Simulating the effects of soil temperature and moisture in the off-rice season on rice straw decomposition and subsequent CH4 production during the growth season in a paddy soil

To understand the effects of soil temperature and moisture on rice straw decomposition during the off-rice season and subsequent CH4 production in rice growth season, we firstly incubated a paddy soil with 13C-labeled straw addition under four temperature (±5, 5, 15, and 25 °C) and two moisture levels (60 and 100 % water-filled pore space (WFPS)) under aerobic conditions for 24 weeks and subsequently incubated those samples anaerobically at 30 °C and under submerged conditions for 4 weeks. Temperature change at ±5 °C was used to simulate the diurnal variation of soil temperature between night (−5 °C, 12 h) and day (5 °C, 12 h) during the corresponding freeze-thaw cycles. Our results showed that both increased soil temperature and moisture significantly promoted straw aerobic decomposition as observed by carbon dioxide (CO2) production, soil organic carbon (SOC) content, and its δ13C value in this incubation experiment. During the anaerobic incubation, straw-amended soil samples remarkably promoted CH4 production and CH4/total decomposed C. Both CH4 and CO2 productions in straw-amended soil samples were lowest at 25 °C and 100 % WFPS and highest at ±5 °C and 100 % WFPS, which could be attributed to a depletion of labile organic C derived from straw during the 24-week aerobic incubation. Total rates of straw C decomposition calculated from decomposed C production, SOC content, and δ13C value in the whole incubation (aerobic + anaerobic) ranged from 29.8 to 48.1, 45.0 to 64.2, and 25.7 to 35.7 %, respectively. These results imply that increasing soil temperature during the off-rice season promotes the rice straw decomposition and leads to a decrease in CH4 production during the subsequent rice growth season under conditions of northeastern Japan.