The possible substrates of aromatic acid (benzoic acid, 2-phenylpropionic acid, 3-phenyl-propionic acid) synthesis were identified by incubating flooded soils and soil suspensions with different organic matter. Rice plants were grown in small pots containing soil supplied with different organic matters to evaluate inhibitory effect on rice seedling growth within 3 weeks after transplanting. The results are summarized as follows: 1) Aromatic acids were found to be metabolites produced in soil microbial fermentation in reduced soils; 2) Accumulation of aromatic acids in the soil depended on the properties of the applied organic matter, and the accumulation loosely correlated with decomposability by organic matter microbes; 3) During a short period, such as 2-3 weeks after flooding, the growth inhibition of rice plants depended on the quantity of easily decomposable components in the applied organic matter not on the C/N value; and 4) Rice seedling growth was inhibited by application of the same organic matter that led to the accumulation of aromatic acids in the soil.
