Many growers are using white clover as a living mulch with few problems and many benefits. The peoples at the Sustainable Agriculture Research and Education have a good publication out about this use of white clover.

http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition/Text-Version/Legume-Cover-Crops/White-Clover

Clovers, using certain bacteria, fix Nitrogen in nodules on the roots of the plants for use by the clover and other plants over time. While that N would be somewhat more available if the clover was dead it does not need to be for other plants to use it and that N is in a more stable form.

http://msucares.com/news/print/agnews/an04/040311.html

kimmq is kimmsr

I have found clover to be a nuisance among growing crops.

That claim that the fixed nitrogen becomes only "somewhat more available" after the clover is dead is incorrect.

The amount of nitrogen returned to the soil during or after a legume
crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant.
Little leaks into the soil for a neighboring non-legume plant. However, nitrogen
eventually returns to the soil for a neighboring plant when vegetation (roots, leaves,
fruits) of the legume die and decompose.

http://www.csun.edu/~hcbio027/biotechnology/lec10/lindemann.html

Most people envision a "pipeline" that transports nitrogen directly from clover to grass. Unfortunately
almost no nitrogen is contributed to grasses this way. Essentially all nitrogen supplied to grasses from
clover is indirect. Because of this indirect route, nitrogen from clover root nodules is not immediately
available to companion grasses. Root nodules must decompose and nitrogen must be converted into a
form available to plants.

http://extension.uga.edu/publications/detail.cfm?number=B1251

When legumes are tilled into the soil, the N stored in their roots is released and made available to the next crop...

http://nmsp.cals.cornell.edu/publications/factsheets/factsheet2.pdf

the nitrogen is shared through mature mycorrhizae, which invade the roots of each host plant and trade with each plant. If you, say, rototill yearly, the myc. never become functional enough to work that way. so it depends on the situation. With a never tilled asparagus bed, with plenty P, it might work. those Cornell pages need updating, and the asparagus bed may need inoculating through the soon to be seeded clover. most endo-myc. will do.

Mycorrhizae does not fix atmospheric nitrogen.

of course not. mycorrhizae take nitrogen from clover and give it to asparagus. works best when you have a long term relationship, such as the case described here. myc. also take P from wherever (possibly even from asparagus, if there are significant amounts too deep for clover to reach) and give it to clover. it is a prairie thing. something something bio-mimicry something something.