The rates of degradation and mineralization of metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4H)‐one] were determined in the surface and subsurface horizons of a Dundee silty clay loam soil (Aeric Ochraqualf) from the Mississippi Delta. Residual metribuzin and metabolites were measured by HPLC or TLC methods during or after 91‐d incubations of treated soil in the laboratory. Metribuzin degradation was faster in the surface soil than in the subsurface soils. Degradation in the surface and the subsurface horizons were not described by first‐order kinetics. Degradation was a second‐order process in the surface soil, but only a half‐order process in the subsurface horizons. Considerably more mineralization of 14C‐metribuzin occurred in the surface (0‐ to 10‐cm depth) soil, with 15 to 20% evolved as 14CO2 after 91 d, vs. 5% from the subsurface soils. Substantial amounts of 14C‐bound residues were formed in both surface and subsurface horizons. Reduced microbial populations and activity corresponded to the reduced degradation of metribuzin in the subsoils.
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Moorman et al. (1989) studied this question.