Soil science was not my field of study in college so I defer to those who know what they are talking about. Lee Reich knows of what he speaks and he wrote about mulch for Fine Gardending Magazine in an article called "Use Mulch to Manage Your Soil Conditions".
Below are excerpts and here is the link to the entire column.
Mulch has many benefits
"The major reason gardeners use mulch is to snuff out weed seeds by shading them. This allows the roots of desirable plants to access soil, water, and nutrients without undue competition. Mulches free of viable weed seeds—such as leaves, good compost, and wood chips—are best. Weed seedlings that sprout in any organic mulch are easily done in if you periodically fluff up and flip over the mulch with a pitchfork.
The second reason to mulch your garden is to conserve water. Organic mulches soften the impact of raindrops so that water can effectively permeate the soil, and all mulches, organic or otherwise, limit evaporation of soil moisture.
The benefits of mulch do not end with water and weeds. As organic mulches decompose, they promote healthy soil, which, in turn, helps fend off disease.
Mulches also regulate soil temperature, acting as insulation to prevent the alternating freezes and thaws that can heave plants out of the ground. Such ground-insulating mulches are especially useful in keeping the roots of newly planted trees and shrubs growing as long as possible into autumn, and keep the soil beneath evergreens unfrozen deeper and longer so that their roots can absorb moisture in winter.
You must consider the plants being grown when you choose your mulch. As organic mulches decompose, they release nutrients that will affect soil fertility. Every year, I blanket my vegetable garden with an inch or two of compost. This mulch is rich in nutrients, and its dark color helps warm the soil so that I can plant early. At the other extreme, in my wildflower garden, I mulch with wood chips. They smother weeds and because they decompose more slowly than compost, they keep the coneflowers, yarrow, and liatris on the relatively lean diet they enjoy. A tidy blanket of dark brown buckwheat hulls looks good for a more-formal garden of delphiniums and roses, but it won’t provide the same amount of nutrients as compost, so I add some fertilizer to nourish my heavy feeders.
Black plastic mulch, though marginally functional, is unattractive and must be covered with some other mulch, usually wood or bark chips. Weeds will invade that layer anyway, and the mulch will slide off to reveal an ugly, black underbelly. Roots can suffer oxygen starvation beneath plastic mulch, and the soil can overheat in hot summers."
If you aren't mulching, now is the right time to begin!
Dr. Reich's book credits include "Landscaping With Fruit", "Weedless Gardening", "The Pruning Book", and "Uncommon Fruits for Every Garden." Here's a link to his site.which includes a northeast U.S. gardening blog.