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Nitrogen uptake in aquatic plants.

     Hi Folks ... 
     Following a brief discussion with the local aquarium shop owner I found 
     myself confused as to aquatic plants' preferred sources of nitrogen.
     On this subject, the bulk of the postings to APD over the last couple of 
     years suggest that the plants will take up NH4+ in preference to NH3, 
     NO2 and NO3. However the Fish Shop Guy swears that NO3 is the preferred 
     source of N for aquatic plants. His argument, which consisted in the 
     main of simple assertion, was less than compelling.
     Unhappily my browsings through the APD back issues has not provided a 
     hard line of argument to take back to the FSG. I have however gleaned 
     1. NH3 and NH4+ exist in equilibrium, and this equilibrium is affected 
     by pH and temperature. 
     2. Colloidal clays offer binding sites for cations (i.e., NH4+). As 
     George Booth has it: "The negative sites [CEC sites offered by colloidal 
     clays] attract and hold the ammonium ions ... until a plant root hair 
     exchanges another [cation] for the ammonium (adsorption) and takes it in 
     to metabolise into amino acids and ultimately protein." 
     3. Another writer (apologies, should have kept the references ...) noted 
     that plants contained an enzyme which allows NH4+ ions to be attached to 
     C 'skeletons' to form amino acids. The writer also noted that plants 
     [all plants? some plants?] could reduce NO3- first to NO2- and then to 
     NH4+ by way of enzyme activity. However this nitrogen reduction process 
     is said to be energy expensive and is bypassed if NH4+ is immediately 
     available. A second caveat in my mind is that this writer sourced his / 
     her comments from a plant physiology text that dealt with terrestrial 
     plants. Do aquatic plants use a different set of reductions?
     4. I am also aware that Diana Walstead wrote an article in a 
     peer-reviewed journal that (as I understand it) reported on experiments 
     involving Spirodela that revealed the plant's preference for N delivered 
     as NH4+. Does anyone have a source for this article? 
     I have a number of other questions. 
     Is there an authoritative text with detailed information on this topic 
     (N uptake in aquatic plants)? 
     Do aquatic plants take up NH4+ solely as a function of cation exchange? 
     Is this function limited to activity at the substrate level or do the 
     plants (or some plants) have an ability to take NH4+ (or nitrogen in 
     another form) straight from the water column? If so, what is the 
     machinery the plant uses to do this? 
     Do aquatic plants take up NH3 at all? NH3 doesn't seem to figure in the 
     account of the nitrogen reduction process mentioned above. Is the 
     beneficial effect of plants on N levels centred on moving N from an 
     occurrence as NH3 to an occurrence as NH4+ (given that the two species 
     exist in equilibrium)? 
     Thank-you for your patience, gentle reader.