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Re: Nitrogen


On Mon, 17 Apr 2000, Tom Barr wrote:
 
> But what about the presence of O2 and CO2 and PH stability? Would these
> factors not encourage N uptake if they are stable as in nature more than in
> our tanks?

Well, if stability implies high levels of CO2 all the time, then I suppose
that the plants would be better off.  But, as Diana Walstad pointed out,
large variations in CO2, O2 and pH are common in nature and plants are
adapted to handle it.

> Isn't Nitrogen in both forms a much coveted nutrient in nature
> and a plant or algae etc can be far better off by having the ability to get
> it all the time rather than just some here and there -even at the expense of
> sacrificing ATP for Photosynthesis or other resources- at a certain point? I
> tested zero NO3 and NH4+ in every location I found plants in a study I did
> and almost none were rooted.

I'm not real surprised by the unreadable levels of nitrogen sources.
natural water that supports good plant growth will probably be lower in
most nutrients than our tank water.  Surprisingly low in many instances.

Despite it's being found at low concentrations, nitrogen is not usually
thought of as growth-limiting.  Competition for nitrogen probably isn't as
great as it is for carbon, which many aquatic plants seem adapted to
scavenge.  There should also be great competition for phosphorus. Humans
think phosphorus is commonly limiting to plant growth in nature, but it's
hard to say what the plants think; they may not agree.

In the specific case of floating plants, I think some of their CO2 and
nutrient supply may come from their ability to host communities of
critters in their roots and fine leaves.  The critters (ranging from
cyanobacteria to fish) produce CO2 and biologically available nitrogen
directly to the roots and leaves of the plant.

I think I saw something like this work in my tanks once.  I had a small
but very healthy A. barteri nana in a sunlit, unfertilized tank where the
pH could rise over 9 in the afternoons and several plant species failed
miserably.  I had a hard time explaining why the nana did so well and
finally concluded that it was probably because shrimp and small fish
clustered in it's exposed roots. The animals gave the plant a direct
supply of CO2 and ammonia so the plant didn't have to compete for the very
limited supply available to the other plants in the tank.
  
> Basically, would it not be limited by the plant/algae needs for
> Photosynthesis products(sugars) against the need for a Nitrogen source? Or
> is this a chicken or the egg dilema? Can a plant switch gears so to speak in
> order to adapt to a given situation and need if things are limited
> (light,Nitrogen, perhaps more factors) to optimize the growth? Seems that
> evolution would have provided this ability.

Evolution provided a lot of ways for individual plants to adapt to
conditions in their environment.  I recently read what was to me a fairly
meaningful description of plants adaptations.  Paraphrasing (because I
don't have the original on hand right now); plants tend to allocate their
resources so that all factors are equally limiting.

I think this means (for instance) that if iron is in short supply that the
plant will take some of the energy it's using to get nitrogen and use it
instead to get iron.  That can lead to a nitrogen deficiency, but the
plant will be able to grow faster than it would without allocating the
extra energy to get iron.

I think one reason our tanks succeed with high nutrient levels is that we
are playing this energy balancing act.  Most of our tanks have relatively
low light compared to the plants' natural setting.  That means that algae
that are specifically adapted to high light conditions won't grow in our
tanks.  It also limits the amount of energy that our plants can allocate
to solve their nutrient problems.  We offset the limited energy supply by
providing nutrients in such high concentrations that the plants need
relatively little energy to get what they need.

> Guess I better start doing some digging for the answers myself, eh? I'll get
> there fairly soon<g>!

Have fun!

Roger Miller