Pedosphere 31(4): 549--559, 2021
ISSN 1002-0160/CN 32-1315/P
©2021 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Degradation of the fungicide metalaxyl and its non-extractable residue formation in soil clay and silt fractions
Roschni KALATHOOR1, Jens BOTTERWECK2, Andreas SCHÄFFER2,3, Burkhard SCHMIDT2, Jan SCHWARZBAUER1
1Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Aachen 52062 (Germany)
2Institute for Environmental Research, RWTH Aachen University, Aachen 52062 (Germany)
3School of the Environment, Nanjing University, Nanjing 210023 (China)
ABSTRACT
      The proportion of organic matter and mineral composition are important factors determining the formation and type of non-extractable residues (NERs) of pesticides in soil. In this study, we investigated the enantioselectivity in degradation and NER formation of the chiral fungicide metalaxyl in soil particle size fractions (silt and clay). Microbial and extracellular enzyme activities during these processes were monitored in incubation of silt and clay samples isolated from sterilized and non-sterilized soil samples collected from a long-term agricultural field experimental site in Ultuna, Sweden. The temporal influence on the fate of the fungicide was noted by short-term (10-d) and long-term (92-d) incubations. Besides the acquisition of quantitative data with gas chromatography-mass spectrometry (GC/MS), stereoselective analyses were performed with chiral GC/MS. Quantitative results pointed to a higher metabolism rate of the pesticide through microbial activity than through extracellular enzyme activity. This was also confirmed by the enantioselective depletion of R-metalaxyl and the subsequent formation of R-metalaxyl acid in microbially active samples from non-sterilized soil. The silt fraction containing a high amount of organic matter exhibited a significant hydrolyzable proportion of metalaxyl NERs that was releasable under alkaline conditions. On the contrary, the clay fraction showed an enhanced affinity for covalently bound residues. Based on our results, we recommend differentiating between reversibly and irreversibly bound proportions of pesticides in persistence and environmental risk assessment because the reversible fraction contained potentially bioavailable amounts of residues that may be released under natural conditions.
Key Words:  chiral fungicide,enantioselectivity,metalaxyl enantiomer,non-extractable pesticide residues,organo-mineral complex,pesticide degradation,extracellular enzymes,soil particle size fractions
Citation: Kalathoor R, Botterweck J, Schäffer A, Schmidt B, Schwarzbauer J. 2021. Degradation of the fungicide metalaxyl and its non-extractable residue formation in soil clay and silt fractions. Pedosphere. 31(4): 549-559.
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