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Greenhouse gas (GHG) emissions from paddy fields depend on water management practices and rice varieties. Lysimeter experiments were conducted to determine the effect of rice varieties (lowland; Koshihikari (KH) and upland; Dourado Precoce (DP)) on GHG emissions under two water management practices: alternate wetting and drying (AWD) and continuous flooding (CF). A repeated cycle of drying and wetting in AWD irrigation was performed by drying the soil to −40 kPa soil matric potential and then rewetting. Consequently, the closed chamber method was used to measure direct emissions of methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2). The result revealed that water management significantly affected CH4 and N2O emissions (p < 0.05), while no significant effect was observed between different rice varieties. Although, AWD irrigation reduced CH4 emissions, it increased N2O emissions compared to CF irrigation, likely due to increased oxygen supply. AWD irrigation decreased GWP by 55.6% and 59.6% in KH and DP, respectively, compared to CF irrigation. Furthermore, CH4 and N2O emissions significantly correlated with soil redox potential and volumetric water content. These results suggest that AWD irrigation might be an effective water management method for mitigating GHG emissions from rice fields in central Japan. [ABSTRACT FROM AUTHOR]

A decline in the demand for using bamboo products has led to abandoned bamboo forests in Japan. To reduce the area of abandoned bamboo forests, the use of bamboo as a construction material and bamboo charcoal as an absorbent have gained considerable attention in recent years. In the literature, many studies have described the formation of biofilms on biochar, leading bamboo charcoal to be considered as a microbial carrier. The aim of this study was to examine the potential of bamboo charcoal as the microbial carrier of lactic acid bacteria (LAB) for phosphate removal. Bamboo charcoal was immersed in a LAB solution for 24 h for LAB to adhere to the bamboo charcoal. Then, the bamboo charcoal was placed in a bamboo fermented solution. Two types of bamboo charcoal, i.e., without pretreatment and dissolved in tap water, were used in the experiments. The experiments were also conducted with and without aeration to determine the effects of oxygenation. The bamboo charcoal without pretreatment displayed an increase in the phosphate concentration, indicating that phosphate was released from the bamboo charcoal. LAB-attached bamboo charcoal demonstrated a much smaller increase in phosphate concentration, suggesting phosphate was consumed by LAB. Experiments with dissolved bamboo charcoal also indicated a reduction in the phosphate concentration. The removal rate of phosphate decreased with an increase in the solution pH, suggesting that alkaline conditions limited the activity of the bacteria. Furthermore, the redox potential of the solution became negative in the solution without aeration. Overall, the results demonstrated that bamboo charcoal could be a LAB carrier; however, the bamboo charcoal must be dissolved prior to its use for phosphate removal. Furthermore, aeration and an acidic during phosphate removal are needed to obtain a higher removal rate when using LAB. [ABSTRACT FROM AUTHOR]

The relationships among eutrophication, anoxia, and microbial distribution were investigated for Nagatsura-Ura Lagoon on the northeastern Pacific coast of Japan. In September 2017, the bottom environment in a small area of the inner part of the lagoon (which has a basin-shaped bottom topology) was eutrophic and anoxic, with high carbon, nitrogen, phosphate, acid-volatile sulfide, and low dissolved oxygen and oxidation-reduction potential. Dissolved oxygen levels improved during the winter. Bacillariophyta (diatoms) were the main organic component according to pigment analysis and next-generation sequencing of nucleic acids in seawater samples. Phylum Proteobacteria was dominant among the bacterial flora in the sediment but the proportions of Class Epsilon-proteobacteria and Chlorobium (a green sulfur-utilizing bacterium) were high in the inner part of the lagoon compared to other stations, and these groups were also present in winter. Apparently groups able to thrive in both anoxic and aerobic conditions were predominant in the inner part of the lagoon. • Diatoms (Bacillariophyta), the predominant phytoplankton, were the major source of organic matter. • A basin-shaped topographic feature of the lagoon affects eutrophication and anoxia. • Phylum Proteobacteria was the predominant bacterial taxon. • Eutrophication and anoxia affect occurrence of Epsilon-proteobacteria and Chlorobium. [ABSTRACT FROM AUTHOR]

