Nitrogen (N) is an essential element for life, constituting a fundamental component of proteins, nucleic acids, chlorophyll and other organic molecules vital for the growth and development of plants and soil microorganisms. Despite being the most abundant element in the Earth's atmosphere (about 78% in the form of gaseous N₂), this form is inaccessible to most organisms. Therefore, the various forms in which nitrogen is presented and transformed in the soil are crucial for the fertility and functioning of terrestrial ecosystems.
As mentioned by Bravo (2000), nitrogen is found in the soil in a variety of forms, both organic and inorganic, each with its own dynamics and availability to plants and microorganisms, understanding these forms and their interactions is fundamental for the sustainable management of soils and the optimization of agricultural production.
The same author points out that most of the nitrogen present in the soil is found in organic forms, constituting between 90% and 98% of the total nitrogen, these forms are the result of the decomposition of organic matter, including plant and animal waste, microbial biomass and humic substances, organic nitrogen is not directly available to plants and must be mineralized by soil microorganisms to become assimilable inorganic forms. The main forms of organic nitrogen in soil include:
- Proteins and Amino Acids: They are the most abundant nitrogenous components in living organisms and their residues. the breakdown of proteins releases amino acids, which can then be transformed by microorganisms.
- Humic Substances: They are complex organic polymers and resistant to decomposition, formed during the humification of organic matter. They contain a significant amount of organic nitrogen that is slowly released through mineralization.
- Microbial Biomass: Soil microorganisms (bacteria, fungi, actinomycetes) contain a considerable amount of nitrogen in their cells, the death and decomposition of these microorganisms release organic nitrogen that can be mineralized.
The inorganic forms of nitrogen are those directly assimilable by plants and some microorganisms, these forms are the result of processes of transformation of organic nitrogen and biological and atmospheric fixation. The main inorganic forms of nitrogen in soil are:
- Ammonium (nh₄⁺): It is the first inorganic form of nitrogen produced during the mineralization of organic matter (ammonification). Ammonium has a positive charge and tends to adsorb to clay particles and soil organic matter, which reduces its mobility and protects it from leaching, plants can absorb ammonium directly, although in many soils it is quickly converted to nitrate.
- Nitrate (no₃⁻): It is the predominant form of inorganic nitrogen in most well-aerated soils. It is produced through nitrification, a two-stage microbial process in which ammonium is oxidized first to nitrite (NO₂⁻) by bacteria of the genus Nitrosomonas and then to nitrate by bacteria of the genus Nitrobacter. Nitrate has a negative charge and is highly mobile in the soil, which makes it more susceptible to leaching (loss by washing with rainwater or irrigation). However, it is also the preferred form of nitrogen by many plants due to its high solubility and easy absorption.
- Nitrite (NO₂⁻): It is an intermediate in the nitrification process and is usually found in very low concentrations in the soil due to its rapid conversion to nitrate. Nitrite is toxic to plants and microorganisms in high concentrations.
The different forms of nitrogen in the soil are interconnected through a series of complex biogeochemical processes, collectively known as the nitrogen cycle, these processes are mainly driven by the activity of soil microorganisms and are fundamental for the availability of nitrogen for plants. The main processes include, mineralization which is the decomposition of organic matter by microorganisms, releasing organic nitrogen in the form of ammonium (ammonification), immobilization, is the reverse process to mineralization, where soil microorganisms incorporate inorganic nitrogen (ammonium and nitrate) into their biomass, making it temporarily unavailable to plants, nitrification is the biological oxidation of ammonium to nitrite and then to nitrate by autotrophic bacteria, denitrification, it is the biological reduction of nitrate or nitrite to nitrogen gases (N₂, N₂O, NO) under anaerobic conditions (lack of oxygen), resulting in the loss of nitrogen from the soil to the atmosphere.
iv class="text-justify">On the other hand, we have biological nitrogen fixation, which is the conversion of atmospheric nitrogen (N₂) into nitrogenous organic forms by certain microorganisms (bacteria and cyanobacteria), either free-living or in symbiosis with plants (such as legumes and bacteria of the genus Rhizobium). atmospheric nitrogen fixation, electric discharges (lightning strikes) can convert small amounts of atmospheric nitrogen into nitrogen oxides, which then reach the soil with rain and become nitrate., absorption by plants the roots of plants mainly absorb nitrate and ammonium from the soil through active transport processes.
The availability and transformations of the different forms of nitrogen in the soil have profound implications, for example, the presence of inorganic forms of nitrogen in adequate quantities is essential for the optimal growth and development of plants and therefore for agricultural productivity,
Dear readers, the various forms of nitrogen in the soil and the complex processes that interconnect them are fundamental for life on earth. understanding the dynamics of nitrogen, from its organic form to its assimilable inorganic forms, and the factors that influence its transformations is crucial to develop sustainable soil management practices that optimize fertility, proper management of nitrogen in soil is a continuous challenge that requires a thorough knowledge of biogeochemical processes and the implementation of strategies that promote efficiency in the use of this essential nutrient.
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| Bibliographic references |
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Bravo, S (2000). Basic aspects of soil chemistry. Editorial Unellez. Barinas: Venezuela.
Urbano, P. (2002). Phytotechnics Plant production engineering. Madrid: Mundi-prensa.
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