Some plants certainly can and do take up simple smaller Amnio acids.
Whether it's preferred or no is really not an issue with a lot of NO3 and NH4 floating around. It's hard for me to really say without measuring it in a direct manner, eg, using 15N isotopes for organic N and seeing where it goes and at what rate.
When you start getting into N limitings tanks/systems, then the organic N as a source of available N becomes more important.
There are two forms of nitrate. Inorganic nitrate is an ion consisting of one nitrogen atom surrounded by 3 oxygen atoms and carries a negative charge. Organic nitrate is different and is a functional group with the formula RONO2. R is the organic part of the molecule which can be very simple or a very long structured chain.
So nitrate is an ion, but it is also a functional group. These two types of nitrate are quite different which is why I was asking about both types.
What prompted me to ask was a certain person on APC is convinced that plants cannot use organic nitrate which I'd say is incorrect. Then other people state that plants don't care if it's organic or inorganic. I'm just curious to know if plants go for the inorganic or organic nitrate first, or are they not bothered. Personally I would think that the nitrate ion is more available.
And you'd be correct, it takes energy to break the organic bond, but this happens naturally via bacteria and fungi..............so in l,ow nutrient situations, this process is critical, if you have plenty of NO3 inorganic floating around.....then probably not.
The other issue, and one that's more critical is amnio acids, which are organic forms of reduced N.
These are what the plant makes from NH4 and NO3, so they require less energy to make into plant parts.
It depends on the system and it depends on what type of organic nitrogen is being discussed.
If thuis scertain person is convinced, they need to explain why they are so sure that aquatic plants cannot use it and state why, test, evidence, research background specific to aquatic plants or at least field crops.
Mere belief and speculation is hardly a standard to debate.
Another more basal question is: are there any real organic NO3 compounds such as nitrate esters and alcohols in your tank to begin with?
If not, this entire topic of debate is mute!
We have mainly organic reduced, not oxidized organic forms of N in our tanks.
So it's much more the R-NH2 groups that are really the issue.
The others are fairly rare in plants and tissues.
So...........we have amino acids, reduced organic N and NH4/NO3.
Without using a tracer, it's rather tough to say whether a plant uses or prefers anything, we might guess, but that's about it without actually testing it.
Testing and seeing if their hypothesis are true are not strong points for most aquarists. Seeing correlation and trying to tie that to cause and their belief is very much a factor however.
Thanks Tom for your detailed answer. One point you make that caught my attention is about the actual amounts, if any, of organic nitrates that are present. This to me rings very true and has often confused me when people state that the source of nitrates in a tank are fish and that these nitrates are organic nitrates. I can't see us having chemicals like methyl nitrate in our tanks. Amino acids make a lot more sense to me.
The big question now is what do nitrate test kits measure then, if there are no addition of nitrate salts or nitrate from the tap water? I presume a lot of the nitrates in our tap water are salts from agricultrual use.
Getting late here now so will do some investigations tommorow.
But isn't all organic matter produced from dead plants and fish waste converted back into ammonia through ammonification? The ammonia is then converted into ionic nitrates by nitrification. So what's the difference between this ionic nitrate derived from organic waste and the ionic nitrate we add as KNO3? I can however see how it is a lot better to dose KNO3 rather than trying to load a system with organics to produce NO3.
If there is unlikely to be any true organic nitrates in the water what other N containing compounds are there likely to be?
There are a few forms that plants will take up besides organic nitrates, those(Organic NO3's) are often toxicants, they will take them up if exposed, but our plants are rarely if ever exposed in aquariums.
But Amino acids are quite another matter.
Those will be taken up and are not toxicants. They are a bit like Excel and/vs CO2.
In general though, waste products such as decayed materials tends to be oxidized and mineralized back into NH4 then perhaps NO3 if the plants and algae did not get the NH4 first.
There is a huge difference between Organic N, NH4, NO2 and NO3 processes vs adding just NO3.
NH4 is toxic, so is NO2, each step consumes a lot of O2 and can be rate limited so it takes time for the bacteria and plants to remove it.
Meantime, fish and algae spores are exposed to higher levels of NH4 etc.
As far as DON, unless it gets mineralized, or is a an amino acid and few other smaller organic N forms, it's going to be less bioavialable