Soil Liquefaction Potential Evaluation with Use of the Simplified Procedure

A simplified method based on both a liquefaction resistance factor, FL and a liquefaction potential factor, PL has been proposed for evaluating soil liquefaction potential. The factor FL indicates the liquefaction potential at a given depth of a site, and the factor PL indicates the one at a site. The effectiveness of the proposed method is investigated by calculating the factors FL and PL at both liquefied and non-liquefied sites during past typical earthquakes in Japan, and carrying out shaking table tests

Appropriate frequency and time of day to measure methane emissions from an irrigated rice paddy in Japan using the manual closed chamber method

Rice agriculture is a large anthropogenic source of atmospheric methane (CH 4 ). The reliable estimation of CH 4 emissions requires frequent measurements to trace diurnal and seasonal variations. To evaluate the appropriate intervals and optimal time of day to measure CH 4 emissions using the manual closed chamber method, this study analysed four datasets of CH 4 fluxes in a Japanese irrigated rice paddy measured at 2-hour intervals using the automated closed chamber method. The typical diurnal variation in the CH 4 flux was observed after the rice's heading stage, during which the daily time-weighted mean CH 4 flux was observed twice, in the 08:00 11:59 and 18:00 21:59 time windows. During the flooded rice-growing period, the CH 4 emissions, which were estimated by weekly measurements once per day during the 10:00 11:59 time window, corresponded to 93 106% of the emissions calculated using the automated measurement method. In contrast, no regular measurement strategies produced a satisfactory estimate of the CH 4 emissions during the non-flooded rice-growing period because of a sharp increase in the CH 4 flux just after the drainage. Consequently, the combination of weekly measurements once per day at approximately 10:00 as local mean time for the flooded rice-growing period and daily measurements once per day during the daytime for 1 week after each drainage event is recommended as a strategy to obtain the estimation with a ±10% error

Effects of free-air CO₂ enrichment (FACE) and soil warming on CH₄ emission from a rice paddy field: impact assessment and stoichiometric evaluation

Paddy fields are an important source of atmospheric CH4, the second most important greenhouse gas. There is a strong concern that the increasing atmospheric CO2 concentration ([CO2]) and global warming are further stimulating CH4 emissions, but the magnitude of this stimulation varies substantially by study, and few open-field evaluations have been conducted. Here we report results obtained at a Japanese rice free-air CO2 enrichment (FACE) site under water and soil temperature elevation during two growing seasons. Our objectives were to evaluate the effects of high [CO2] (ambient + 200 μmol mol−1) and elevated soil temperature (+ 2 °C) on CH4 emissions under completely open-field conditions. We found about 80% enhancement in total seasonal emissions by the additive effects of FACE and warming, indicating a strong positive feedback effect of global warming. The enhancement in CH4 emission from the FACE-effect alone (+ 26%) was statistically non-significant (P = 0.19). Nevertheless, observed positive correlations between CH4 emissions and rice biomass agreed well with previous studies, suggesting that higher photosynthesis led to greater rhizodeposition, which then acted as substrates for methanogenesis. Soil warming increased the emission by 44% (P < 0.001), which was equivalent to a Q10 of 5.5. Increased rice biomass by warming could only partly explain the enhanced CH4 emissions, but stoichiometric analysis of the electron budget indicated that even a moderate enhancement in organic matter decomposition due to soil warming can cause a large increase in CH4 production under conditions where Fe(III) reduction, which was little affected by soil warming, dominates electron-accepting processes. At later rice growth stages, advanced root senescence due to elevated temperature probably provided more substrate for methanogenesis. Our stoichiometric evaluation showed that in situ Fe reduction characteristics and root turnover in response to elevated temperature should be understood to correctly predict future CH4 emissions from paddy fields under a changing climate. Challenges remain for determination of in situ root-exudation rate and its response to FACE and warming