As a host of micro-organisms in the soil goes to work digesting it, they suck almost all available nitrogen from the top 2 to 3 inches of soil. There's where the majority of new feeder roots grow. Proponents of dyed mulches suggest adding extra nitrogen fertilizer to supply what the plants need. Experts argue it would take huge amounts to counteract the effects of the mulch. Also, since nitrogen is volatile and moves quickly through the soil, fertilizer would have to been applied liberally and frequently to overcome nitrogen deficiency during the life of the mulch.
Most fertilizers contain phosphorus and potassium in addition to nitrogen.
Dr. Francis R. Gouin, professor emeritus at the University of Maryland and a world renowned expert on composting, raises even more troubling questions about the safety of dyed mulches. His concern has to do not with the dyes, which the manufacturer claims are non-toxic to plants, but with the possible contaminants of the wood itself. Asbestos and lead-based paints could possibly be introduced, he believes, if construction materials from older buildings being demolished were recycled as mulch. From the perspective of this horticulturist, the whole issue seems a no-brainer. I could understand, perhaps, if someone was trying to create a special effect and confined the use of colored mulches to paths.