| Metsulfuron-methyl is a sulfonylurea herbicide commonly used for the control of a number of broadleaf and grass weeds in cereal crops, characterized by very low application rates, high herbicidal activity, and good crop selectivity. However, the extremely low residues of sulfonylurea herbicides in soil may cause damage and high phytotoxicity to rotation or substitution crops, and the leaching potential in soil is relatively high. Therefore, it is of significant value to study the rapid degradation of sulfonylurea herbicides in soil. In the present study, a series of laboratory experiments were conducted to determine the rapid biodegradation of metsulfuron-methyl and the eco-chemistry mechanism in rhizospheric ecosystem by the method of soil chemistry and environmental microbiology, and the in-situ co-remediation technology of organic pollutants contaminated soil by plant and soil fungi were also presented. The main results were summarized as follows: 1. The effects of metsulfuron-methyl on microbial population and enzyme activities in wheat (Triticum aestivum L.) rhizospheric soil collected from the southern China were investigated. It was found that the growth of common bacteria was significantly inhibited by 2 μg·g-1 metsulfuron-methyl (P<0.01), but the number of tolerant bacteria just decreased in the first 30 days and recovered slowly with respect to the strengthening of rhizospheric effects and the degradation of metsulfuron-methyl. The amounts of common fungi only changed in a small extent after application of the herbicide, however, the population of tolerant fungi increased obviously, the difference of which between the treated soil and the control was significant at the probability level of 0.01 on day 30. Actinomycetes were the inferior microorganisms in metsulfuron-methyl polluted soil. The inhibition on the growth of common actinomycetes was as much as 90 percent on day 30, and the tolerant actinomycetes were even subthreshold of detection. It indicated that actinomycetes can be served as the evaluation factor of metsulfuron-methyl contamination. The ratio of fungi in the total three main types of microorganisms was only a little decrease at the end of experiment, it illuminated that the soil quality wasn't obviously reduced. The toxicity ofmetsulfuron-methyl on soil microorganisms was at the order of actinomycetes > bacteria > fungi, which demonstrated that fungi were the tolerant microbes in metsulfuron-methyl contaminated soil. The reproduction of aromatic compounds-decomposing bacteria was enhanced by the cooperation of metsulfuron-methyl domestication and plant growth, this provided theoretical basis of the co-remediation of aromatic compounds polluted soil using plants and microorganisms. However, among the nitrogen transforming physiological groups, the growth of azotobacter wasn't sensitive to the toxicity of metsulfuron-methyl, but the nitrite bacteria and denitrifying bacteria were seriously restrained although the growth of wheat roots can release the inhibition of metsulfuron-methyl to some degree. Similarly, the population of sulfur transforming physiological groups, including sulfur-oxidizing bacteria and sulfur-reducing bacteria, were also inhibited strongly by the pollutants, and at the same time, the rhizospheric effects were also remarkable. The activities of hydrogen peroxidase, polyphenol oxidase and dehydrogenase all led a decrease in the contaminated soil and suffered the toxicity of metsulfuron-methyl. In addition, the principal component analysis of the total difference of soil microbial characteristics showed that the effects of metsulfuron-methyl on the microbial growth were great in the early stage of incubation, and drive to insignificant with respect to time, which correlated to the degradation of the target herbicide. On the other hand, the activities of rhizospheric microbe were higher than that of non-rhizosphere all along, which indicates that the rhizosphere of wheat had the potential of detoxification of metsulfuron-methyl.2. The degradation of metsulfuron-methyl in domesticated wheat rhizospheric soil was investigated. The results showed that, the degradation dynamics of metsulfuron-methyl in soil followed the first order reaction kinetics, and the degradation velocity constant was about 0.020-0.037 d'1 and half time was about 19-45 d. The planting of wheat significantly promoted the degradation of metsulfuron-methyl, and the difference of degradation velocity constant between the rhizospheric samples and non-rhizospheric samples were significant at the 0.05 probability level. What's more, the degrading capability of microbe were enhanced by domestication, and the removal ratio of metsulfuron-methyl in the demesticated soil was 6-12 percent more than that without demestication. In addtion, theapplication rate of metsulfuron-methyl negatively correlated with its degradation velocity, which meant that low concentration was benefit for degradation. As a results, it was possible that to accelerate the degradation of the xenobiotics in soil using wheat rhizosphere, and some microbes that can degade metsulfuron-methyl effectively might be found in the demesticated soil.3. The process of isolation, selection and determination of metsulfuron-methyl degrading microbes from the soil collected from southern China were described in detail, and the degradation characteristics in pure culture of the obtained strains were also investigated. 