It specifically takes nitrogen - this can be avoided by adding nitrogen, at first directly underneath the bark before spreading (use Growmore granules) and then additional dressings every few weeks during the growing season. Any dead material on the surface of the soil will have the same effect, but using dead vegetable matter is still better than hard things like slate, for instance,because at least once its rotted down, it adds its nutrients to the soil. If no mulch is used, there is usually what is known as the leaf litter layer which builds up on its own anyway.

I shred bark and store it for 6 months before using it as a mulch. this avoids the problem aparently , I followed advice from the RHS.