This pdf (right click file save as) is research done into the use of woodchip as a mulch cover.
3. The main-plot treatments that included hardwood chips as a soil amendment were particularly beneficial in terms of effects on crop yields, on several chemical and physical soil properties, and on indices of soil biological population and activity.
Under continuous cropping to vegetables, topdressing with hardwood chips after planting (the chips to be plowed under the following year) was an outstanding treatment with respect to effects on yields and maintenance or buildup of soil nitrogen and structure.
Wood Chips in Vegetable Production