In some plants the nitrate is transported in the xylem. Fixation in the atmosphere Lightning Artificial fixation. The Haber process. Note the expense of this process and the possible production of CO2. Most nitrogen comes in the form of protein but some comes from nucleic acids.
Excess nitrogen is converted into urea in the liver.
Return to the soil Decomposers break down the protein of dead organisms into amino acids. Some of these are used for their own growth and the rest is converted into ammonia. This process requires oxygen. Denitrifying bacteria obtain energy by reducing NO3- to N2 and thrive in anaerobic conditions. Eutrophication means enrichment.