ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 3. ALLELOPATHIC INTERACTIONS OF HIGHER PLANTS

3.3. Nitrogen nutrition of plants and inhibition of nitrification by vegetation

Plants are capable of absorbing chemical elements necessary for their development from the environment. Among these, the primary role belongs to organogenic elements, which form The basis of Organic compounds: carbon (averaging 45% of the total), oxygen (42%), hydrogen (6.5%), and nitrogen (1.5%).

Nitrogen plays an exceptionally crucial role. Plant Tissues contain inorganic nitrogen, as well as low-molecular-weight and high-molecular-weight nitrogen-containing organic compounds. The bulk of nitrogen is concentrated in Proteins (80–95% of its total content), Nucleic Acids (up to 10%), and Amino Acids and amides (up to 5%).

Microbially mediated Nitrogen Fixation is a biochemically advantageous interaction between Bacteria and plants. At the same time, the inhibition of nitrification by vegetation is a classic example of allelopathy.

Nitrogen fixation by plants. Plants utilize only mineral nitrogen, which is predominantly present in the form of NH⁺₄ and NO^-₃ ions, accounting for a mere 0.5–2.0% of the total nitrogen reserves in the soil.

NO^-₃ ions are highly mobile in the soil, readily leaching into deeper soil layers and Water bodies. Meanwhile, NH⁺₄ is less mobile and becomes fixed in soils rich in clay minerals.

The majority of nitrogen found in organic residues as part of nitrogenous organic compounds (proteins, amino acids, urea, etc.) is utilized by plants only after being transformed by ammonifying microorganisms. These microbes contain Enzymes that break down proteins into amino acids, which are then deaminated to form NH₃. One of the possible Pathways of ammonia formation during amino acid transformation involves The production of an oxy acid and NH₃ through The addition of H₂O.

Ammonification during the Mineralization of organic nitrogen culminates in nitrification—a process carried out by bacteria. In The First stage, Nitrosomonas bacteria oxidize ammonia to nitrite, for example:

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In the second stage, Bact. nitrobacter bacteria oxidize nitrite to nitrate:

Using the energy derived from oxidation, nitrifying bacteria are capable of assimilating atmospheric CO₂ or carbonates and utilizing them for the synthesis of organic compounds.

Nitrifying bacteria can oxidize ammonium nitrogen derived not only from organic residues and manure, but also from fertilizers. This can lead to excessive leaching and loss of a significant portion of the applied nitrogen. To prevent such losses in crop production, nitrification inhibitors are used—such as 2-chloro-6-(trichloromethyl)-pyridine—which suppress the accumulation of nitrates in plants at levels toxic to humans and animals.

The content of plant-available nitrogen is also altered by denitrification processes carried out by anaerobic microorganisms, which reduce NH₃ to gaseous N₂.

The ROOT System of plants is capable of directly absorbing both ammonium nitrogen and nitrate salts. It has been established that under slightly acidic conditions (pH 5.0), nitrates are assimilated more efficiently, whereas under neutral conditions (pH 7.0), ammonium salts are preferred. When utilizing the latter, a certain amount of CARBOHYDRATES is required within the plant; without these, The formation of amides is hindered, leading to the accumulation of plant-toxic ammonia.

Molecular atmospheric nitrogen (N₂) can be converted into plant-accessible forms through chemical or biological fixation. Chemical nitrogen fixation occurs As a result of photochemical processes and electrical discharges in the atmosphere. These phenomena bind N₂ in negligible quantities, which are insufficient to sustain plant Nutrition.

Biological Nitrogen Fixation is carried out by bacteria. Some of these (Azotobacter, Beijerinckia, certain strains of Clostridium, etc.) are heterotrophs. Some live alongside microorganisms that decompose Cellulose and other Polysaccharides, utilizing organic residues as a carbon source. Others (Azotobacter, Beijerinckia) typically colonize The surface of higher plant roots and feed on their root exudates. In temperate climates, free-living nitrogen-fixing bacteria yield 1–20 kg of nitrogen per hectare annually.

Another group of bacteria (primarily of the genus Rhizobium) lives in Symbiosis with higher plants, forming nodules on the roots of legumes. Symbiotic nitrogen fixers also include certain actinomycetes and cyanobacteria. Besides legumes, plants such as alder, oleaster, sea buckthorn, and certain other trees and shrubs are capable of this type of nitrogen fixation, with nodules sometimes developing on their leaves as well.

