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> Thomas Barr wrote:
> > > > http://www.plantphys.net/article.php?ch=e&id=158
> ...
> > This NO3=NH4 dynamic is in line with the notion of NH4
> > limitation/competition with macrophytes and algae is perhaps
> > a larger role
> > than previously considered or investigated.
> Tom, can you clarify what you mean above?

"The form in which N was supplied did not influence plant growth at 360
µmol mol?1 (ambient) CO2 , but had a dramatic effect at 700 µmol mol?1
(elevated) CO2 (Figure 6). Leaf area in the elevated CO2 treatment relative
to the ambient CO2 treatment increased 49% under NH4+ nutrition, but only
24% under NO3? nutrition. Total plant biomass increased 78% under NH4+
nutrition, but only 44% under NO3? nutrition. As a result, the plants
receiving NH4+ were more responsive to CO2 enrichment than those receiving
NO3? ."

If you add CO2(we add much more relative to this study) then this increases
the plant competition(uptake) for NH4 vs the algae perhaps. What is not
known is what this type of effect would do on algae, but a CO2 enrichment
and NH4 appears to influence plants significantly vs NO3.
The effects of CO2 addition/enrichment on algae presence might be a
function of NH4 and not the carbon needs of the plants so much. 
It is important to note that this study is not aquatic and not the same
plants as we keep.

Tom Barr

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