Effects of continuous and intermittent flooding on greenhouse gas emission from rice paddy field under RCP-8.5 scenario in South Korea estimated by DNDC model

Methane is a potent greenhouse gas that is over 25 times greater than carbon dioxide, and rice paddy soil represents one of the largest artificial source of methane emission. Improvements in water-management techniques such as intermittent flooding are expected to mitigate methane emissions by increasing soil redox potentials. The objectives of this study were to estimate methane emission of nationwide paddy field (1km² scale grid cell) in South Korea under RCP-8.5 scenario using the process-based model (DNDC). To do this, 1) input parameters (daily climate data, soil pH, soil organic matter contents, soil bulk density, and soil clay contents) were collected from domestic national organizations (Korea Meteorological Administration and Rural Development Administration). 2) Simulations of the DNDC model were performed from the paddy field for rice cultivation with continuous and intermittent (7-days intervals) flooding. 3) Following the 5-year spin-up from 2011 to 2015, model was simulated an 80-year from 2016 to 2095 for each grid cell. Total methane emissions were 7.9 and 15.7 Mt CO₂-eq yr^-1 in the 2020s (average of 2016 to 2025) and 2090s (average of 2086 to 2095), respectively with continuous flooding (water management method currently used in Korea). According to RCP-8.5, during the flooding period, the temperature increased from 25.3^oC in the 2020 to 30.8^oC in the 2090, suggesting that the microbial activity index of methanogen steadily increased until 2090. In the case of intermittent flooding, methane emissions were 1.0 and 1.9 Mt CO₂-eq yr^-1 in the 2020s and 2090s, respectively, which was much lower than the continuous flooding as the redox potential increased at 7-day intervals. Compared to continuous flooding in the 2090s, carbon dioxide emissions of the intermittent flooding was promoted by approximately 0.9 Mt CO₂-eq yr^-1 because of the increased contact between soil and air during drainage conditions. Nevertheless, the global worming potential of intermittent flooding was about 40% of the continuous flooding in the 2090s.