Plant Physiology - Musiyenko M.M. 2001

Plants and the Biosphere. Ecological Cycles of Matter and Energy Flow
The Nitrogen Cycle

Equally important was the incorporation of nitrogen, a key organogenic element, into the biogeochemical cycle. The total amount of nitrogen in the atmosphere is 3.8 × 10¹⁵ t, whereas in the waters of the World Ocean it is 2.0 × 10¹³ t.

A vast amount of free (N₂) nitrogen cannot be utilized directly by plants. This nitrogen can be assimilated only by so-called nitrogen-autotrophic organisms through Biological Nitrogen Fixation, which plays the most crucial role in the nitrogen cycle. Nitrogen fixation is carried out both by free-living nitrogen-fixing Bacteria—such as heterotrophs (Azotobacter, Clostridium), photoautotrophs (Chromatium, Chlorobium, Rhodospirillum), and cyanobacteria (Nostoc, Anabaena)—and by symbiotic nitrogen fixers, such as the nodule bacteria of the genus Rhizobium, which live in Symbiosis with higher plants. In the biosphere, annual nitrogen fixation amounts to 175 × 10⁶ t. This is predominantly biological fixation, with only a minor fraction fixed As a result of electrical discharges and photochemical processes.

All other organisms are nitrogen-heterotrophic, meaning they depend entirely on the availability of nitrogen-containing compounds, mostly of a proteinaceous nature. They influence the nitrogen cycle only after assimilating it into their cellular structures. When nitrogen enters a plant as NO₃, it is reduced to ammonium, as it exists in the Cells in the maximally reduced ammonium form. The mass of nitrogen bound in terrestrial biomass is 14,020 million tons, and that of other ash elements is 34,062 million tons. All terrestrial vegetation annually incorporates 2,562 million tons of nitrogen and 2,762 million tons of ash elements into the cycle. In the phytoplankton biomass of the World Ocean, these elements are present in amounts thousands of times smaller; however, due to the rapid and repeated reproduction of such organisms, 2,762 million tons of nitrogen and 12,274 million tons of ash elements pass through them annually.

As a result of the vital activity and death of organisms, large amounts of nitrogen-containing organic substances enter the soil and Water, where they are mineralized and can be reused by plants and microorganisms. The Mineralization of organic nitrogen is carried out through ammonification and nitrification.

Ammonification is the microbial decomposition of nitrogen-containing Organic compounds (Proteins, urea) resulting in The formation of free ammonia. This is one of the fundamental Stages of the Nitrogen Cycle in nature, enriching the soil with forms of nitrogen accessible for plant uptake:

The ammonia produced during ammonification, along with that synthesized during nitrogen fixation, is partially oxidized by nitrifying bacteria into nitrates and nitrites through The process of nitrification. This is a microbiological process in soil and water whereby reduced nitrogen compounds from organic matter are converted into oxidized inorganic ones—specifically, converting ammonium salts and ammonia into nitrates, which are the primary form of nitrogen Nutrition for plants. Such highly exergonic two-stage reactions are carried out by nitrifying bacteria (Nitrosomonas, Nitrobacter). They utilize The energy released from The oxidation of ammonium or nitrites for carbon dioxide assimilation and other endergonic processes. Both genera of bacteria inhabit well-aerated soils. Thus, photoautotrophic plants, heterotrophic organisms, and nitrifying bacteria form the nitrogen cycle (Fig. 220).

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Fig. 220. The nitrogen cycle

These stages do not complete the cycle, as they are coupled with another vital link in the biogeochemical loop—denitrification. This is the reverse process in which denitrifying microorganisms (Pseudomonas, Paracoccus, Bacillus, Thiobacillus) reduce oxidized nitrogen compounds (nitrites and nitrates

of the soil) back into molecular nitrogen. In the absence of oxygen, denitrifying bacteria use nitrites and nitrates as TERMINAL ELECTRON ACCEPTORS (anaerobic nitrate Respiration). Through denitrification, bound nitrogen is removed from the soil and water, releasing gaseous N₂ into the atmosphere. Denitrification closes the nitrogen cycle in the biosphere.

During its migration, nitrogen assumes various valence states (ranging from -3 in ammonia to +5 in nitrates), enabling it to participate in numerous oxidation-reduction reactions. For instance, in organic compounds that make up living organisms, nitrogen is in a reduced state (with a valence of -3), whereas in the atmosphere and hydrosphere, it can exhibit electroneutral or positive valences. The nitrogen cycle, synchronized and balanced with the carbon cycle, has become one of the most critical factors in shaping a clean ecological environment on Earth.

Also vital for the overall balance of the carbon cycle and a clean ecological environment is the active, balanced incorporation of such elements as phosphorus, sulfur, potassium, and others into the cycle of life. Indeed, owing to their specific properties—discussed in previous chapters—and their highly uneven distribution in the environment, they flow through the stream of life and subsequently accumulate in dead organic residues for reuse by subsequent generations.