Studies on the anoxic dissipation and metabolism of pyribambenz propyl (ZJ0273) in soils using position-specific radiolabeling

Abstract

Pyribambenz propyl (ZJ0273) is a polycyclic herbicide with increasing use, although studies show that it tends to be persistent in soil and pose phytotoxicity to rotational crops. This study employed an improved ring-specific (14)C labeling method to characterize its anoxic metabolism, with (14)C positioned on the benzoate, pyrimidyl or benzyl rings. Separation and identification of the metabolites were achieved by liquid chromatography (LC), ultralow-level liquid scintillation spectrometry, and LC-mass spectrometry (MS). Results show that the anoxic degradation follows first-order kinetics and the half-lives are approximately 38.7, 50.2 and 70.7d for loamy, saline and clayey soils, respectively. A total of five radioactive intermediates (M1-M5) were detected, and due to the loss of radiolabels, different radiochromatograms were obtained from different labels, i.e., radioactive M5 was only detected for pyrimidinyl-(14)C; M3 and M4 were only detected for pyrimidinyl-(14)C and benzyl-(14)C, while M1 and M2 were detected for all labels. Based on their appearance pattern and fragmentations from LC-MS, the structures of M1-M5 were identified, and they were proposed to form by reactions such as de-estering, hydrolysis, acylation, CN cleavage, and demethylation. All metabolites have been previously detected in aerobic soils except M4, which is a demethylation product from M3, and identified as 2-(4-hydroxy-6-methoxypyrimidin-2-yloxy)benzoic acid. The results show that ZJ0273 is more persistent in anoxic soils, and its degradation pathways and intermediates are different from aerobic metabolism and differ with the soil types, suggesting that soil-specific and farming practices may be important considerations in the use of this herbicide. The ring-specific labeling provides full molecular information about the referred compound and guarantees the reliability of the results, and can be used as an effective tool for metabolite profiling of polycyclic compounds.

Keywords: Anoxic degradation; Herbicide; Metabolites; Pyribambenz propyl (ZJ0273); Ring-specific (14)C labeling.