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Short chain organic acid dynamics in solution of flooded soil treated with ryegrass residues

Short chain organic acids (C2 to C5), toxic for paddy rice, can be produced in flooded soils treated with organic residues. In order to study the liberation of organic acids into the solution of a flooded soil treated with residues of a ryegrass crop, an experiment was carried out in the greenhouse. The trial was planned in random blocks with three replicates using the following treatments: (a) without ryegrass residues, (b) ryegrass residues kept on the soil surface, and (c) ryegrass residues incorporated into the soil. The organic acids acetic, propionic, butyric, isobutyric, valeric, and isovaleric, were evaluated during eight weeks by gas chromatography. Results showed that the use of ryegrass residues increases the organic acids concentration in the soil solution. The highest concentrations were obtained at 10 cm deep with incorporated ryegrass. However, the organic acid concentration at 2 cm deep with ryegrass kept on the surface was equally high, probably at toxic levels for paddy rice. Acetic acid was the one presented at the highest concentration in the soil solution for all treatments.