The air around us contains a vast amount of nitrogen, making up about 78% of the atmosphere. Despite this abundance, nitrogen is frequently a limiting nutrient in soil, meaning its scarcity can hinder plant growth. This apparent contradiction arises because atmospheric nitrogen is not in a form that most living organisms, including plants, can directly utilize. This article explains the scientific reasons behind this paradox, detailing how nitrogen becomes available, how it is lost from soil, and the methods used to manage its levels for healthy ecosystems and agricultural productivity.
The Unavailability of Atmospheric Nitrogen
Atmospheric nitrogen exists primarily as dinitrogen gas, N2. It consists of two nitrogen atoms joined by a strong triple covalent bond. This strong bond makes N2 largely inert and unreactive. Plants cannot directly absorb N2 from the atmosphere or soil.
Most biological systems lack the enzymes and energy to cleave this bond. Even with abundant atmospheric nitrogen, plants cannot access it in this elemental form. The challenge is converting this stable gas into reactive compounds for biological systems.
Transforming Atmospheric Nitrogen for Soil Use
Nitrogen must transform to become available for plant uptake. Biological nitrogen fixation, primarily by microorganisms, is a key pathway. These bacteria possess the enzyme nitrogenase, breaking the triple bond in N2 to convert it into ammonia (NH3).
Symbiotic nitrogen-fixing bacteria, like Rhizobium species, live in nodules on leguminous plant roots (e.g., peas, beans, clover). They provide the plant with fixed nitrogen in exchange for carbohydrates. Other free-living soil bacteria, including Azotobacter and cyanobacteria, also fix nitrogen into ammonia.
Following ammonia production, nitrification transforms nitrogen into forms usable by plants. Nitrifying bacteria like Nitrosomonas convert ammonia into nitrites (NO2-). Nitrobacter then oxidizes nitrites into nitrates (NO3-), which are highly soluble and the primary form absorbed by most plants.
Industrial nitrogen fixation also converts atmospheric nitrogen. The Haber-Bosch process synthesizes ammonia from atmospheric nitrogen and hydrogen under high temperature and pressure. This process forms the foundation for synthetic nitrogen fertilizers, important for global food production. Lightning also breaks the N2 bond, converting a small amount of atmospheric nitrogen into nitrogen oxides that enter the soil via rainfall.
How Soil Nitrogen Levels Decrease
Even after nitrogen is converted, its presence in soil is not permanent, as several processes lead to its depletion. As plants grow, they absorb available nitrogen compounds (e.g., nitrates, ammonium) from the soil to synthesize proteins, nucleic acids, and chlorophyll. This biological uptake reduces accessible nitrogen.
Water movement through soil can cause nitrogen loss via leaching. Nitrates, negatively charged and highly soluble, do not bind readily to soil particles. They can be washed below the root zone by heavy rainfall or excessive irrigation, especially in sandy soils.
Denitrification is another biological process removing nitrogen from soil and returning it to the atmosphere. Under anaerobic or waterlogged conditions, bacteria convert nitrates into gaseous forms like dinitrogen gas (N2) or nitrous oxide (N2O). These gases escape from the soil, making nitrogen unavailable for plants and representing a loss of fixed nitrogen.
Physical processes like erosion also contribute to nitrogen depletion. Topsoil loss due to wind or water runoff removes nitrogen-rich organic matter, a reservoir of slowly available nitrogen. Volatilization can lead to ammonia gas loss from the soil surface, especially when urea-based fertilizers are applied without proper incorporation, as ammonia escapes into the atmosphere.
Managing Nitrogen Levels in Soil
Maintaining adequate nitrogen levels in soil is important for plant growth and agricultural productivity. Synthetic nitrogen fertilizers are a common strategy. These products (e.g., urea, ammonium nitrate) supply readily available nitrogen to the soil, supplementing natural processes and meeting crop demands.
Crop rotation leverages plants’ natural ability to enhance soil nitrogen. Farmers often include legumes in rotation cycles, allowing symbiotic bacteria in their root nodules to fix atmospheric nitrogen. When harvested or incorporated into the soil, these legume crops leave a nitrogen residue that benefits subsequent non-leguminous crops.
Adding organic matter is another method for managing soil nitrogen. Materials like compost, animal manure, and plant residues decompose, releasing nitrogen slowly. This provides a continuous nitrogen source and improves soil structure, enhancing nutrient retention.
Cover cropping involves planting non-cash crops (e.g., rye, clover) when the main crop is not growing. These cover crops prevent nitrogen loss through leaching and erosion by protecting the soil surface and absorbing residual nitrogen. Some cover crops, particularly legumes, can also fix nitrogen, further enriching the soil. Modern agricultural practices emphasize soil testing and precision agriculture. These methods help farmers determine exact nitrogen requirements and apply fertilizers efficiently, minimizing waste and environmental impacts.