I think that the plants are taking up ammonium, bypassing the Nitrogen cycle that results in NO3. So the plants are getting their Nitrogen without our being able to detect it. Sorry it took me this long to come up with a clear-headed solution
I honestly don't have a clue about why Nitrogen is dosed in high-light tanks if this is the case.
Steven
Steven, in a high-tech setup, all nutrients have to be increased along with the light in order to balance. Nitrogen is one nutrient, and we know that most aquatic plants prefer to uptake nitrogen in the form ammonium (ammonia). The amount of nitrogen required to balance is beyond what the fish and organic breakdown will provide, so, like all other nutrients including carbon (CO2), it has to be added somehow. Most aquarists would not like to be adding ammonia, so nitrate is the usual form of nitrogen. The plants, being driven to photosynthesize full out, will take up the nitrate when ammonia/ammonium is no longer available.
When you are talking low-tech or natural method planted tanks, such as you and I both have, dosing nitrate is foolish because there is more than sufficient ammonia/ammonium available to balance the moderate light and less carbon (CO2) in particular. The fact that we also employ filtration and a part of this is biological, meaning the nitrifying ammonia-oxidizing bacteria are also taking up ammonia/ammonium, shows just how much there is naturally. We discourage excess biological filtration in natural planted tanks so we can be confident that there will be sufficient ammonia/ammonium for the plants' needs, in balance with everything else. And beyond this, plants can take up toxic ammonia aside from using it as a nutrient, and they have a fair capacity for this (according to Tom Barr).
Back to your initial question, you have answered that here. Plants out-compete the bacteria for ammonia/ammonium, so little gets into the nitrification cycle. Our basic aquarium test kits will not detect these small amounts, thus you see (or should) zero ammonia, zero nitrite, and very low if not zero nitrate.
Nitrate is also being used by different bacteria in the substrate. Walstad in her book refers to her own experiments proving that bacteria in the substrate significantly reduced nitrates, but these remained high in non-substrate containers of the same water.
But again back to your initial question, nitrate at zero is not unusual in planted tanks set up as low-tech or natural, and nitrates are not required by the majority of plants in such setups. And for those that do, the fish load most of us maintain likely ensures there will be sufficient.
Byron.