Production of volatile fatty acids (VFA) in flooded soils amended with rice straw and azolla

Studies in the laboratory and screenhouse were conducted to determine the decomposition products, particularly low molecular weight volatile fatty acids (C2 to C4) in Tarlac loam, Guadalupe clay and Luisiana clay amended with either azolla or rice straw under flooded condition. Each soil was amended with azolla at 15 t/ha and rice straw at 7.5 t/ha to increase the carbon content of the soil and compare the accumulation of organic acids. Rice variety IR 72 was used in the screenhouse experiment to determine the effect of organic acids. Under laboratory conditions, volatile fatty acids were detected during the early stages of the decomposition period: butyric acid was observed from 2 until 25 days of incubation whereas propionic acid persisted up to 65 days. Higher amounts of organic acid were detected when the soils were amended with azolla or rice straw compared to control. Higher amounts of volatile fatty acids were produced in water logged soil planted with rice compared to unplanted soil. In cropped soil, acetic acid was observed after 9 days of incubation and decreased to zero at 67 days. It was detected again at 90 days. Acetic acid was in the order of Luisiana clay Tarlac sandy loam Guadalupe clay from 9 to 67 days. Likewise, butyric acid in cropped soil was at its peak level at 9 and 24 days of decomposition in all soils except for Luisiana clay which was at its peak up to 44 days and minimal amount was detected when propionic acid was at its peak of production. Higher amounts of total volatile fatty acids were produced when rice straw was higher after 90 days in azolla treated Tarlac sandy loam. In contrast, a higher amount of volatile fatty acid was produced when Luisiana clay was amended with azolla. A reduction in growth of rice was observed when soils were amended with either azolla or rice straw as shown in the reduced plant height apparently at 23 and 35 days after transplanting in Guadalupe clay and was more pronounced in the amended Luisiana clay. However, growth reduction in plants grown on the different soils studied were not significantly different.