Denitrification in soil amended with thermophile-fermented compost suppresses nitrate accumulation in plants

NO 3 − is a major nitrogen source for plant nutrition, and plant cells store NO 3 − in their vacuoles. Here, we report that a unique compost made from marine animal resources by thermophiles represses NO 3 − accumulation in plants. A decrease in the leaf NO 3 − content occurred in parallel with a decrease in the soil NO 3 − level, and the degree of the soil NO 3 − decrease was proportional to the compost concentration in the soil. The compost-induced reduction of the soil NO 3 − level was blocked by incubation with chloramphenicol, indicating that the soil NO 3 − was reduced by chloramphenicol-sensitive microbes. The compost-induced denitrification activity was assessed by the acetylene block method. To eliminate denitrification by the soil bacterial habitants, soil was sterilized with γ irradiation and then compost was amended. After the 24-h incubation, the N 2 O level in the compost soil with presence of acetylene was approximately fourfold higher than that in the compost soil with absence of acetylene. These results indicate that the low NO 3 − levels that are often found in the leaves of organic vegetables can be explained by compost-mediated denitrification in the soil.