From the Central Ohio Agronomy Report #13 @ http://licking-cms.ag.ohio-state.edu/agriculture/licking-county-agronomy-report

I stated in another post that we observed a benefit from legume covers.  We believed it was from N-fixation.  Our OSU Soil Fertility Specialist cannot confirm that although there was a yield benefit.

This would be one of a few better balanced opinions.

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Cover Crops and Nitrogen –
Can I Capture Some Nitrogen for Next Spring?
Robert Mullen, Soil Fertility
Ohio State University Extension

Nitrogen prices are an issue on most producers' minds these
days. It appears as if the higher prices are going to be with us
for a while and you may be thinking of alternative methods of
supplementing nitrogen for corn. One possibility is growing a
cover crop after your wheat this summer and allowing it to
grow into next spring. The question is what kind of nitrogen
value might you expect?

Ohio State University has established field trials the last couple
of years to help answer this question. Experimental plots have
been at OARDC's Northwest, Western, and Wooster locations.
At the Northwest Research Station in 2007 we did not observe
any nitrogen contribution from red clover interseeded into
wheat for a subsequent corn crop. We did, however, observe a
rotational benefit of 18 bu/acre when red clover was
established compared to no cover crop. At the Western
Research Station in 2007, we did not observe a rotational
benefit associated with the establishment of red clover after
wheat harvest nor did we observe a nitrogen contribution.
From a separate study at the Western Research Station in 2007,
we did see what appeared to be a rotational benefit from field
pea established in late September, but we did not see similar
results for soybeans, early planted field peas (late August), or
annual ryegrass. We did not observe a nitrogen contribution
from any of the cover crops. At a fourth trial conducted at the
Wooster research location, we did not observe a nitrogen
contribution or rotational benefit with annual ryegrass seeded
after wheat harvest.

Take home message – based upon last year's findings we
cannot definitively identify a nitrogen contribution from the
establishment of any cover crop after wheat prior to corn. We
did see some rotational benefits (2 out of 4 locations) with
legumes, but at the two sites were non-legumes were evaluated
we observed no corn yield benefit. There are some research
publications from other areas of the U.S. that show nitrogen
contributions from legume cover crops, but our research does
not corroborate those findings. Establishment of cover crops
solely for the purpose of supplementing nitrogen should be
approached with caution. Legume cover crops have a greater
probability of supplementing some nitrogen (not based on our
research, but others have reported this) than non-legumes, but
it is not a sure thing. Additionally, establishment of the cover
crop is not a guarantee either. The earlier the crop is
established the greater the chance of good growth prior to the
onset of winter.
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