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Sulfur-based denitrification may be a key biogeochemical nitrate (NO3−) removal process in sulfide-rich regions, but it is still poorly understood in natural terrestrial ecosystems. We examined sulfur-driven NO3− reduction using streambank soils in a headwater catchment underlain by marine sedimentary rock in Akita, Japan. In a catchment exhibiting higher sulfide content in streambed sediment, we sampled two adjacent streambank soils of streambank I (two layers) and of streambank II (eight layers). Anaerobic long-term incubation experiments (40 days, using soils of streambank I) and short-term incubation experiments (5 days, using soils of streambank II) were conducted to evaluate variations of N solutes (NO3−, NO2−, and NH4+), N gases (NO, N2O), and the bacterial flora. In both experiments, two treatment solutions containing NO3− (N treatment), and NO3− and S2O32− (N + S treatment) were prepared. In the N + S treatment of the long-term experiment, NO3− concentrations gradually decreased by 98%, with increases in the SO42−, NO2−, NO, and N2O concentrations and with not increase in the NH4+, indicating denitrification had occurred with a high probability. Temporal accumulation of NO2− was observed in the N + S treatment. The stoichiometric ratio of SO42− production and NO3− depletion rates indicated that denitrification using reduced sulfur occurred even without additional S, indicating inherent S also served as an electron donor for denitrification. In the short-term incubation experiment, S addition was significantly decreased NO3− concentrations and increased NO2−, NO, and N2O concentrations, especially in some subsoils with higher sulfide contents. Many denitrifying sulfur-oxidizing bacteria (Thiobacillus denitrificans and Sulfuricella denitrificans) were detected in both streambank I and II, which dominated up to 5% of the entire microbial population, suggesting that these bacteria are widespread in sulfide-rich soil layers in the catchment. We concluded that the catchment with abundant sulfides in the subsoil possessed the potential for sulfur-driven NO3− reduction, which could widely influence N cycling in and NO3− export from the headwater catchment.

With global population growth, substantial increases in food demand have aggravated the problem of methane emissions, which most researchers have been focusing on reducing in paddy fields. Since methane is a clean energy source and the main component of natural gas, we established a novel and efficient on-site biomethane production and collection system using rice straw in the paddy field without any other methane fermentation tan, implying that this system will contribute to reduction of the greenhouse effect. We have termed this the “GET system”: the efficient production of biomethane gas (G) as renewable energy (E) from a tanbo (T), which means “paddy field” in Japanese. The results showed that, during mesophilic (20–40 °C) digestion, the GET system could generate up to 100 L/day m−2 of biomethane at a methane concentration of more than 60% from rice straw, which was at the optimum application amount of 14 kg m−2 in the study. Moreover, the resulting 25.7% of carbon in rice straw was simultaneously stored in the soil as fermentation residue, improving soil fertility during the next rice-growing season and creating a carbon sink. The GET system is therefore a game-changing technology, offering a potential solution to the demand for food and energy without the emission of methane, thus reducing atmospheric carbon dioxide (CO2). Promotion and use of the GET system as effective clean technology will play a key role in global sustainable development in the near future.