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Rice bran application is practically used by Japanese organic rice farmers as a common measure in controlling Monochoria vaginalis, a known weed in organic rice production. To elucidate the inhibitory mechanisms of rice bran application on M. vaginalis germination, we focused on volatile fatty acids (VFAs) and aromatic carboxylic acids (ACAs) as organic acids produced through rice bran decomposition in the soil, which can possibly suppress M. vaginalis germination. Both solo solutions of 1 mM VFAs (propionic or isobutyric acid) and 0.2 mM ACAs (3-phenylpropionic or 4-phenylbutyric acid) exhibited M. vaginalis germination suppression, especially at pH 4.6, the lowest pH tested. In the 0.5–3 mM VFA solutions at pH 4.6, germination was strongly suppressed in the order of isobutyric > propionic > butyric > acetic acid. With the 0.05–0.2 mM ACA solutions at pH 4.6, the germination was strongly suppressed in the order of 3-phenylpropionic > 4-phenylbutyric > 2-phenylpropionic acid. Benzoic acid did not exhibit significant germination suppression. The solutions of 0.5 mM VFAs and 0.05 mM ACAs exhibited no suppression when applied separately; however, the certain mixture of a VFA and an ACA at the aforementioned concentration, for example, 0.5 mM isobutyric acid and 0.05 mM 3-phenylpropionic acid, exhibited significant germination suppression. These results showed the several combinations of VFAs and ACAs at even non-effective concentrations synergistically suppressed M. vaginalis germination. Abbreviations:2PPA: 2-phenylpropionic acid; 3PPA: 3-phenylpropionic acid; 4PBA: 4-phenylbutyric acid; ACA: aromatic carboxylic acid; EC: electrical conductivity; M. vaginalis: Monochoria vaginalis; VFA: volatile fatty acid

Organic paddy rice (Oryza sativa L.) cultivation can be negatively impacted because of the weeds growth like Monochoria vaginalis. Although the rice bran application is useful in combating weed, its performance is not consistent. In this study, laboratory experiments were conducted to elucidate the effects of rice bran on the suppression and promotion of germination of M. vaginalis. Suppression experiments involved incubating mixtures of flooded soil and rice bran. This was followed by a germination experiment in the collected soil solutions. A negative and significant correlation was observed between germination and the electric conductivity (EC) of the solution. None of the seeds germinated when the EC of the Gray Lowland soil was over 130 mS m−1. Conversely, the seeds in the Andosols’ solution germinated even when EC was over 130 mS m−1. Thus, the solutions of the Andosols had different mechanisms of suppression than the Gray Lowland soils. To understand the promotion of germination, air-dried soil was prepared after the flooded incubation of rice bran and soil mixture, simulating a long-term application of rice bran. Germination was analyzed in a filtrate of the mixture of air-dried soil and distilled water. The addition of rice bran significantly promoted germination. There was a negative and significant correlation between germination and dissolved oxygen (DO) suggesting that hardly decomposable components like antioxidants in rice bran decreased O2 and stimulated germination because the seeds of M. vaginalis require very little O2 for germination.

We investigated the effect of poultry manure (PM) on the occurrence and early growth of Monochoria vaginalis in relation to soil solution electrical conductivity (SSEC). PM was applied at rates corresponding to 0 g of nitrogen (N) m−2 (PM-0), 1 g N m−2 (PM-1), 3 g N m−2 (PM-3), and 5 g N m−2 (PM-5). At 7 d post-seeding, the soil solution was sampled to measure EC, and also the emergence and growth of M. vaginalis were evaluated. The emergence rate of M. vaginalis decreased with increasing application rate of PM and SSEC. SSEC was significantly negatively correlated with the emergence rate of M. vaginalis seedlings. The average leaf number and length of M. vaginalis did not differ between PM-0, PM-1, and PM-3, but were significantly lower in PM-5. In summary, PM would allow to better control the emergence and early growth of M. vaginalis.

Field experiments were conducted to analyze the relationship between the settled soil volume in water (SSVW) and the growth of Monochoria vaginalis (Burm. f.) Kunth under organic farming conditions. SSVW corresponds to the mud volume per dry matter weight. Soil was sampled from the superficial layer of the topsoil (