Utilization of the Great Duckweed (Spirodela polyrhiza) as a Living Mulch in Organic Rice Farming

Abstract

The use of floating plants with shading effect as living mulch for weed control in organic paddy rice farming was considered, and a comparative study was conducted focusing on Spirodela polyrhiza and Lemna aoukikusa. The shading rate of Spirodela polyrhiza is higher than that of Lemna aoukikusa. It was found that Spirodela polyrhiza has a higher light shading rate than Lemna aoukikusa, and more effectively shaded a specific wavelength range that promotes weed germination. This result suggests that Spirodela polyrhiza is a more effective floating plant for weed control than Lemna aoukikusa. The weed control effect of the mulch was verified by pot tests, and it was confirmed that the mulch suppressed the germination and growth of weeds, including the weed, Monochoria vaginalis which is considered to be difficult to control. The results of the rice field trials showed that the mulch suppressed the increase in daily maximum water temperature, which resulted in lower average water temperature until the panicle formation stage of rice plants, but had no effect on daily minimum water temperature, floodwater of dissolved oxygen concentration, pH, electrical conductivity, and redox potential. The weed control effect of the duckweed mulching was confirmed in the field, suppressing weeds, including Monochoria vaginalis, and the nitrogen produced by the decomposition of duckweed mulching increased the inorganic nitrogen concentration in the surface soil. The nitrogen produced by the decomposition of the duckweed mulching increased the concentration of inorganic nitrogen in the surface soil, which may have contributed to the growth and yield improvement of rice plants. These results suggest that organic farming of paddy rice using living mulch with duckweed may be an effective technology because it suppresses weed germination and provides nitrogen to rice plants, which may contribute to the growth and yield improvement of paddy rice.