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Re: Aquatic Plants Digest V2 #1123

> << But isn't it possible to limit N in the _water column_, while still not
>  allowing N to be limited in the substrate? >>
> Nitrogen is the single most common element in the atmosphere.  It will be
> almost four times more common in the water column than oxygen, because of the
> forces that cause exchange of gases at the surface.
> Bob Dixon

Plants can't use nitrogen gas as a source of nitrogen, regardless of
whether its atmospheric or dissolved.  It must first be "fixed" as
nitrate, nitrite, ammonia or possibly organic molecules (probably amines).
Nitrogen fixation doesn't seem like a very common feat - its done by a few
bacteria, by some cyanophytes and by lightning.  Lightning isn't likely in
your aquarium, and I don't think that biological fixation generates very
much free fixed nitrogen.

Atmospheric nitrogen won't effect nitrogen limitation.  If you want to
change the state of limitation from nitrogen limited to not nitrogen
limited then you need to either add fixed nitrogen or you need to lower
the concentration of something else 'till its more limiting than nitrogen.

If you aren't flushing water through your substrate, then probably you can
have a nitrogen-limited water column and enough nitrogen in the substrate
so that growth of rooted plants isn't nitrogen-limited.

Roger Miller