Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2007
Publication Date: 1/13/2009
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2220.pdf
Citation: Wang, H.Z., Gan, J., Zhang, J.B., Xu, J., Yates, S.R., Wu, J.J., Ye, Q.F. 2009. Kinetic Distribution of 14C-Metsulfuron-methyl Residues in Paddy Soils under Different Moisture Conditions. Journal of Environmental Quality. 38(1):164-170. Interpretive Summary: Metsulfuron-methyl is widely used for pre- and post-emergence control of annual grasses and broadleaf weeds in cereal, pasture, and plantation crops. However, this compound has been shown to persist in the environment and may cause phytotoxic effects to beneficial plants. Metsulfuron-methyl degrades in soil by hydrolysis and microbial degradation and is affected by soil pH. In cereal-growing regions of China, the pH of paddy soils range from 5 to 9, so metsulfuron-methyl is predominantly anionic. The anionic form is more soluble in water and less susceptible to hydrolysis, which may increase its persistence and leaching potential. Soil moisture has also been shown to play an important role in the fate of herbicides. Studies have shown that increasing soil water content can stimulate the microbial degradation of many herbicides. The objectives of the study were to study the degradation of extractable and soil-bound residues of 14C-metsulfuron-methyl in six Chinese paddy soils under different moisture conditions, and to better understand how moisture content influences the relationship between soil properties and the extractable and bound residues in soils.
Technical Abstract: Rice paddy soils undergo several cycles of drying and wetting during a growing season. A laboratory study was conducted to determine the effect of soil moisture conditions on the distribution and kinetics of extractable and bound residues of 14C-metsulfuron-methyl in six Chinese paddy soils during 84 d incubation at 15 oC with moisture contents varying from 20 to 50% of the field water-holding capacity. The amount of extractable residues consistently increased, and the bound residues decreased with increasing soil moisture content. At 84 d after the treatment, extractable residues and bound residues accounted for 34.5-84.4% and 11.6-53.3% of the applied radioactivity in the soils, respectively. Soil pH and soil microbial biomass carbon were the most predominant factors affecting the formation and relative distribution of the herbicide residues between the extractable and bound residue forms. In high pH soils, the level of bound residues decreased, and that of extractable residues increased, suggesting an increased leaching risk for metsulfuron-methyl in alkaline soils. High precipitation rates, along with the common practice of liming in the southeastern regions of China, may lead to enhanced herbicide leaching as well as phytotoxicity to rotation plants, and should be considered in the overall pest management practices.