Article in Nature suggests some of the N in moon soil comes from early earth because the 14N/15N isotope ratio for lunar N is 30% from what it would be if it arose purely from solar wind. They propose that in the absence of a geomagnetic field on the early earth, there was a significant earth wind i.e. the surface N on the moon arose from a combination of ions carried in an earth wind (I have cut out a quote from the paper below that explains why). The surface N of the moon is, they suggest a combination of earth wind and solar wind. This gives a potential measurement of when the earth’s geomagnetic field kicked in; the authors’ best guess is that this must have been around or after 3.8-3.9 billion years ago. They suggest a test for this hypothesis; collecting and analyzing rock from the far-side of the moon. This may help pinpoint the origin of life since without the protection of the earth's magnetic field it is gassumed any emerging life forms would have been baked by solar radiation. (I'm not happy with that argument because it makes a lot of assumptions, all it takes is for the first organisms to hve evolved under a rock or in a cave or at the bottom of an ocean if there was one etc etc.).
http://www.nature.com/nature/journal/v436/n7051/pdf/nature03929.pdf

The editorial commentary is: - http://www.nature.com/nature/journal/v436/n7051/full/436631a.html

Terrestrial nitrogen and noble gases in lunar soils
M. Ozima, K. Seki, N. Terada†, Y. N. Miura, F. A. Podosek & H. Shinagawa.
Nature 463, 656

In the presence of the GMF (geomagnetic field), the SW (solar wind)
approaching the Earth is stopped at the magnetopause (at a distance of ,10 Earth radii, 10RE), where
the GMF magnetic pressure balances the SWdynamic pressure. If the
GMF did not exist or were much weaker, the SW would approach
much closer to the Earth, until its dynamic pressure was balanced by
ion pressure at the ionopause, about 500 km above the surface. The
SW must thus have interacted directly with the ionized (by solar
radiation) upper atmosphere. Atmospheric ions around the ionopause
could then have been picked up by the incoming SW and
carried away from the Earth6. This SW-induced ion escape mechanism
from a non-magnetic planet has been observed for Venus12,
which has no permanent magnetic dipole. Thus, if the GMF were
absent or extremely weak, we would expect substantial ion escape
from the Earth’s ionosphere. Some fraction of the escaping ions
would have hit the Moon.