Nitrogen assimilation by plants

Elmerich, C. (2001) Nitrogenase biochemical, molecular and genetic aspects, Nitrogen Assimilation by Plants 149-167. 

Moeot-Gaudey JF, ed. (2001) Nitrogen assimilation by plants. Science Publishers, Enfield. 

De Saussure also noted the contribution of atmospheric deposition, adding that nitrogen assimilated by plants also comes from vegetable and animal materials held in suspension in the atmosphere, which deposit upon the plant (Conant et al. (2), p. 431). 

In recent years greater attention has been given to nitrogen containing pesticides and the possibility of their nitrosation in soil. The N-nitrosamines that form may arise from the parent pesticide or from a pesticide metabolite. The reaction calls for favourable pH conditions (pH 3-4) and excess nitrite. Under field conditions, the nitrosable residues are usually present in traces and only small quantities of these will actually be nitrosated in soils. However, the possibility exists that the small amounts of N-nitrosamines could be assimilated by plants. 

Liebig,13 thought that the nitrogen of the atmosphere could be directly assimilated by plants, the value of manure residing wholly in the mineral content of its ash. Lawes disagreed with this view, as did Boussingault, whose opinion was that a manure s value lies mostly in the azotized portion. 

Animal and human wastes have long been used as fertilizers, especially in Europe and Asia, particularly China. Even some American Indians are said to have planted a dead fish in each com hill to increase yields. Such materials contain small percentages of nitrogen and other plant nutrients that are assimilable by plants. Today, the use of raw sewage on crops persists in Asia and Europe, but volumewise is not of great significance. In the United States and most European countries, the use of raw... 

Nitrate (NO3 ). This is a negatively charged ion (or auiou), and is highly soluble in water. Nitrate is the preferred form of nitrogen nutrition for most species of plants. If nitrate is not assimilated by plants or microorganisms, it is readily leached from soils. 

Ammoniaeal compounds.—Carbonate, nitrate, and nitrite of ammonium occur in small quantity (0.1 to 6.0 parts j>er million of NHa) in air, as products of the decomposition of nitrogenized organic substances. They are absorbed and assimilated by plants. 

This mechanism of loss of nitrogen is now considered as one of the more important channels of loss from soils. As already pointed out, soil organic matter is constantly undergoing decomposition with the formation of ammonia as the end product of the degradation process. The ammonia rarely accumulates, however, for if not assimilated by plants or microorganisms it is oxidized rather rapidly to nitrite, and then to nitrate. In the absence of a crop, nitrate nitrogen may accumulate to the extent of 20, 50 or even 100... 

This unfavorable effect of low pH is usually intensified by the nitrogen fertilizers themselves, because most of them increase soil acidity. This may be due to the added acid, as in the case of S04 in ammonium sulfate. It may also be due to the NOs added as neutral salts, or to that formed in the nitrification process. If the NOs formed by nitrification is assimilated by plants and converted into protein, there is no appreciable direct effect on soil pH, but if the acid is leached out in the form of nitrate salts then the increase in soil acidity is marked. Even anhydrous ammonia, which is strongly basic, will produce acidity to the extent that it is nitrified and lost to the drainage waters as calcium nitrate or as some other neutral salt. Even if biologically-fixed nitrogen, present in plants as protein, undergoes decomposition to ammonia and oxidation to nitrate, it increases acidity if leaching occurs. Most of the increase in soil acidity is due to the removal of bases as nitrate salts. The possible reactions are explained in detail by Allison (1931). 

The quantity of nitrogen assimilated by the algae and the molds is ordinarily considered as being too small to be taken into account. Only bacteria are of real interest from the point of view of the enrichment of the soil in combined nitrogen. And yet, of all these, the most important, as we shall see, are those which are capable of living in symbiosis with the plant. 

Ammonification This process converts organic nitrogen to ammonium, which can then be converted to other forms of nitrogen that can be assimilated by plants or organisms. The process of ammonification is facilitated by bacteria or fungi. 

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