Duckweed (Lemna minor L.) as a natural-based solution completely offsets the increase in ammonia volatilization induced by soil drying and wetting cycles in irrigated paddies.

One of the primary pathways of nitrogen loss in rice fields, ammonia (NH) volatilization resulting in low nitrogen use efficiency and contributes significantly to near-surface atmospheric pollution. Duckweed (Lemna minor L.), a common small floating plant in rice fields, often completely covers the water surface. However, the extent to which this biotic cover affects ammonia flux remains unclear. A three-year field experiment was conducted to investigate the effects of duckweed cover on NH volatilization in rice fields under two different irrigation management practices (Flooding irrigation vs. alternate wetting and drying irrigation). In the duckweed-free paddies, alternate wetting and drying irrigation significantly increased the cumulative ammonia emissions over the full observation period by 16.6 %, 22.5 % and 7.8 % in 2020, 2021, and 2022, respectively, compared to flooding irrigation. Compared to the duckweed-free paddies, the presence of duckweed significantly mitigated cumulative NH volatilization in rice fields, regardless of the irrigation regimes. Under flooding irrigation, the reduction in NH volatilization with duckweed cover reached 6.3 %, 33.2 % and 37.6 % over three consecutive years. The reduction was 23.3 %, 48.2 % and 41.8 % under alternate wetting and drying irrigation, demonstrating that duckweed achieved greater reductions in NH volatilization under alternate wetting and drying irrigation than flooding irrigation. An independent incubation experiment revealed that physical coverage, ammonium ion absorption and surface water temperature reduction were primary factors contributing to duckweed-induced NH emission mitigation, accounting for 50.9 %, 28.4 %, and 20.7 %, respectively. The present study indicates that duckweed might prove a promising nature-based solution for mitigating the potential environmental risks of excessive reactive nitrogen outputs from rice paddies, and for promoting the broader application of alternating wet and dry irrigation.