Nitrogen is essential for all life, fundamental to biological molecules. It is a component of amino acids (protein building blocks) and nucleic acids (DNA, RNA). This element is also found in chlorophyll, the pigment for photosynthesis in plants, and in compounds for energy transfer within cells. Nitrogen continuously cycles through various forms, moving between the atmosphere, land, and aquatic environments.
The Nitrogen Cycle: An Overview
The nitrogen cycle describes the process by which nitrogen transforms into various chemical forms as it moves through the atmosphere, terrestrial, and marine environments. These conversions involve biological and physical processes, making nitrogen available to support diverse life. Atmospheric nitrogen (N2) is about 78% of Earth’s air but is largely unusable directly by most organisms, including plants. The cycle converts this abundant atmospheric nitrogen into forms organisms can incorporate. Microorganisms play significant roles throughout these transformations.
Nitrogen’s Gateway to Life: Fixation
Nitrogen fixation is the entry point for atmospheric nitrogen into biological systems. Atmospheric nitrogen (N2) is stable due to a strong triple bond, making it inert for most life forms. For organisms to use it, this inert gas must be “fixed” or converted into reactive compounds like ammonia (NH3) or ammonium (NH4+). Most natural nitrogen fixation (about 90%) is carried out by specialized microorganisms called diazotrophs.
These bacteria, including symbiotic Rhizobium in legume root nodules and free-living Azotobacter and Klebsiella, possess an enzyme called nitrogenase that converts nitrogen into ammonia. Cyanobacteria (blue-green algae) also fix nitrogen in aquatic environments. Other forms of fixation occur, such as atmospheric fixation by lightning, which converts nitrogen gas into nitrates that dissolve in rain. Industrial processes, notably the Haber-Bosch process, also fix nitrogen to produce ammonia for fertilizers, significantly impacting global food production.
How Plants Use Nitrogen: Assimilation
After nitrogen is fixed into usable forms, plants absorb it from the soil. This process is known as assimilation. Plants primarily take up inorganic nitrogen compounds, such as ammonium (NH4+) and nitrate (NO3-), through their root systems. Transporter proteins in plant roots facilitate the uptake of these ions.
After absorption, plants incorporate these inorganic nitrogen compounds into organic molecules for their growth and development. This includes synthesizing amino acids (protein building blocks) and nucleic acids (DNA, RNA). Plants consume fixed nitrogen, relying on fixation processes; they do not initiate the conversion of atmospheric nitrogen themselves. Assimilation ensures nitrogen becomes part of the plant’s biomass, making it available to other organisms in the food web.
The Cycle’s Other Essential Steps
Beyond fixation and assimilation, other processes are essential for the continuous circulation of nitrogen within ecosystems. Ammonification occurs when decomposers (primarily bacteria and fungi) break down organic nitrogen from dead organisms and animal waste. This decomposition releases nitrogen back into the soil as ammonia (NH3) or ammonium ions (NH4+). This step recycles nitrogen locked in organic matter, making it available for subsequent transformations.
Nitrification is a two-step process performed by specialized nitrifying bacteria in the soil. First, ammonia-oxidizing bacteria (Nitrosomonas) convert ammonium (NH4+) into nitrites (NO2-). Then, nitrite-oxidizing bacteria (Nitrobacter) oxidize these nitrites into nitrates (NO3-). Nitrates are the most common form of nitrogen absorbed by plants, making nitrification important for plant nutrient uptake.
Denitrification is the final major process, returning nitrogen to the atmosphere. Denitrifying bacteria (Pseudomonas) carry this out under anaerobic (low-oxygen) conditions. These bacteria convert nitrates (NO3-) back into atmospheric nitrogen gas (N2) or nitrous oxide (N2O), completing the cycle. Denitrification ensures nitrogen is continually recycled between the biosphere and the atmosphere, maintaining its overall balance.