Suppression of chlorate degradation in longan plantation soil after multiple applications of chlorate and accelerated degradation by sugar amendment

Potassium chlorate is widely used as an active substance for flower induction in longan plantations for off-season production of fruits in northern Thailand. Contamination of groundwater with residual chlorate in soil is a cause for concern because of its toxicity to human health. Based on the assumption that potassium chlorate might adversely affect chlorate-reducing soil microorganisms due to its disinfecting nature, the effect of repeated applications of chlorate on chlorate decomposition in soil was examined in a laboratory study. In the longan plantation field soil which had received an annual application of potassium chlorate for three years, chlorate decomposition was slower than that in the soil without application. The suppression of chlorate decomposition was confirmed in the soil to which potassium chlorate was applied repeatedly in the laboratory. However, even in such soils that had received multiple applications of potassium chlorate, chlorate decomposition was remarkably enhanced by soil amendment with glucose or sucrose. Sugar amendment seemed to be an effective way of cleaning-up the residual chlorate in longan plantation field soils.