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 s ummer vegetable crop rotational system might influence the potential of sur face and subsurface layers of a Willamette silt loam soil to mineralize the herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D). On three occasions betw een 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 cropw inter cover crop (cereal rye) rotation. Composite samples of soil were prep ared from the four replicates of each of the two held treatments, and the m ineralization 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 tr eatment than the winter fallow treatment on five of the six sampling occasi ons. 2,4-D mineralization characteristics of the 80-100-cm soil differed be tween the cover crop and winter fallow treatments and also differed between sampling dates. In February, 2,4-D mineralization rates developed slowly a nd persisted at suboptimal rates for at least 10 d in both treatments. In A pril and June, daily 2,4-D mineralization rates increased more rapidly in s oil from the cover crop treatment than in samples taken from the winter fal low treatment. Although no differences were detected in mineralization char acteristics of subsurface soil sampled from the two treatments while the sw eet corn summer crop was growing (July), treatment differences were again d iscernible immediately after the summer crop was harvested (September). Our findings identified a potential bonus of using a winter cover crop, i.e. e nhancing the potential of subsurface soil microflora to mineralize herbicid es that might leach from the surface. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.