Nitrogen uptake refers to the fixation of useable nitrogen forms, which in plants are nitrate (NO3-) and ammonia (NH3). Plants are unable to acquire nitrogen in its gaseous form (N2). Plants possess ammonia transporters which are used for ammonia uptake and nitrate transporters for nitrate which are powered by a proton gradient.
Nitrogen is a critical element for plant growth. In fact, often continued plant growth is limited by nitrogen availability. Plants are dependent on processes in the nitrogen cycle for converting nitrogen into useable forms which they can uptake.
Nitrogen mineralization and nitrification are two processes in the nitrogen cycle which are beneficial to plants. Nitrogen mineralization increases the availability of ammonia within the soil. Microbes convert dissolved organic nitrogen into ammonia in this process. Nitrification is when nitrate is created through the transformation of ammonia. In this process ammonia is first converted to nitrite (NO2-), the rate limiting step, and then to nitrate.
To increase their efficiency for gathering useable nitrogen, some plants form symbiotic partnerships with cyanobacterial nitrogen fixers. For example, legume species such as soya bean have a partnership with Rhizobium bacteria.