Walnut trees, black walnut & English walnut and other closely related trees, have a chemical compound called juglone, which inhibits growth of other plants.  This gives walnut a competitive edge, so other plants have a more difficult time growing because of juglone. 

We do not recommend people to collect leaves, nuts, or twigs from walnut trees and put into compost exactly for this reason.  Grass, groundcovers, many plants will not survive under walnut trees.  Best advice it to cover the roots of trees with 2-4 inches of organic mulch such as bark nuggets that will prevent weeds, moderate moisture and temperature of soil as far out as the drip line of the tree.

Here is a link to an extension article on juglone and effects on other plants. Click on http://www.wvu.edu/~Agexten/hortcult/fruits/blkwalnt.htm