Effect of organic matter and chemical fertilizers on the degradation of benthiocarb and mcpa herbicides in the soil

Abstract

The degradation of herbicides in soils amended with rice straw, compost and NPK chemical fertilizers was investigated in the laboratory under upland, oxidative-flooded (aerobic-flooded) and reductive-flooded (anaerobic-flooded) conditions using ¹⁴C-benthiocarb and ¹⁴C-MCPA labelled at the phenyl ring. Rice straw, compost and NPK amendments promoted the degradation of these herbicides under upland and oxidative-flooded conditions and large amounts of ¹⁴CO₂ were evolved. Under reductive-flooded conditions, the degradation of the herbicides was remarkably slow and the various soil amendments had practically no effect on the degradation of the two herbicides. 4-Chlorobenzoic acid, desethylbenthiocarb, benthiocarb sulfoxide and 4-chlorobenzyl methyl sulfone were found to be the major degradation products of benthiocarb. Rice straw amendments appeared to increase the amount of benthiocarb sulfoxide but the amount of desethylbenthiocarb was reduced by both rice straw and compost, especially under upland conditions. 4-Chloro-2-methylphenol, which was the major degradation product of MCPA, was produced in relatively large amounts in the rice straw amendment under oxidative-flooded conditions and in the NPK amendment under upland conditions.