[APD] RE: N #4

["Thomas Barr" <tcbiii at earthlink_net>]

> Just a question here: NH4 --> NO2 is an exothermic reaction, as is NO2 -->
> NO3 (after all, that's where the bacteria
> colonies get their energy from). So, if I offer both NH4 and NO3 to
plants,
> wouldn't the plants preferentially take
> up NH4 because it takes less energy for them to get at the N?

Sure, 8 electrons less. NO3=> NH4 is one of the most expensive
conversions/reductions in the plant metabolism.

But the NH4 transportor that takes in the NH4 only can go so fast.
A healthy plant does best when it comes to NH4 removal.
The plant can only take in so much, plants are good at storing NO3 in their
vacuoles.
There is also an issue with cation/anion balances, too much of one drives
the system slower than a nice balance of both.

Most submersed aquatic plant research had all or nothing ratio of NH4/NO3
rather than an optimal ratio.. 

I detail this out more in TAG.
Just wait and read that.

NH4 while valuable to plants, is a real prize for an algae spore.
Think about an Elephant getting a single peanut, then a mouse. Would would
get fat and happy from that?
Who could go further, grow more from that?
The algae spore will gain a lot from that relative to a large plant. 

> If this assumption is correct, then that might help to explain why healthy
> and actively growing plants
> help to control algae: a healthy plant is biologically active and would
have
> "a go" at any NH4 as
> soon as it appears, thereby removing it from the water and depriving algae
> spores of an essential nutrient.

Well "a sort of essentail" cation that helps induce spores into vegetative
mode.
Niether is truely essential, since both can live on NO3 but the spores
don't seem to grow
without the free meal of NH4.

But yes, this is the only limited nutrient cation that seems to play an
significant role in terms of plant/algae dynamics.

> On the other hand, a plant that isn't growing well would take some time to
> activate and remove
> the NH4, thereby allowing the spores to get at it first and establish
> themselves as algae. Then, once the
> algae are established, it doesn't matter much any more what the plants are
> doing because, once established,
> algae can get their N from NO3 just as well as the plants can, and the NH4
> concentration becomes a
> secondary factor.

You got it.
 
> If all this (admittedly surmise) is correct, that would help to explain
why
> a well-planted and fertilized
> tank tends not to get algae and why, once algae are present, it's so
> difficult to get rid of them again.

Yep, took awhile but seems to fit well with the model.
Regards, 
Tom Barr

> Michi Henning              Ph: +61 4 1118-2700
> ZeroC, Inc.                http://www.zeroc.com



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