04-13-2013, 10:27 AM
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In her book, Walstad cites the scientific studies that determined the preference of various plant species for nitrogen forms. Most aquatic plants prefer ammonium, and in studies where ammonium, nitrite and nitrate were present the plants took up all ammonium first. Then they turned to nitrite. And last to nitrate. Walstad suggests this is likely because plants have to change nitrite and nitrate back into ammonium before they can use it as nitrogen, and this takes more energy/work by the plant, which has the effect of reducing their energy devoted to photosynthesis (growing and reproducing). Plants inherently want to devote energy into photosynthesis, so they have evolved methods to focus on this.
As I think someone said, in well-planted tanks that use the natural method, nitrite will never be detectable with our test kits (it will be too minimal) and nitrate will usually remain very low. My tanks run < 5 ppm nitrate, which is between zero and 5 with the API kit which may mean nearer zero, and many with similar tanks have zero nitrate continually. Using common sense, I assume this is because the plants are grabbing most of the ammonia/ammonium before bacteria/archaea have the opportunity, which means very little ammonia/ammonium gets changed into nitrite and thus very little nitrite to nitrate. Walstad does cite scientific studies which show that plants out-compete bacteria for ammonia/ammonium.
Now, obviously all other factors must be present for this to work. Meaning adequate light to balance the nutrients, and sufficient nutrients (all 17). If something, say carbon (CO2) is insufficient, plants can't photosynthesize fully so any excess ammonia would then presumably be taken up by the bacteria/arachea. Plants can and do take up ammonia as a toxin as well as a source of nitrogen, so this too enters the equation.