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Submerged rice paddies are a major source of methane (CH 4 ) which is the second most important greenhouse gas after carbon dioxide (CO 2 ). Accelerating rice straw decomposition during the off-rice season could help to reduce CH 4 emission from rice paddies during the single rice-growth season in cold temperate regions. For understanding how both temperature and moisture can affect the rate of rice straw decomposition during the off-rice season in the cold temperate region of Tohoku district, Japan, a modeling incubation experiment was carried out in the laboratory. Bulk soil and soil mixed with 2% of δ 13 C-labeled rice straw with a full factorial combination of four temperature levels (−5 to 5, 5, 15, 25°C) and two moisture levels (60% and 100% WFPS) were incubated for 24 weeks. The daily change from −5 to 5°C was used to model the freezing-thawing cycles occurring during the winter season. The rates of rice straw decomposition were calculated by (i) CO 2 production; (ii) change in the soil organic carbon (SOC) content; and (iii) change in the δ 13 C value of SOC. The results indicated that both temperature and moisture affected the rate of rice straw decomposition during the 24-week aerobic incubation period. Rates of rice straw decomposition increased not only with high temperature, but also with high moisture conditions. The rates of rice straw decomposition were more accurately calculated by CO 2 production compared to those calculated by the change in the SOC content, or in its δ 13 C value. Under high moisture at 100% WFPS condition, the rates of rice straw decomposition were 14.0, 22.2, 33.5 and 46.2% at −5 to 5, 5, 15 and 25°C temperature treatments, respectively. While under low moisture at 60% WFPS condition, these rates were 12.7, 18.3, 31.2 and 38.4%, respectively. The Q 10 of rice straw decomposition was higher between −5 to 5 and 5°C than that between 5 and 15°C and that between 15 and 25°C. Daily freezing-thawing cycles (from −5 to 5°C) did not stimulate rice straw decomposition compared with low temperature at 5°C. This study implies that to reduce CH 4 emission from rice paddies during the single rice-growth season in the cold temperate regions, enhancing rice straw decomposition during the high temperature period is very important. ARTICLE HISTORY

We have developed a method for staining the root systems of neighboring plants distinguishably in pot culture to facilitate studies of the interactions between plants. Pot soil was desiccated until the plant wilted, and then the shoot was cut and a dye solution (Fantasy) was pressure-injected into the roots at 0.05 MPa (gauge). All the roots, including fine roots of double-planted tomato (Lycopersicon esculentum var. Momotaro), in a 10-cm-diameter pot were well stained, but the root hairs were not. The volume of dye and the time required for staining were 6.4 mL and 5 h, respectively (root length 179 m plant; root fresh weight 2.62 g plant −1 on average). The root distribution of double-planted tomato in a 40 L container was well determined. The volume of dye and the time required for staining were 100 mL and 40 h, respectively (root length 1,514 m plant; root dry weight 4.93 g plant −1 on average). Each plant's roots accounted for 37-52% of the total root weight at an intermediate position between the two plants, whereas at the other positions, the roots of either plant were dominant (82-99% of total root weight). In principle, the proposed method is applicable to whole plant species with vascular systems.

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