Suppressive and promotive effects of rice bran on germination of Monochoria vaginalis in organic rice (Oryza sativa L.) production

In organic paddy fields, one of the toughest weeds to control is Monochoria vaginalis (Burm. f.) Kunth. The application of fresh rice (Oryza sativa L.) bran to soil suppresses the germination of this weed. In stark contrast, the scarcely decomposable component of this material stimulates the germination. This review focuses on the elucidation of these conflicting traits. One of the chemical factors for the suppression is toxic substances such as organic acids. In addition, rice bran application enhances the piling of small soil particles by tubificids on the soil surface. This layer acts as a physical factor that slows germination, presumably because its fine grains prevent light from reaching the seed. The newly accumulated soil with small particle was sampled for the analysis of the settled soil volume in water (SSVW). This value corresponds to the mud volume in water per dry weight. The electrical conductivity (EC) of the soil solution and the SSVW of the soil surface are useful indicators to evaluate the chemical and physical factors that control germination, respectively. EC and SSVW have been traced in paddy fields where rice bran was applied. The chemical and physical factors were active during the earlier and latter halves of the month after transplanting, respectively. These factors worked in a complementary manner. Paddy soil is typically equipped with the ability to reduce oxygen (O-2). This trait enhances the number of M. vaginalis plants because hypoxic conditions are preferable for germination. Anaerobic soil conditions are not required for the expression of O-2-reducing power. A long-term application of rice bran might intensify this power because a scarcely decomposable component of rice bran contains a significant amount of O-2-reducing substances. The presence of conflicting factors that suppress and enhance germination might destabilize the efficacy of the rice bran.