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Re: NO3 and some resolve

Tom Barr wrote:

> >>> Something is using up
> >>> the NO3 because the test detected less than 5 mg/l.  So I added another 1/4
> >>> tsp KNO3 to get between 5 to 10 mg/l of N03.
> >>
> >> This is clear evidence of plant useage.
> >
> > Well, almost.   Don't forget about denitrification.  There are cases
> > where denitrification can consume a pretty large amount of nitrate.  In
> > that case there can be declines in nitrate concentrations without much
> > plant use.
> 
> Yes, perhaps. Can you tell how much? Difficult. Check out the end of this
> rant if your interested in a way though.

In Chin See Ming's case (what started this), probably there was no
significant denitrification because the tank setup was too new to build
up the necessary organic mass.

>  I had a batch denitrifier way back when. I was looking for the holy grail
> of never doing water changes.

I tried to do the same thing with a paludarium in a circulating range
with one of my tanks.  My idea was to combine plant uptake with
bacterial 
dentrification in an organic substrate.  To make that work I pumped
water slowly through the substrate. I even built my own variable-speed
centrifugal pump to run the whole thing. I could measure differences in
the nitrate content going into and out of the paludarium, but it never
had the effect that I wanted on the nitrate content in the tank, because
the necessary flow rates were too low.  I know now that it probably
would have worked better if I had scratched the idea of using bacterial
denitrification and used a higher flow rate, a higher density of plants
and faster-growing plants.
 
> I have not seen one yet nor tested a tank as such that the denitrification
> is or was a cause for any concern regarding NO3 usage as far as I could
> tell.

I don't think I have either, but that doesn't mean it isn't possible. 
People on this list have reported nitrate consumption rates that are out
of balance with the amount of growth they get in their tanks.  After a
lot of thought I finally concluded that unless I rejected their methods
and their test results that those reports were probably evidence for
significant denitrification.

We see fairly direct evidence for denitrification any time bubbles rise
out of a substrate.  The bubbles almost certainly started as nitrogen
gas resulting from denitrification.  Bubbles of pure nitrogen gas would
only carry about 1 mg of N2 per cc, so if all the N2 produced by
denitrification went into the bubbles, then a significant rate of
denitrification would produce a *lot* of bubbles.  Most of the nitrogen
gas probably remains dissolved in the water until it reaches the surface
and escapes, rather than forming bubbles.

>  Some tanks are anaerobic in large parts but there little flow in and out of
> these areas as well. How does one get large removal rates without
> sacrificing low flows in/out of the gravel?

That's the fundamental problem with denitrification.  The main thing
that's necessary is reactive organic matter.  In most tanks that just
means mulm.  Anaerobic bacterial processes -- even process much more
extreme then mere denitrification -- have been found to happen inside
floccs less than a mm across suspended in aerated water.  So it really
doesn't appear that with enough organic matter it should be all that
difficult to get denitrification going at signficant rates.  Some sewage
treatment plants denitrify really remarkable amounts of nitrate.

We can get some denitrification in a mature substrate due to slow
plant-induced circulation or maybe even due to circulation induced by
heating cables.  Dirty filters are also possible denitrifiers.

I agree that most of the time when people report nitrate use in their
aquariums that it's the plants that are responsible.  Still, I recall
reported cases where plant growth doesn't seem to balance nitrate loss,
so denitrification appears to be significant.  That just means that when
nitrate drops it isn't always *perfectly* clear that the plants are
taking it up.

For natural aquatic environments I think there's a substantial body of
research data -- including some using isotopic tracers -- that quantify
the relative rates of denitrification and plant or algae uptake.  Those
studies are probably not relevant to planted aquariums.  Your research
would be new :)


Roger Miller