Nodule bacteria living in symbiosis with legumes fix the largest amounts of nitrogen—ranging from 100 to 400 kg/ha, and up to 500–600 kg/ha per year in alfalfa. In the course of their metabolic activity, they consume plant mono- and Disaccharides.

Organic compounds contain exclusively ammonium nitrogen. Consequently, the nitrate ions absorbed by plants must be reduced to ammonia. Nitrate reduction takes place in the leaves and roots. First, nitrate is reduced to nitrite by the action of nitrate reductase, and subsequently to ammonia by nitrite reductase.

Ammonia entering the plant can be assimilated through the amination and amidation of various compounds, primarily the amination of 2-ketoglutaric acid to form glutamic acid with the participation of Glutamate dehydrogenase.

Another pathway of ammonia assimilation, which is primary for most plants, involves The transfer of the amino group to α-ketoglutaric acid via Glutamine Synthetase, and the amide group via glutamate synthase.

In addition to α-ketoglutaric acid, which plays a major role in the primary binding of NH₃, oxaloacetic, pyruvic, glyoxylic, and certain other keto and aldehyde acids are of significant importance. Through amination, these acids yield aspartic acid, Alanine, Glycine, and Other Amino Acids, respectively. Via Transamination reactions mediated by aminotransferases, they subsequently donate their amino groups to Other Compounds.

Nitrogen absorbed by plants in the form of NH⁺₄ is mostly assimilated in the roots and transported to the aerial Organs of plants as amides. The synthesis of these amides involves organic acids produced during Glycolysis and The Tricarboxylic Acid Cycle.

Inhibition of nitrification by vegetation. As shown above, nitrification—the biochemical process in which soil ammonia is converted by nitrifying bacteria into HNO₃, which is then taken up by plants as NO^-₃—plays a vital role in the nitrogen nutrition of plants.

It has been established that the total nitrate content in soil planted with vegetation is significantly lower than in unplanted soil of the same type. This suggests that nitrate production in the soil decreases in the presence of plants. A classic example is The Study of nitrate levels in soils sown with cereals. During the first season after sowing, lush vegetation is observed, but plant growth soon declines. By the third, and especially the fourth season, plant growth becomes so poor that it ceases to contribute to the restoration of soil fertility. Grass growth can be improved by plowing or applying nitrogen fertilizers, but in both cases, the decline in plant growth eventually recurs. While sufficient nitrates are present in the soil during the period of vigorous growth, their levels drop drastically or become undetectable in subsequent seasons of poor plant growth.

Thus, perennial grasses, as well as other fast-growing plants, inhibit nitrification (though, as shown, not ammonification). This is likely due to the toxic effect on nitrifying bacteria exerted by toxins secreted by plant roots. Indeed, root extracts from Tragopogon plumosus have been found to inhibit the viability of Escherichia coli, Bacillus subtilis, Staphylococcus aureus, and Streptococcus haemolyticus. Direct inhibition of nitrifying bacteria by root-secreted toxins has been demonstrated for needle and thread grass (Stipa comata), tall oat-grass (Arrhenatherum elatius), rapeseed (Brassica napus), common wheat (Triticum aestivum L.), lettuce (Lactuca sativa), onion (Allium cepa), and many other plant species.

Significant results regarding the inhibition of nitrification by vegetation have been obtained in studies of climax (from the Greek climax, meaning ladder)

ecosystems—that is, those characterized by a final, relatively stable state of vegetation that develops as a result of natural succession in plant cover. It has been revealed that soils at the Cytology/cytology/16.html">Early stages of succession show low levels of ammonium nitrogen, which increase substantially at the climax stage. Conversely, nitrate levels are high at the initial stage and decrease sharply at the climax stage. Since large populations of nitrifying bacteria are observed in early-stage soils and few in climax-stage soils, it is likely that nitrifiers are suppressed by climax vegetation within these ecosystems.

Numerous studies conducted across diverse geographical regions indicate that the inhibition of nitrification by vegetation is a widespread phenomenon. Across various stages of succession, it promotes the accumulation of nitrogen in the ammonium form and creates favorable conditions for the germination of climax plant species that require an abundant supply of readily available nitrogen.