Degradation dynamics of pyroxasulfone: exploring soil health impacts and dietary risk assessment

Pyroxasulfone is a widely used pre-emergent herbicide for weed control in various crops. However, moderate to high soil persistence of pyroxasulfone can cause phytotoxicity to succeeding crops and disrupt soil microbial communities. Thus, the present study aims to evaluate the persistence of pyroxasulfone and its impact on soil health, followed by an assessment of its phytotoxicity on succeeding crops such as Solanum tuberosum (potato), Pisum sativum (pea), Spinacia oleracea (spinach), Brassica oleracea var. capitata (cabbage), Trigonella foenum-graecum (metha), Trigonella corniculata (methi), and Brassica rapa subsp. trilocularis (toria). A 3-year field study (2020-2022) was conducted to examine pyroxasulfone degradation with application rates ranging from 76.5 to 255 g ha^-1. Soil enzymatic activities (DHA, APA, urease) and microbial populations (bacteria, fungi, actinomycetes) were measured over time, while carryover effects on subsequent crops were evaluated through growth parameters (germination, root/shoot length, biomass). Degradation of pyroxasulfone was strongly influenced by application rates and prevailing weather conditions. Although residues were below maximum residue limits at harvest, differential sensitivity was observed in succeeding crops, with peas showing a higher susceptibility to pyroxasulfone than methi or cabbage. The application of pyroxasulfone initially reduced soil enzymatic activities and microbial abundances and this inhibition effect gradually recovered over time. Dehydrogenase activity was the most sensitive and bacterial count exhibited highest reduction in abundance among the studied soil activities. The differential sensitivity observed among succeeding crops and short-term suppression of soil activities underscores the importance of optimizing plant-back periods to mitigate residual herbicide effects and continuous monitoring of soil health in herbicide-treated fields. These insights can help to improve herbicide management practices and guide regulatory agencies in minimizing environmental impact while ensuring effective weed control.