Plants absorb nitrogen from the soil in the form of nitrate (NO3-) and ammonia (NH3). In aerobic soils where nitrification can occur, nitrate is usually the predominant form of available nitrogen that is absorbed. However this need not always be the case as ammonia can predominate in grasslands and in flooded, anaerobic soils like rice paddies Plant roots themselves can affect the abundance of various forms of nitrogen by changing the pH and secreting organic compounds or oxygen. This influences microbial activities like the inter-conversion of various nitrogen species, the release of ammonia from organic matter in the soil and the fixation of nitrogen by non-nodule-forming bacteria.
Xisca Nicolas wrote:
And what is the form of N in rain?
Then diluted urine commonly used is useless or even bad? What happens to it in the soil?
Many soil bacteria possess the enzyme urease, which catalyzes the conversion of the urea to ammonia or ammonium ion and bicarbonate ion, thus urea fertilizers are very rapidly transformed to the ammonium form in soils. Among soil bacteria known to carry urease, some ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas, are also able to assimilate the carbon dioxide released by the reaction to make biomass via the Calvin Cycle, and harvest energy by oxidizing ammonia (the other product of urease) to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils because of its negative charge and is a major cause of water pollution from agriculture. Ammonium and nitrate are readily absorbed by plants, and are the dominant sources of nitrogen for plant growth.
John Elliott wrote:Very interesting video...
Nitrogen is assimilated into plants in two forms, as ammonium (NH4+) and nitrate (NO3-). When a plant takes in NH4+ and it enters the cytoplasm of a cell, one of the H+ in its ionic structure quickly combines with an OH+ typically floating around due to the relatively high pH. The result is a water molecule and an ammonia molecule (NH3). This reaction changes the pH of the cytoplasm by decreasing the OH+ concentration (that is, increasing the H+), which can quickly create toxic conditions and mess with the transport of electrons needed for photosynthesis and respiration. In order not to have this happen, ammonia is either converted into organic molecules or transported to a vacuole, where the conditions are acidic, meaning there are plenty of H+ to return it to ammonium and thus a non-toxic state.
Nitrate is also converted into organic molecules. At first, this typically occurs in the roots, but as more and more nitrate accumulates, it is moved to shoot cells for assimilation. The conversion to organic molecules or assimilation causes NO3- to become nitrite (NO2-), which is toxic to plant cells.