• Variation of hydrological connectivity of karst was assessed by intensive wells. • Chemicals and isotopes explain heterogeneity in surface–groundwater connectivity. • Coral limestone is porous and heterogeneous for water percolation and storage. Carbonate aquifers are critical for the health and well-being of human communities. Despite considerable efforts on the structure of karst aquifers, the variable and invisible flow paths or the vertical and spatial heterogeneity of karst systems remain too complex to be fully understood. An artificially constructed small limestone aquifer with an extensive network of observation wells in the Miyako Island, Japan, provides a unique opportunity to illustrate the vertical and spatial hydrological connectivity for water movement and storage. Water chemical data at two different depths over six years, including electrical conductivity (EC), dissolved oxygen (DO), oxidation–reduction potential (ORP), major ions (Na+, K+, Ca2+, Mg2+, SO 4 2−, and HCO 3 −), δ18O and δ2H capture water mixing processes during groundwater recharge in the aquifer influenced by irrigation pumping and recharge by precipitations throughout drying and wetting cycles. Results suggest that the intensive weathering of coral limestone under the impact of agricultural fertilization, as indicated by high EC values and high concentrations of Ca2+ and HCO 3 −. The high DO and low ORP values imply a lack of organic decomposition activity, which is typical in the karst groundwater condition. More importantly, groundwater recharge in the small aquifer was found to be influenced by following three processes. Spatially, both Cl− and NO 3 −−N indicate the large heterogeneity of water mixing as rainfall or drainage water percolates differently to various parts of the aquifer. Vertically, there are apparent differences in the concentration of Cl− and NO 3 −−N between the top and bottom layers in several wells, suggesting that, even within a single well, the vertical profile can be weakly connected between the top and the bottom layers. The bottom layer should have been recharged by lateral water inflow from fractures or conduits. And thirdly, the even distribution of Cl− over the long precipitation season from October 2016 and March 2021 and another nearly even distribution of NO 3 −−N in the low fertilization season in March 2021, demonstrate that the aquifer system is basically connected and porous. In summary, the insights gained from our study reveal that: 1) coral limestone can be treated as a well-developed porous aquifer and 2) the hydrological connectivity is highly heterogeneous and influenced by rock properties, including formation structure, degree of weathering, and clay content. Such findings are essential for understanding the hydrogeological properties of coral limestone aquifers globally. [ABSTRACT FROM AUTHOR]

Biodiesel fuel (BDF) is a potentially carbon-neutral fuel that could play a potentially important role in preventing global warming. However, its high production cost poses a challenge for many BDF producers. To establish an efficient method for BDF production and increase its cost competitiveness, the production efficiencies of 35 BDF plants in Japan, which produce BDF from waste cooking oil, were evaluated. Moreover, the cost reduction potential associated with improved efficiency was estimated. The empirical analysis revealed that (1) approximately 92% of the BDF plants have inefficient production; (2) they exhibit two predominant types of inefficiencies, technical and scale inefficiencies, and (3) improvement of production inefficiency can lead to an average production cost reduction of 3.52 yen per liter of BDF. To increase the production efficiency, it is important to improve the quality of the waste cooking oil used and increase the production scale. It is recommended that operators of inefficient BDF plants learn the production activities of the most efficient plants identified in this study. Furthermore, government policies focused on efficient BDF plants are essential to increase BDF production with limited resources. • DEA for analyzing production efficiency of biodiesel fuel (BDF) plants using waste cooking oil in Japan. • Approximately 92% plants were inefficient, considering production technology and scale. • Improvements in production inefficiencies can reduce the production cost per liter of BDF by 3.52 yen on average. • Decision-makers in inefficient plants must refer to the technology of the most efficient plants. • Sharp energy policies prioritizing technological top runners were recommended to ensure the economic competitiveness of BDF. [ABSTRACT FROM AUTHOR]