Juglone is present in the roots, bark and leaves of walnut, especially black walnut. It's also present in related species like pecan and butternut. It is extremely toxic to some plants, while others are unaffected. It can not only suppress growth in some plants, it can outright kill some of them. Its extract has been used as an herbicide. I would not use crushed walnut shells as a soil amendment for bonsai because of this. It would be like playing Russian roulet with your trees.
http://ohioline.osu.edu/hyg-fact/1000/1148.html