4 strains of aerobic heterotriphic bacteria, 9 strains of fungi and 20 strains of actinomycete capable of utilizing metsulfuron-methyl as sole carbon and energy sources was isolated from a metsulfuron-methyl treated soil with the enrichment culture method. A fungal strain was selected as the highest-effective one according to the maximum tolerance concentration of 1200 mg-r'and metsulfuron-methyl degrading rate of 0.0716 g metsulfuron-methyl g"1 cells- h"1, and was identified as one unknown strain of Penicillium sp. on basis of colony growth, morphology and biochemical characteristics. Through liquid pure culture, the optimal metsulfuron-methyl degrading condition of DS11F was determined to be 22.6 mgT1 of metsulfuron-methyl concentration, 12.25 mgT1 of inoculum concentration, 7.0 of pH and 30°C of temperature. When supernatant of soaked compost at the concentration of 284 mg COD -I"1 was added into the liquid medium as an additional C source, the degradation rate of metsulfuron-methyl increased 9.2 percent, but the effect of glucose addition was insignificant. The results showed the isolated strain Penicillium sp. can effectively degrade the herbicide metsulfuron-methyl.4. The response of wheat roots physiology and morphology to the inhibition of metsulfuron-methyl was studied. The main components of low molecular weight organic acids in wheat root exudates were acetic acid, oxalic acid and succinic acid, among which the concentration of acetic acid was the highest for about 91.1 ugg*1 r.d.w. 17 types of amino acids can all be detected in wheat root exudates except HIS and ARG, and the concentration of THR, GLU, PRO, ALA, VAL, MET, LEU, PHE and LYS were more than 24.5 ug-g'1 r.d.w. Root dry weight and root surface area were the sensitive factors of wheat morphology inhibited by metsulfuron-methyl. Treatments of metsulfuron-methyl at concentration of 0.05mg-1'1, which approached the field application rate, didn't cause toxicity on wheat growth, but changed the secretion of wheat roots, including most amino acids, saccharide and low molecular weight organic acids. After the inhibition of metsulfuron-methyl of at the concentration of 0.05, 0.2 and 1.0 mgT1 for 48 h, SRE, GLY, GYS, PHE and LYS presented relative higher increase, and oxalic acid also led a obvious increase, but the secretion of acetic acid was significantly inhibited and reduced to about 11.9 percent at the lowest metsulfuron-methyl treatment concentration of 0.05 mg I"1. It demonstrated that acetic acid was the sensitive factors in wheat root exudates to the inhibition of metsulfuron-methyl. The time response of root exudation to metsulfuron-methyl was very rapid within 3 h and the secretion of acetic acid also deceased continuously. Transmission electron microscope observation on the cellular structure in the elongation of wheat root tip indicated that some cell organ, including cell wall, vacuole, nucleolus and chromosome had changed, and this might be the reason of the variation in root exudation but the mechanism involved need to be further studied. 5 . The model of rapid biodegradation of metsulfuron-methyl in wheat rhizospheric soil was established by means of combining the soil inoculation of the selected highest-effective degrading strain Penicillium sp. and the amendment of root exudates. The wheat rhizosphere were well simulated by setting up a hydroponic system that allowed the aseptic wheat root exudates flow onto columns containing soils previously contaminated with metsulfuron-methyl, and the fungus Penicillium sp. were inoculated into the soil, with the bulk and sterile samples as control. The removal ratio of metsulfuron-methyl in the amended inoculated experimental apparatus was significantly higher than that in the other apparatuses without root exudates amendment or fungal inoculation. The degrading half life of metsulfuron-methyl in simulated inoculated rhizospheric soil, inoculated non-rhizospheric soil, bulk rhizospheric soil and bulk non-rhizospheric soil were 8.6 d, 15.4 d> 23.1 d and 31.5 d, respectively. The tolerant fungi and the inoculums grew well. In 10 days, the population of Penicillium sp. in amended inoculated soil increased for about 6 fold from the initial inoculation rate 5.0 lgCFU-g'1 d.s., and was more than 40 percent of the tolerant fungi which was 6.20 + 0.08 lgCFU-g"1 d.s. at the same time. What's more, when a large number of metsulfuron-methyl had been degraded, the growth of Penicillium sp. and tolerant fungi reducedrapidly, which didn't evoke the problem of ecological risk. By means of applied metsulfuron-methyl again into the inoculated soil, the sustainability on metsulfuron-methyl degradation of the inoculums was investigated. It can be observed that the reproductions of tolerant fungi and Penicillium sp. kept relatively stable, and were about 5.0-6.0 Ig CFU-g"'d.s. and 4.0-5.0 lg CFU-g"'d.s, respectively throughout the second incubation period. Degrading velocity constant of metsulfuron-methyl in simulated inoculated rhizospheric soil were a little less than that obtained during the first period, but was also significantly higher than the other treatments (PO.01). On day 74, the removal rate of metsulfuron-methyl in simulated inoculated rhizospheric soil, inoculated non-rhizospheric soil, bulk rhizospheric soil and bulk non-rhizospheric soil were 69.4 percent, 48.7 percent, 43.3 percent and 34.3 percent, respectively. It can be inferred that the inoculums can stably grow in soil and sustainably degraded the target herbicide, and the horizontal transfer of metsulfuron-methyl degrading gene also promoted the capability of indigenous microorganisms on metsulfuron-methyl and enhanced the effectiveness of bioremediation of metsulfuron-methyl contaminated soil. As a result, in this study we discussed the rapid degradation of metsulfuron-methyl in wheat rhizospheric soil and its eco-chemical mechanism, and systematically established the in-situ bioremediation model of metsulfuron-methyl contaminated soil. The purpose of this study satisfied the practical situation and was of great value.
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