Re: NO3 losses

[Roger Miller <roger at spinn_net>]

Tom Barr responded to Antonio Trias:

> > When I said N is secuestred by the biomass I wanted to  explain  the 
> > bigest
> > part of N plant uptake is IN the tank, IN the plants you leave, in its
> > rizomes
> > roots and leafs, and not only on the part  you are trimming. (The plants,
> > fishes and microfauna, they  not have the same weigth the day you set-up
> > your
> > tank or after one year)

> But relative to the amount of N added and removed from the water column,
> this is an insignificant amount unless you are removing large amounts
> weekly/monthly or fish/critters/animals etc.
> Also consider you are adding food for these aniumals also and some of the
> waste ends up as plant "food" while some fish/animals eat some small amount
> of algae.
> The amount of N uptake from the water column is very low relative to the
> plants and or algae.

Tom, 

It may not be "very low", depending on the situation.  Anyone who wants to 
calculate a nitrogen balance for a planted tank needs to consider the amount 
of nitrogen in the biomass.  Particularly in fish, as their nitrogen content 
is rather high and the process of growing out young fish can lead to uptake 
of quite a bit of nitrogen.
 
Similarly, a nitrogen balance needs to consider the amount of nitrogen that 
is added to an aquarium when new plants and livestock are added to the tank.  
Don't forget the nitrogen content of the detritus that gets washed out of 
filter material.  The N content of detritus can be quite a bit higher than 
the content in undecomposed material, so a small amount of detritus cleaned 
out now and then might contain a disproportionate amount of nitrogen.

> > In a tank with a deep substrate is a lot of space for denitrifcation, 
> > everyone
> > of us can constate ours sustrates bubbling, and these bubbles are mainly
> > N2.

> CO2 and O2 are far larger % of the gas evolved from the substrates. Roots
> bring O2. O2 is used by Roots and expelled into the region around the roots.
> ASll this O2 allows a large aerobic bacterial colony to grow. These aerobic
> respire and give off CO2 gas.

While O2 or CO2 may be present in substrate bubbles I doubt O2 or CO2 is the 
dominant gas in substrate bubbles -- the exception being bubbles formed by 
algae growing under the gravel line against the well-lit side of an aquarium. 
I doubt that CO2 could ever reach levels necessary to form bubbles.  It is 
just too soluble.  While plants may aerate their rhizosphere (the actual size 
of the area they aerate being disputable) I don't think they do it by pumping 
oxygen out of their roots at levels necessary to form bubbles.

Nitrogen is normally present in water at about 80% of the concentration 
necessary to maintain bubbles of pure nitrogen, so it doesn't take very much 
of an increase in N2 gas concentrations from denitrification to start bubbles 
forming.

> > More space is available in every mature biofilm, even in very  oxigen rich
> > waters.-
> > The lost of N by denitrification,  (I,m remembering because at home I do
> > not
> > have bibligraphy) should be more or less 50%,

> 50%? Well the highest levels of denitrification I've found was around 2-3%
> in Danish Estuaries.

I didn't have to do more than turn around and grab a volume off my office 
shelf to find out that dentrification losses from fertilized soils easily 
exceed 50% and that dentrification losses in unstratified lakes may consume a 
substantial part (20-30% if I recall correctly) of the influent nitrogen.  
Our aquariums are heavily fertilized and it seems likely to me that some of 
our aquariums lose a substantial amount of nitrogen to denitrification.  
Denitrification requires a near absence of oxygen and a readily available 
organic substrate.  Those conditions are easily attained in the substrate and 
in filter media.  Furthermore, the significant denitrification can occur in 
microenvironments -- little anaerobic islands in otherwise aerobic conditions.

> A very simple solution to test this simply requires you to remove the
> substrate and grow Riccia and/or plants lacking substrate root
> attachments(Just have all the nutrients in the water column in an empty
> tank). No substrate, no N2 losses.

You may easily show that aquariums exist where there is no denitrification.  
That isn't the point.  The point is that anyone who tries to calculate the 
nitrogen balance for an aquarium needs to consider denitrification.  Not 
doing so would be as negligent as ignoring the nitrogen content in fish food.


Roger Miller