Formation and distribution of14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied14C was still present in the soil. By 2 mo after treatment (MAT),14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the14C applied was atrazine 16 MAT. Atrazine was the predominant14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Summary: Metribuzin efficacy and dissipation were determined in two silty clay loam soils following preplant (PP), pre‐emergence (PRE) and split (PP+PRE) application to tilled and no‐till soybeans in rotation with corn at Clay Center and Lincoln, Nebraska. A similar experiment was conducted in tilled and no‐till soybeans in rotation with wheat at Lincoln. Corn and wheat residue in no‐till plots reduced the amount of metribuzin that reached the soil by approximately 54 and 89%, respectively. No differences in weed control or soybean yield were observed between tillage treatments or time of metribuzin application in the corn‐soybean rotation. However, both weed control and yield were reduced in the wheat‐soybean rotation. Most of the metribuzin remained at the 0–5 cm depth, and dissipation was exponential. The mean metribuzin half‐life at the 0–5 cm depth across locations, tillage treatments, application time and rates was 11 days. The metribuzin half‐life was 4–19 days following PP application and 3–17 days following PRE application. The metribuzin concentration did not exceed 65 μg kg−1 at the 5–10 cm or 10–20 cm depths in any treatment, indicating that little metribuzin had leached from the surface soil after PP or PRE application. The finding of a higher metribuzin concentration at 5‐20 cm depth in tilled plots than in no‐till could be attributed to higher initial soil concentrations in the absence of crop residue.

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Formation of14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT),14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the14C applied 16 MAT and was the predominant14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.