The ultimate action that a plant could do with ammonia is oxidize it to
nitrate and then pump it down through the roots and into the soil. Bacteria
in the soil would love that source of oxygen. They could use it to break
down polyphenols that are bound with copper, zinc and iron. The bacteria
would free more of those trace minerals than it needs, and whatever is in
excess would go back to the plant. So, it is an open question. I will try to
find an answer on pubmed, because I can't nail down the best way to
demonstrate that plants actually perform something that would be so
advantageous. There could be radiolabelling studies. Someone might just
provide a plant with soil that is free of nitrate to begin with, then
measure levels of nitrate in the soil and in the water after a year.
