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Re: denitrification and nitrate reduction (technical)
I am a marine biologist, not a microbiologist, but I do have a lot of applicable
physiology and biochemistry. I freely admit the following discussion is idle
speculation on my part. :-)
Mark Fisher wrote:
>>> Just like oxygen respiration, denitrification and nitrate/nitrite
>respiration allows for the complete oxidation of an organic molecule to CO2 and
>>> Whether or not ammonia or nitrogen gas is produced is dependent upon the
>kinds of bacteria present in the substrate, not the amount of carbon (or
>whether fertilizer is encased in clay). <<
>Ahh, I think I misunderstood what was told to me. But you are saying
>that neither reduction process will occur without the presence of the
>organic (carbon based) molecule? Therefore encasing the fertilizer in
>clay should delay denitrification until the nitrate diffuses out,
Perhaps. But what is to prevent the bacteria from colonizing the
outside of the clay ball, where they have access to both reduced carbon
(food) and nitrate, as it slooowly leaches out? Also, a penetrating
plant root would just open the flood gates to internal colonization.
Paul Sears' comment about osmocote containing urea is a good one.
However, I suppose it is also possible the ammonia cleaved from the urea
could be easily converted to nitrite, where it could then enter into the
usual denitrification pathway. As I mentioned before, though,
denitrification is strongly inhibited by oxygen, so a low-oxygen
environment may lead to the accumulation of ammonia, in this case.
>I think we will find the production of both ammonia and nitrogen gas
>occurring for several months in substrates containing organic matter or
>nitrate fertilizer. My limited experience suggests that the ammonia
>production occurs mainly in the first month whereas nitrogen gas is
>produced for several months. Does this make sense?
Yes, it makes a lot of sense. Ammonia production yields only one ATP per
nitrate molecule, while the production of N2 or N2O yields several ATP per
nitrate. Denitrification is a "better" long-term metabolic strategy than
nitrate respiration, as it has a higher energy yield per unit input.
Plus, as ammonia builds up in the substrate, it may favor the
proliferation of bacteria who can convert the ammonia into nitrite, and
into the denitrification pathway.
>You said the kinds of bacteria present determine which product is
>released. What factors determine which bacterial colonies are favored?
>Is it purely random?
Many genera of bacteria possess the ability to respire aerobically and
anaerobically; some of the denitrifyers are quite common, like
Bacillus, Pseudomonas, and Thiobacillus. They are on your hands, your
fish, the filter media, etc. (remember sneezing during your last water
change?) In the short-term, I suppose it depends on who gets there
first (assuming your substrate was sterile to begin with), but in the
long run, a real mixture could develop. I also imagine the bacteria
found at the bottom of the substrate would be quite different from
those found nearer to the surface.