Integrating alternate wetting and drying irrigation with duckweed for potential microplastic mitigation in rice ecosystems

Water-saving irrigation in rice fields can reduce water input and enhance water productivity, while widespread microplastics (MPs) contamination introduces an environmental risk. Duckweed is a hydrophyte that commonly floats in rice fields and can capture MPs. Regular harvesting of these contaminated plants prior to decomposition could prevent secondary pollution and ensures the permanent removal of accumulated MPs from the paddy ecosystem. However, it remains unknown whether water-saving irrigation, such as alternate wetting and drying irrigation (AWD), can promote MPs capturing through duckweed physio-anatomical modifications. In this study, MPs-contaminated rice-duckweed system was exposed to two irrigation regimes, i.e. conventional continuous flooding irrigation (CF) and AWD. The results showed that AWD led to duckweed capturing up to 16.0 -fold more MPs than under CF. We discovered that this increase was due to the considerably higher number of MPs particles per unit surface area (or per unit length) captured by duckweed, despite the reduction in duckweed coverage rate (42.4 %-48.8 %) and root length (23.2 %-67.7 %) during dry periods of AWD. We further revealed that the greatly increased MPs numbers per unit area on the frond of duckweed under AWD was primarily due to improved adhesion and physical interception, achieved through processes such as sticking, trapping, and entangling, facilitated by the rougher surface of duckweed. Notably, experimentally validated detachment curves of MPs, well fitted with linear with lower plateau models (P < 0.01), revealed that AWD resulted in greater capturing of MPs on abaxial fronds than CF. Our work uncovers a potential microplastic mitigation method in rice ecosystems by using duckweed combined with AWD in future.