Influence of tillage practice on major pathways of CH4 emission in rice paddy field

 Generally, during the paddy rice cultivation period, CH₄ produced in the soil is reported to be released to the atmosphere through three pathways: diffusion (<1%), bubbles (<10%), and via rice (> 90%). However, there are few studies have measured gas diffusion coefficient for soil below surface of the water, and there is no study has provided an accurate understanding of CH₄ dynamics in paddy fields. Furthermore, there are few studies that understanding the CH₄ dynamics in fertilizer-free and pesticide-free paddy fields, which is mainly controlled by inter-tillage practices. Therefore, this study aimed to clarify the effects of tillage and the number of inter-tillage and the presence or absence of fertilizer and pesticide on the CH₄ dynamics in rice paddy soil. This study compared three types of CH₄ flux, which were total CH₄ flux from rice paddy field measured by transparent chamber with plants, and soil derived CH₄ flux measured by dark chamber without plants, and gas diffusion flux calculated as a product of the gas diffusion coefficient and measured soil gas concentration gradient at the depths of 0-5 and 5-10cm. And they were compared with in the five rice cultivation periods (flooding, mid-drying, intermittent irrigation, drainage, and fallowing) and in the four treatment plots (conventional farming (CF), and fertilizer- and pesticide-free farming with zero-inter-tillage(T0), two-inter-tillage(T2), and five-inter-tillage (T5)). The CF was conducted according to the regional recommendation for tillage, fertilization and pest and weed control. The results showed that the peak of total CH₄ flux was observed in the mid-drying and intermittent irrigation periods in all treatments, and that the CH₄ flux via rice plant accounted for 60-90% of the total CH₄ flux. The CF showed significantly highest CH₄ emission during the periods, and the increase of the number of inter-tillage tended to increase the CH₄ emission. In the drainage period, the CH₄ flux by bubbles in the CF and T5 accounted for more than 80% of the total CH₄ flux. In the fallowing period, the diffusion CH₄ flux at the depth of 5-10cm increased in all treatments, but the low total CH₄ emission and increased CO₂ emission. This study revealed that incorporation of organic matter into soil in conventional rice farming tended to increase CH₄ emission. The main pathway of CH₄ emission from rice paddy field was via rice, and it was influenced by tillage significantly. The decomposition of organic matter from rice straw and weeds incorporated into soil was the source of the bubble of CH₄. Furthermore, it seemed that the most of diffusively transferred CH₄ was easily oxidized to CO₂.