Nitrogen is absorbed by the roots of plants in the form of negatively charged nitrate (NO3-) and positively charged ammonium (NH4+) ions.
In anaerobic soils NH4+ is readily converted to NO3-, and NO3- is the predominant form of nitrogen absorbed by crop roots.
However under anaerobic conditions such as flooded paddy fields, ammonium is the predominant form in the soil and is the main source for rice.
Roots can absorb simple amino acids which has relevance for Triple D products.
Urea is absorbed by roots but is also very readily converted in the soil to ammonium or nitrate .