Degradation and Sorption of Metribuzin and Primary Metabolites in a Sandy Soil

Abstract

Leaching to the ground water of metabolites from the herbicide metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5-one] has been measured in a Danish field experiment in concentrations exceeding the European Union threshold limit for pesticides at 0.1 microg/L. In the present work, degradation and sorption of metribuzin and the metabolites desamino-metribuzin (DA), diketo-metribuzin (DK), and desamino-diketo-metribuzin (DADK) were studied in a Danish sandy loam topsoil and subsoil from the field in question, using accelerated solvent extraction and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Fast dissipation of metribuzin and the metabolites was observed in the topsoil, with 50% disappearance within 30 to 40 d. A two-compartment model described degradation of metribuzin and DA, whereas that of DADK could be described using first-order kinetics. Part of the dissipation was probably due to incorporation into soil organic matter. Degradation in subsoil occurred very slowly, with extrapolated half-lives of more than one year. Sorption in the topsoil followed the order DA > metribuzin > DK > DADK. Subsoil sorption was considerably lower, and was hardly measurable for metribuzin and DK. Abiotic degradation was considerably higher in the topsoil than the subsoil, especially concerning the de-amination step, indicating that organic matter may be related to the degradation process. The present results confirm observations of metribuzin and transformation product leaching made in the field experiment and demonstrate the need for knowledge on primary metabolites when assessing the risk for pesticide leaching.

Biodegradation of soil-applied pesticides by selected strains of plant growth-promoting rhizobacteria (PGPR) and their effects on bacterial growth

“…Metribuzin exhibited a biphasic dissipation curves consisted of a phase of a more rapid dissipation occurring during the first 14 days of incubation followed by a second phase of slower dissipation rate. Our results are in accordance with previous investigations which reported that the degradation of metribuzin in soil samples was described using two-compartment model with a very rapid degradation rate at the beginning followed by a slow prolonged dissipation (Henriksen et al 2004). The half-lives of metribuzin at low fortification level were 129.8, 128.1, 131.9 and 132.7 days in soil amended with B. amyloliquefaciens IN937a, B. pumilus SE34, B. subtilis FZB24 and B. subtilis GB03 and at high level they were 139.0, 140.6, 138.4 and 138.0 days, respectively (Table 3).…”

Sugarcane and pinewood biochar effects on activity and aerobic soil dissipation of metribuzin and pendimethalin

“…For the sand soil, WC400 also appeared to slow formation of DA. The conversion of metribuzin to DA was about 5-10%, similar to values reported elsewhere (Henriksen et al, 2004). Considering that no DADK was observed, the DA formed was relatively stable.…”

“…Considering that no DADK was observed, the DA formed was relatively stable. The compound had a similar DT 50 in topsoil as metribuzin, 29 and 32 d, respectively (Henriksen et al, 2004).…”

Phytotoxicity and persistence of metribuzin residues in black soil

“…These findings are further supported by Henriksen et al's [29] works where they found that metribuzin degraded rapidly and the majority of the metribuzin residue remained in the surface soil. Metribuzin residues persisted upto harvest as detected by sorghum and cucumber root and shoot bioassays and upto 97% dissipation was noticed by cucumber root bioassay ( Figure 6).…”