Nitrogen assimilation

Organisms like plants, fungi and certain bacteria that can fix nitrogen gas (N2) depend on the ability to assimilate nitrate or ammonia for their needs.

[4] Plant roots themselves can affect the abundance of various forms of nitrogen by changing the pH and secreting organic compounds or oxygen.

[6][7] Nitrogen is transported from the root to the shoot via the xylem in the form of nitrate, dissolved ammonia and amino acids.

Ammonia (both absorbed and synthesized) is incorporated into amino acids via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway.

[12] This may help avoid the transport of organic compounds down to the roots just to carry the nitrogen back as amino acids.

To maintain a pH balance, the plant must either excrete it into the surrounding medium or neutralize it with organic acids.

Plants like tomatoes take up metal ions like K+, Na+, Ca2+ and Mg2+ to exactly match every nitrate taken up and store these as the salts of organic acids like malate and oxalate.

[21] Better fertilizers, improved crop management,[21] selective breeding,[22] and genetic engineering[20][23] can increase NUE.

Nitrogen use efficiency can be measured at various levels: the crop plant, the soil, by fertilizer input, by ecosystem productivity, etc.

Different plants use different pathways to different levels. Tomatoes take in a lot of K + and accumulate salts in their vacuoles, castor reduces nitrate in the roots to a large extent and excretes the resulting alkali. Soy bean plants moves a large amount of malate to the roots where they convert it to alkali while the potassium is recirculated.