogenic growth on H_2/CO_2. Four cultures were grown, each in medium with a distinct hydrogen-isotopic composition (δD-H_(2)O). The δD value of H_2 was quantified in the residual gas exiting the growth chambers and found to decrease concurrently with net H_2 consumption, indicating rapid isotope exchange between H_2 and H_(2)O. An isotopic mass balance was used to constrain the efficiency with which H_2 was activated by the cell and the reducing equivalents catabolized, which we term the H_2 utilization efficiency. Results indicate that H_2 utilization efficiency in these cultures is less than 20% during the growth phase, and less than 2% after the growth phase. The gross rate of cellular H_2 activation was similar in the growth phase and afterward. Biomass harvested at the end of each experiment was used to analyse the D/H of individual membrane lipids. Values of δD were highly correlated between lipids and water (δD-lipids = 0.59 × δD-water – 381‰; R2 = 0.995), indicating the source of lipid hydrogen is in isotopic equilibrium with water. Results are consistent with two possibilities: (i) water is the sole source of hydrogen to lipids, and the fractionation during biosynthesis is significantly larger than previously observed (α = 0.59), or (ii) hydrogen from H_2 is incorporated into lipids, but only after reaching isotopic equilibrium with H_(2)O. Fatty acids were strongly depleted in deuterium relative to all other organisms studied thus far, and such large depletions may prove useful as biomarkers for studying H_2 cycling in anoxic environments as well as in the geological record