Climate change mitigation through irrigation strategies during rice growing season is off-set in fallow season

Non-continuous flooding irrigation practices, such as alternate wetting and drying (AWD) and mid-season drainage (MSD), have been implemented in rice agroecosystems to reduce water use and mitigate climate change. Draining fields reduces methane (CH₄) emissions, as soil aeration decreases the abundance and activity of soil methanogens. Mitigation effects during the growing season have been widely studied. However, there is a knowledge gap regarding potential effects these growing season practices might have on subsequent fallow season emissions. This is relevant when assessing overall annual CH₄ emissions, particularly in systems in which fallow seasons account for a significant part of these. A field experiment was implemented in the Ebro Delta region (Catalonia, Spain) with the objective of identifying potential effects of growing season AWD and MSD on CH₄ emitted during the following flooded fallow season, in comparison to continuously flooded fields. Both emissions and the structure of soil microbial communities were analyzed for rice field plots under the assessed irrigation strategies during the growing season and later for a continuously flooded mesocosm across the fallow season. Both practices achieved an average 86% decrease in CH₄ fluxes when compared to continuous flooding during the growing season. AWD resulted in the highest fallow season emissions, leading to increases in overall annual cumulative CH₄ emissions (+8%), global warming potential (+30%) and yield-scaled global warming potential (+70%) compared to continuous flooding. Growing season AWD decreased the relative abundance of both methanogens and methanotrophs in the fallow season. Reduced methanotroph communities might lead to lower CH₄ consumption, resulting in higher fallow season emissions and offsetting the mitigation effect achieved during the growing season. Under the studied conditions, MSD represented a more effective mitigation strategy. These results highlight the importance of considering both rice growing and fallow season when assessing climate change mitigation strategies.