free nitrogen (the gas) is not available for use by plants. nitrogen fixing bacteria
(like those found in nodules of clover, etc.) must fix the nitrogen to ammonia. that
is an anaerobic process. http://www.backyardnature.net/econitro.htm
Ingrid

One reason for a deeper VF would be (assuming a bog design with an actual
soil bed), to allow anaerobic conditions to occur in the deeper soil layers,
which allows for the reduction of nitrates back to nitrogen, which is more
efficiently utilized by all the plants.