Effect Of Metolachlor On Biological Activities In Plant Rhizosphere And Non-rhizosphere Soil

Plants can absorb chemicals from soil and degrade them. Enhancement of chemicals adsorption in the rhizosphere has been observed. The rhizosphere of plants is a special area. The roots can release certain kinds of organic compounds, which are beneficial to the growth and activity of microorganisms. It has been reported that the number of microorganisms in the rhizosphere is typically an order of magnitude higher than in non-vegetated soil. This often results in greater rate of microbial degradation of organic pollutants in the root zone compared with the edaphosphere. Moreover, plant uptake may result in the accumulation of chemicals in the rhizosphere, which would help to degrade chemicals in the soil.In order to investigate the influence of plant roots and rhizosphere microorganisms on bioremediation of soils contaminated with organic pollutants, the effect of metolachlor on microbial numbers and enzymes activities and its degradation in two plants(celery and spinage) rhizosphere and non-rhizosphere soil were studied with rhizobag technique. The results showed that bacterial numbers were inhibited by metolachlor in celery rhizosphere soil initially, the numbers increased after 7 days. There was some difference between the effect on celery and spinage rhizosphere soils by metolachlor. Fungi numbers were stimulated by metolachlor, and the higher the metolachlor concentration the greater the influence, which indicated that fungi can utilize metolachlor as carbon and energy sources. Fungi numbers in celery rhizosphere soil were higher than those in spinage rhizosphere soil in the same concentration of metolachlor. Both bacteria and fungi numbers in rhizosphere were higher than those in non-rhizosphere soil.The effect of metolachlor on catalase activities was different under different treatment in celery rhizosphere soil. Dehydrogenase activities were stimulated in celery rhizosphere soil, while they were inhibited in spinage rhizosphere soil by herbicide. Both catalase and dehydrogenase activities in celery rhizosphere soil were higher than those in spinage rhizosphere soil in the same treatment. Generally, enzyme activities in rhizosphere were higher than those in non-rhizosphere soil.The degradation of metolachlor in soil followed first- order kinetics. Half lives(ti/2) of metolachlor at 1 mg-kg"1 n 2.5 mg-kg"1 n 5 mg-kg'1 in celery rhizosphere were shorted to be 86.22%, 78.95% and 77.97% of those in non-rhizosphere soils, their corresponding degradation rates were 1.16, 1.27 and 1.28 times of those in non-rhizosphere soils, respectively. Half lives(ti/2) of metolachlor at 1 mg-kg"\2.5 mg-kg"1 % 5 mg-kg"1 inspinage rhizosphere were shorted to be 79.34%, 79.67% and 79.96% of those in non-rhizosphere soils, their corresponding degradation rates were 1.27, 1.26 and 1.26 times of those in non-rhizosphere soils, respectively. The results indicated that degradation of metolachlor was stimulated by plant roots in rhizosphere. Plants and their associated rhizosphere microorganisms could be a good method for bioremediation of soils contaminated with organic pollutants.