Abstract
Objectives of the present experiment were to confirm the inhibitory effect of volatile fatty acids (VFA) on denitrification found by previous experiments and to clarify its mechanisms. Formic and acetic acids were examined for their effects on denitrification and anaerobic decomposition of organic substrates (these organic acids themselves, easily decomposable soil organic carbon and glucose) in two soils : Konosu soil (Grey lowland soil, Aquept) and Tanashi soil (Black volcanic ash soil, Andept). The organic acids were confirmed to inhibit decomposition of the organic substrates (direct effect) even in the absence of nitrate. In addition, denitrification in the presence of organic acids was found to strengthen the inhibition effect on the microbial decomposition of the organic substrates (indirect effect). This was caused by formation and accumulation of nitrite and was enhanced by both the increased reaction rate of denitrification and by the increased concentration of the added organic acids. Konosu soil was more vulnerable to the direct effect than Tanashi soil. However, these two soils were rather similar in regard to the indirect effect. The added formic acid appeared more inhibitory than acetic acid if their amounts were calculated by the amount of organic carbon but rather similar if their amounts were calculated by the number molecule.
