Metribuzin degradation in soil: I—effects of soybean residue amendment, metribuzin level, and soil depth

Plant residue and soil depth effects on metribuzin degradation were investigated. Dundee silt loam soil collected at depth increments of 0–10 cm (SUR) and 10–35 cm (SUB) was treated with labeled [5 −14 C]metribuzin. Samples were assayed at several time points up to 140 days after treatment. Soybean residue was added to half of the SUR samples (RES), with remaining SUR unamended (NORES). None of the SUB samples were amended with soybean residue. Metribuzin mineralization to 14 CO 2 proceeded more slowly in RES and SUB than in NORES and SUR, respectively. Extractable components in SUR samples included polar metabolites, plus deaminated metribuzin (DA) in the RES, and parent metribuzin in the NORES. Deaminated diketometribuzin (DADK) and metribuzin comprised major 14 C components extracted from SUB, while in SUR, faster degradation of metabolites resulted in metrizubin as the primary identifiable compound. Unextractable 14 C increased until day 35 for both RES and NORES, after which it remained constant for NORES. but declined for RES. A corresponding rise in RES polar 14 C suggested that as soybean residue decomposed, 14 C bound in the residue was released as extractable polar material. Soil with soybean residue accumulation may alter metabolite degradation patterns, but does not impede initial metribuzin degradation. Depth differences in metribuzin degradation were attributed to reductions in microbial activity with increasing soil depth.