Greenhouse Gas Mitigation Potential of Alternate Wetting and Drying for Rrice Production at National Scale – A Modelling Case Study for the Philippines

Worldwide, rice production contributes about 10% of total greenhouse gas (GHG) emissions from the agricultural sector, mainly due to CH4 emissions from continuously flooded (CF) fields. Alternate Wetting and Drying (AWD) is a promising crop technology for mitigating CH4 emissions and reducing the irrigation water currently being applied in many of the world's top rice-producing countries. However, decreased emissions of CH4 may be partially counterbalanced by increased N2O emissions. In this case study for the Philippines, the national mitigation potential of AWD is explored using the process-based biogeochemical model LandscapeDNDC. Simulated mean annual CH4 emissions under conventional rice production for the time period 2000 - 2011 are estimated as 1,180163 Gg CH4 yr-1. During the cropping season, this is about +16% higher than a former estimate using emission factors. Scenario simulations of nationwide introduction of AWD in irrigated landscapes suggest a considerable decrease of CH4 emissions by -23%, while N2O emissions are only increased by +8%. Irrespective of field management, at national scale the radiative forcing of irrigated rice production is always dominated by CH4 (>95%). The reduction potential of GHG emissions depends on, e.g., number of crops per year, residue management, amount of applied irrigation water and sand content. Seasonal weather conditions also play an important role, since the mitigation potential of AWD is almost double as high in dry as compared to wet seasons. Furthermore, this study demonstrates the importance of temporal continuity, considering off-season emissions and the long-term development of GHG emissions across multiple years.

全球范围内，水稻生产约占农业部门温室气体（GHG, Greenhouse Gas）总排放量的10%，其主要排放源为持续淹水（CF, Continuously Flooded）稻田产生的甲烷（CH4）。干湿交替灌溉（AWD, Alternate Wetting and Drying）是一项极具应用前景的作物种植技术，可在全球多数主要水稻生产国削减甲烷排放并降低灌溉用水量。然而，甲烷排放的减少可能会被一氧化二氮（N2O）排放的增加部分抵消。本研究以菲律宾为案例，基于过程型生物地球化学模型LandscapeDNDC，探究了AWD技术的全国温室气体减排潜力。研究估算，2000-2011年常规水稻生产模式下的年均模拟甲烷排放量为1180±163 Gg CH4 yr⁻¹。在种植季内，该数值较此前基于排放因子的估算结果高出约16%。针对灌溉稻田全国范围内推广AWD技术的情景模拟显示，甲烷排放量可显著降低23%，而一氧化二氮排放量仅增加8%。无论田间管理方式如何，在国家尺度上，灌溉水稻生产的辐射强迫始终以甲烷为主（占比超95%）。温室气体减排潜力受多种因素影响，例如一年种植季数、秸秆管理方式、灌溉用水量以及土壤砂粒含量等。季节性天气条件同样发挥关键作用：AWD的减排潜力在旱季几乎是雨季的两倍。此外，本研究还阐明了时间连续性的重要性，需考量休季排放以及多年间温室气体排放的长期演变。