Winter cover crop enhances 2,4-D mineralization potential of surface and subsurface soil

A study was conducted to determine if the use of a winter cover crop in a summer vegetable crop rotational system might influence the potential of surface and subsurface layers of a Willamette silt loam soil to mineralize the herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D). On three occasions between April and September 1994, and three occasions between February and June 1995, soil samples were recovered from the Ap horizon (0–20 cm), and from within the argillic B (80–100 cm) horizon of field plots managed in either a summer vegetable crop–winter fallow rotation, or a summer vegetable crop–winter cover crop (cereal rye) rotation. Composite samples of soil were prepared from the four replicates of each of the two field treatments, and the mineralization of 6 mg 2,4-D kg−1 examined under laboratory conditions. 2,4-D was mineralized more quickly in the 0–20-cm soil from the cover crop treatment than the winter fallow treatment on five of the six sampling occasions. 2,4-D mineralization characteristics of the 80–100-cm soil differed between the cover crop and winter fallow treatments and also differed between sampling dates. In February, 2,4-D mineralization rates developed slowly and persisted at suboptimal rates for at least 10 d in both treatments. In April and June, daily 2,4-D mineralization rates increased more rapidly in soil from the cover crop treatment than in samples taken from the winter fallow treatment. Although no differences were detected in mineralization characteristics of subsurface soil sampled from the two treatments while the sweet corn summer crop was growing (July), treatment differences were again discernible immediately after the summer crop was harvested (September). Our findings identified a potential bonus of using a winter cover crop, i.e. enhancing the potential of subsurface soil microflora to mineralize herbicides that might leach from the surface.