Degradation and movement of chlorate in longan plantations.

摘要

In 1998 it was found that chlorate was able to induce flowering in longan trees (Dimocarpus longan L.). Since then potassium chlorate have been used in most longan plantations all over Thailand. The maximum rate in the early years of application was approximately 500 g for a medium size tree. An environmental impact study on the effects of chlorate application was carried out in the year 2000. It was revealed that if farmers apply once a year chlorate is unlikely to accumulate over the years and the long-term effect on soil environment was nil. However, due to reasons which are being investigated the application rate has gradually been increased. In 2003 the maximum application dose for a medium size tree was approximately 2,000 g which is out of the scope of the above mentioned study. Consequently, there has been concern on the accumulation and leaching of residues through soil profiles and the risk of ground water contamination in the long run. Thus, a re-study on the environmental impact has been done. Movement and degradation of the heavily applied chlorate in four control plots on four representative soils were monitored over a period of two years. Samples of these four profiles at in various depths, ranging from 0 to 200 cm were taken and samples have been incubated with chlorate and chlorate degradation has been monitored under laboratory conditions. Additionally, residual chlorate on 42 commercial longan plantations was monitored. The results suggested that organic matter content was the most important factor influencing chlorate degradation. The degradation was observed to be very slow in soils with less than 0.2% of organic carbon, regardless of their textures. It was also found that chlorate moved relatively fast in well drained sandy soils. Therefore, a few months after application almost none of it was found at the depth above 80 cm but significant concentrations of the residues were found at a depth of 150-230 cm at two year period in sandy soils of the control plots. In loamy and clayey soils significant chlorate residues were found in a depth of less than 100 cm. Almost none of it was found deeper than 140 cm within the study period. Similar distributions of chlorate residues were found in soil profiles under the tree canopies in the commercial orchards with these kinds of soils. Consequently, methods to control the application rate have to be investigated. Furthermore, measures to eliminate chlorate residues after flowering of longan trees should be imposed in order to eliminate long term impacts and leaching of residues to the ground water in the long run. Application of any form of sugars was proven to be able to enhance the degradation of chlorate residues.