Environmental Risk Assessment Of Bound Residues Of Metsulfuron-methyl In Soil And Mechanism Of Genotypic Difference In Responses Of Rice To Bound Residues Of Metsulfuron-methyl In Soil

Dependence of human beings on chemicals, especially synthetic organic compounds increased gradually with the rapid development of agriculture and industry, improvement of life quality, and the fast growth of population. Organic compounds mainly consisted of pesticides, herbicides, and industrial organic wastes like phenols, oils, polychlorinated biphenyl, benzopyrene. These organic compounds enter into soil or sediments in water at last, while most of these are considered as bound residues in soil or sediments. Bound residues in environment could be released again and pose impacts on plants, animals, or human beings through food chains. Metsulfuron-methyl, one of sulfonylurea herbicides, was widely used in wheat fields during period of the ninth five-year plan in China. Our previous results showed that 11.4 %-55.7 % of metsulfuron-methyl used could tightly bind with organic matter and form bound residues in soils. Bound residues of metsulfuron-methyl in soil were likely to result in some ecological risk, especially in rice fields due to the formation of loose bound residues of metsulfuron-methyl combined with soil organic matter. However, systemic reports about these are very few. The present research illustrates the environmental risk of bound residues of metsulfuron-methyl in soil, and proves the probability of agricultural reduction resulted from use of metsulfuron-methyl. A set of experiments were conducted to investigate effects of bound residues of metsulfuron-methyl on soil enzymes, microbial biomass, and microbial diversities,and the genotypic different responses of rice to bound residues of metsulfuron-methylin soil. The major results were summarized as follows:1. Ecological risk assessment of bound residues of metsulfuron-methyl in soil.Ecological risk of bound residues of metsulfuron-methyl in soil has been found. Effects of bound residues of metsulfuron-methyl in soil on soil enzymes such as urease, hydrogen peroxidase, acid phosphatase, and sucrase, microbial biomass, and microbial diversity are dependent on the incubation time when bound residues of metsulfuron-methyl exist in soil. (1) Effects of bound residues of metsulfuron-methyl on soil enzymes. At 7 days after treatment (DAT), urease was induced, and hydrogen peroxidase was inhibited significantly by bound residues of metsulfuron-methyl, however sucrase and acid phosphatase were not changed. At 14 DAT, hydrogen peroxidase and acid phosphatase were inhibited significantly by bound residues of metsulfuron-methyl in soil. Urease and sucrase were induced and inhibited significantly by bound residues of metsulfuron-methyl at the levels of 0-0.281 mg kg"1, and 0.50 mg kg'1, respectively. At 28 DAT, soil enzymes such as urease, hydrogen peroxidase, acid phosphatase, and sucrase were induced by bound residues of metsulfuron-methyl in soil. At 56 and 98 DAT, effects of bound residues of metsulfuron-methyl on soil enzymes are similar to that at 28 DAT. (2) Effects of bound residues of metsulfuron-methyl on soil microbial biomass. Effects of bound residues of metsulfuron-methyl in soil on microbial biomass and ratio of microbial biomass carbon to nitrogen changed with time. At 7 DAT, microbial biomass decreased in soil treated with bound residues of metsulfuron-methyl at levels of 0.05 -0.156 mg kg"1 and increased at the levels of 0.281-0.50 mg kg"1 compared with that of control. At 14 DAT, microbial biomass decreased significantly with the increase of bound residues of metsulfuron-methyl in soil compared with that of control. At 28 DAT, microbial biomass increased with the increase of bound residues of metsulfuron-methyl in soil. At 56 and 98 DAT, microbial biomass increased significantly at lower level of bound residues of metsulfuron-methyl and decreased significantly at higher level of bound residues of metsulfuron-methyl in soil. The significant decrease in ratio of microbial biomass carborn to nitrogen has also beenfound, indicating that microbial communities changed under the stress of bound residues of metsulfuron-methyl. (3) Effect of bound resi...