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Re: plant metabolism and nitrate uptake

> Date: Fri, 14 Aug 1998 11:27:27 -0700 (PDT)
> From: Anthony Ciarochi <Anthony.Ciarochi at eng_Sun.COM>
> Subject: Plant metabolism question

Louis Lin just asked a question about nitrate uptake by plants, so this
lets me get two opinions out in one reply.

> A question for any botanists, biologists, or 'smart people' (er, I mean
> AND/OR :-).

Ok, but I'll answer anyway.

> I recently read (either in this list's archives or on the Nature
> Aquarium page) that biological filtration is much less critical in a
> well-planted tank, as the plants are able to metabolize most any waste
> products produced by the fish.  Does this really mean that plants are
able to
> metabolize ammonia and nitrite, in addition to nitrate?

The fish produce ammonia/ammonium only.  The nitrite and nitrate are
produced later by bacteria in your filters.  But more to the point, yes
plants will metabolize ammonia and nitrate.  I'm not sure about nitrite,
but I have no reason to believe that plants wouldn't use that as well.
Even if they don't metabolize nitrite the bacteria will quickly convert
the nitrite to nitrate, which the plants do use.

Four of my six planted tanks have no biological filtration.  None have
detectible ammonia.  In at least one case (the only one I've measured in a
long time) there's no detectable nitrate or nitrite, either.

In order to safely reduce biological filtration in a tank with potentially
sensitive fish you need to be sure that you have and can maintain robust
plant growth in the tank.  Its also safer if the tank carries a low
nitrogen load (i.e., not much feeding).  The first time I removed
biological filtration from a tank I monitored the ammonia in the tank
carefully for several days, as much as twice a day.  I did get detectible
ammonia for a short time but it quickly disappeared.

Louis Lin asked if plant uptake of nitrate could increase pH in the same
way that nitrification tends to decrease it.  I think it does, but the
case isn't real simple.  The net reaction appears to require uptake and
metabolism of at least one hydrogen ion for each nitrate ion that's used
by the plants.  I once read (in Raven's "Energetics and transport in
aquatic plants") an argument that under high pH conditions the requirement
for H+ to accompany nitrate uptake could be growth limiting for aquatic

Roger Miller