Inefficient N absorption, Km values for NO3- and NH4+ absorption by higher plants may be orders of magnitude higher than those of algae and fungi (Galvan et al., 1996; Unkles et al., 2001).

What does that mean? That enzyme kinetics is such that plants need several orders of magnitude higher concentration of nutrients such as NO3 to grow well versus the algae and fungi. Folks seem to think they can out wit algae and starve them to death. No........that's rather impossible if you also have healthy growing plants..........plants can remove NH4 to very low levels though and still have high NO3 levels available.

Although NO3- is the predominant form of N in well-aerated and pH-balanced soils as well as aquariums, NH4+ is an important and commonly underestimated N source for many plant species, in part because in mixtures of NO3- and NH4+, NH4+ inhibits NO3- uptake. In Lemna sp.(Duckweed), in rice (Oryza sativa), and in Arabidopsis, NH4+ influx consists of both saturable high-affinity influx (HATS) and non-saturable low-affinity influx (LATS) that operate at low and high NH4+ concentrations, respectively (Ullrich et al., 1984; Wang. et al., 1993; Rawat et al., 1999). Such HATS NH4 transportors appear to operate at the 1/2 constants of 500 nanomoloar ranges........very low indeed versus the LATS which operate at the 40 micromolar range once induced and adapted for.

About 80X difference in uptake "efficiency". As plants adapt and get settled into their environments, alteration of the N can cause profound effects. Having low stable levels or high stable levels works well. Having varied levels through time does not. Likewise, algae also respond to changes, but they upregulate these super high affinity enzymes that can scrounge the smallest trace amounts of NH4 and NO3 that are far below the ranges of any hobby test kit and and way below limiting levels for aquatic plants.

Plants leach NH4 into the water, they also transport it up through the sediment, bacteria, other senscencing algae release NH4 and NO3. While limiting this and changing the nutrients can influence algae biomass, this party is short libved and the resistent tough algae that's well adapted to supper low nutrients stick it out and hangs on for dear life.

Plants? They are still trying to up and/or down regulate their enzymes and genes to deal with all these changes.

As aquarist, having good reliable data for NO3 and NH4 is very problematic.
We can supply the plants with high or low levels that are stable, but in between can be difficult.

EI and non CO2 methods are very robust for such reasons.
ADA's substrate is as well, it supplies a stable source of N to the roots and low levels to the leaves. Adding EI+ ADA/soil/nutrient rich sediment will provide for more stability for both sources and for the whole plant. The goal is to keep the NH4 low as possible and the NO3 at good higher range, say 10-30ppm or so.

Note, CO2 supply is critically linked and co regulated with Nitrogen uptake and amino acid biosynthesis (Kolbe et al, 2006).

Regards,
Tom Barr

Here is an example of the uptake rates due to the various HATs and LATs: