Response of duckweed to different irrigation modes under different fertilizer types and rice varieties: Unlocking the potential of duckweed (Lemna minor L.) in rice cultivation as "fertilizer capacitors"

Duckweed (Lemna minor L.) is a common plant in paddy fields, absorbing nutrients, heavy metals, and agrochemical residues from irrigation water, thus playing a pivotal role in ecosystem sustainability. To reveal the combined effects of irrigation modes, rice cultivars, and fertilizer types on duckweed, tank experiments were conducted in two rice-growing seasons (2019 and 2020) in Jiangsu Province, China. The experiment included three factors, each consisting of two levels: rice cultivars (R) (japonica rice (J) and indica rice (X)), fertilizer types (F) (organic fertilizer (O) and chemical fertilizer (C)), and irrigation modes (I) (controlled irrigation (1) and shallow-frequent irrigation (2)). Results indicated that organic fertilizer significantly increased the total phosphorus concentration by 396% and promoted DCR by 703% in floodwater as compared with chemical fertilizer. The significantly higher tiller numbers of indica rice increased the shade level, repressing maximum electron transfer rate (ETRmax) of duckweed, and consequently reduced DCR by 32.8% as compared with japonica rice. A cyclic pattern of Production-Death-Reproduction of duckweed under controlled irrigation was observed, which depended on the periodic cycle of Appear-Disappear-Reappear of the floodwater. Setting the irrigation lower limit to 87.9% saturation water content(θs) can benefit the regeneration of duckweed after re-watering. Similar to a capacitor that could store and release electric charge in circuit system by regulating voltage, under the application of organic fertilizer (battery), duckweed (capacitors) efficiently absorbs, decomposes, and subsequently reabsorbs nutrients by the regulation of controlled irrigation (switch), releasing fertility gradually for the rice plant (electrical appliance).