Effects of chlorpyrifos and quinalphos on dehydrogenase activities and reduction of Fe3+ in the soils of two semi-arid fields of tropical India

The impact of seed dressing and standing crop treatment (SC) with chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro pyridyl) phosphorothioate] and quinalphos [O,O-diethyl O-quinoxalin-2-yl phosphorothioate] on the oxidative capability of the soils of two semi-arid regions of tropical India were studied by monitoring their dehydrogenase activities (DHA) and capacities for reduction of iron (Fe3+) for three consecutive crop seasons. In the sandy loam, the metabolites of quinalphos inhibited both reduction of Fe3+ and DHA whereas chlorpyrifos was found stimulatory. The metabolites of chlorpyrifos namely 3,5,6-trichloro-2-pyridinol (TCP) and 3,5,6-trichloro-2-methoxy pyridine (TMP) stimulated DHA but were significantly inhibitory to reduction of Fe3+. The stimulatory effect of TCP and TMP suggested their utilization as substrates by soil microbes. In the loamy sand, chlorpyrifos and quinalphos inhibited the overall DHA within a field but stimulated DHA of the rhizospheric microbes. Standing crop treatments with quinalphos and chlorpyrifos caused prolonged inhibitory effects on reduction of Fe3+ while seed treatments caused fluxes of shorter durations. The inhibitory effects on reduction of Fe3+ were more prolonged with chlopyrifos than with quinalphos. Pesticide treatments at the recommended doses increased the yield of groundnut (Arachis hypogaea L.) pods that had no residual pesticides. Adverse effects of agronomic importance on DHA and reduction of Fe3+ were not observed during this study.