Modeling aerobic decomposition of rice straw during the off-rice season in an Andisol paddy soil in a cold temperate region of Japan: Effects of soil temperature and moisture

Submerged rice paddies are a major source of methane (CH₄) which is the second most important greenhouse gas after carbon dioxide (CO₂). Accelerating rice straw decomposition during the off-rice season could help to reduce CH₄ 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 δ¹³C-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) CO₂ production; (ii) change in the soil organic carbon (SOC) content; and (iii) change in the δ¹³C 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 CO₂ production compared to those calculated by the change in the SOC content, or in its δ¹³C 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 Q₁₀ 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 CH₄ 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.