There is a misconception going around that using mulch will tie up the nitrogen in your soil, weakening your plants. This isn't completely accurate. While it is true that there is a nitrogen deficiency at the thin topmost layer of the soil, just beneath the mulch, this is a good thing. This thin deficient layer inhibits weed seed germination.
The nitrogen deficiency does not extend past this very thin layer so it won't impact the roots of plants. In fact, nutrients in mulched soil actually increase in the root zone and in the plant foliage itself. Since mulch also provides protection from extreme temperatures, reduces pest and disease issues, and reduces watering needs, it is highly recommended.
Applying natural mulches, such as wood chips, leaves or shredded bark (not bark chunks) will benefit your soil and plants. We don’t recommend large bark chunks as mulch since they are too big to provide the weed prevention, beneficial insect habitat and water loss protection that smaller mulches offer. Since large pieces break down significantly slower, they don’t tend to benefit the soil as much as our recommended mulches do. To install your mulch, simply water your soil, spread your mulch 10 - 12 cm (4 - 5 in) deep and water it again. Mulch must not be mixed into your soils or it is no longer considered mulch. Once you mix it into your soils, it is now considered a soil amendment. Mixing wood chips and bark into your soil is not recommended as this will tie up significant amounts of nitrogen in your soil.