Nitrogen is not only a crucial component of the Earth’s atmosphere—making up about 78% of it—but an essential element for plant growth. Plants rely on nitrogen to synthesize proteins, enzymes, and chlorophyll, which are vital for their development and photosynthesis. However, plants cannot use atmospheric nitrogen (N2) directly; they depend on nitrogen fixation, a process that converts N2 into ammonia (NH3) or other nitrogen compounds, often facilitated by nitrogen-fixing bacteria in the soil or plant root nodules. Plants can then absorb these compounds from the soil through their roots. Nitrogen availability significantly impacts plant health and productivity, and its proper management, through fertilizers or crop rotation, is essential in agriculture to ensure optimal plant growth and food production.
Nitrogen-Fixing Plants
A nitrogen-fixing (N-fixing) crop is a natural and sustainable means of enhancing soil fertility and supplying plants with the nitrogen they require in an eco-friendly way. N-fixing crops, such as legumes (e.g., peas, beans, clover), have a unique ability to form symbiotic relationships with nitrogen-fixing bacteria in their root nodules. These bacteria convert atmospheric nitrogen into ammonia and other nitrogen compounds, which enrich the soil with readily available nutrients for subsequent crops.
Crop rotation is a valuable agricultural practice that leverages the benefits of N-fixing crops. By alternating nitrogen-fixing crops with nitrogen-demanding crops like grains or vegetables, farmers can optimize soil health and reduce the need for synthetic fertilizers. This sustainable approach mitigates the environmental consequences associated with excessive fertilizer use, such as nutrient runoff and water pollution, and increases crop yields.
Advantages Of Fixing Nitrogen With Cover Crop
Cover crops, plants grown primarily to protect and improve soil health in between main crop cycles, offer numerous advantages in sustainable agriculture and soil management. These crops:
- Participate in N-fixation
- Protect soil from erosion by covering it or holding it in place with strong root systems
- Improve soil fertility when used as green manure
- Retain soil moisture
- Help in weed management with crop residues
- Serve as forage and grazing material for poultry and cattle
- Attract pollinators at the crop flowering time
Best Cover Crop For Nitrogen Fixation
The three best cover crops for nitrogen fixation are:
- Clovers: White, red, silver, silver river
- Beans: Fava, alfalfa, French (green), sweet, groundnuts, runner, field, black-eyed, soybeans, cream, purple-hulled, lupins, chickpeas, cowpeas, lentils
- Vetches: American, tufted, wood, hairy.
These are used by farmers globally but tend to be used abundantly in the Southern Great Plains region of the United States in particular.
Clover crops are used in spring and fall for nitrogen fixation. Additionally, Texas A&M University has developed resilient white clover varieties capable of withstanding rusts and droughts. It is important to note that clover can withstand mild winters, but may not be suitable for areas that experience more severe winter conditions.
Peas and beans serve a dual purpose: nitrogen-fixing cover crops during the summer and a food source when harvested. In either role, they contribute to soil enrichment with plant-friendly nitrogen. It is important to research when each pea or bean variety grows best before choosing which to use as a summer cover crop. Southern peas, for example, thrive in warmer climates where soil temperatures do not fall below 60℉.
Vetches—flowering plants belonging to the legume family, like broad bean and bird vetches—are known for withstanding colder climates, making them ideal winter cover crops. The most resilient type of vetch is a hairy vetch. Most varieties of vetches must be planted in the early fall to allow them to establish robust root systems prior to the first frost of the winter months.
Why Is Nitrogen Important to Plants?
Plants require nitrogen to synthesize proteins, enzymes, and chlorophyll, all of which are vital for their growth and function. Proteins are the workhorses of plant cells, involved in countless processes like photosynthesis, cell division, and defense mechanisms. Enzymes, which are specialized proteins, catalyze chemical reactions necessary for plant metabolism. Chlorophyll, the green pigment in leaves, relies on nitrogen for its structure and function in photosynthesis, the process that allows plants to convert sunlight into energy and produce carbohydrates. Without sufficient nitrogen, plants exhibit stunted growth, yellowing of leaves (chlorosis), and reduced crop yields.
What Is the Nitrogen Cycle?
The nitrogen cycle is a continuous, natural process that facilitates the movement of nitrogen from the atmosphere to Earth, through soils, and back to the atmosphere. This continuous circulation of nitrogen is vital for fostering the growth of healthy crops and sustaining ecosystems. Nitrogen undergoes transformations, transitioning between various chemical forms during this cycle, ensuring a constant supply of this essential nutrient to support plant development and overall ecosystem health.
What Are the Stages of the Nitrogen Cycle?
The nitrogen cycle is comprised of 5 stages: Nitrogen fixation, mineralization, nitrification, immobilization, and denitrification.
Nitrogen Fixation
Nitrogen fixation is the first stage of the nitrogen cycle, in which atmospheric nitrogen gas (N2) is converted into ammonia (NH3) or other nitrogen compounds that plants can utilize. Nitrogen fixation occurs through two primary mechanisms: biological and non-biological.
Biological nitrogen fixation involves specialized microorganisms, such as nitrogen-fixing bacteria, which establish symbiotic relationships with certain plants or operate independently in the soil. These bacteria possess enzymes that break the strong chemical bonds in atmospheric nitrogen, converting it into a form accessible to plants. In this way, nitrogen-fixing plants like legumes, such as soybeans and clover, can harness these microbial partners to enrich the soil with nitrogen compounds, improving their own growth and benefitting neighboring plants in the process.
Non-biological nitrogen fixation, on the other hand, occurs through industrial processes like the Haber-Bosch method, where high temperatures and pressures are used to convert atmospheric nitrogen into ammonia for synthetic fertilizers, supporting agricultural productivity worldwide.
Mineralization
The second stage of the nitrogen cycle is mineralization, in which organic nitrogen compounds are transformed into inorganic forms, such as ammonium (NH4+), making nitrogen available for plant uptake and returning it to the soil. This process primarily occurs during the decomposition of organic matter, including dead plants, animal remains, and other organic substances in the soil. As microorganisms break down these organic materials, they release ammonium ions into the soil, converting the organic nitrogen into a form that plants can easily absorb and utilize for their growth.
Mineralization ensures a continuous supply of nitrogen to support healthy plants and ecosystems. It is an essential part of nutrient cycling, as it replenishes the soil with the necessary nitrogen nutrients, contributing to soil fertility and promoting healthy crop growth. Proper management of organic matter and nutrient recycling through mineralization is essential for sustainable agriculture and ecosystem preservation.
Nitrification
The third stage of the nitrogen cycle is Nitrification. Nitrification is responsible for the conversion of ammonium (NH4+) into nitrate (NO3-), which is a more readily available form of nitrogen for plants. This transformation occurs in two distinct steps carried out by different groups of bacteria. First, ammonium is oxidized into nitrite (NO2-) by ammonia-oxidizing bacteria. Subsequently, nitrite is further oxidized into nitrate by nitrite-oxidizing bacteria.
Nitrification is essential for ensuring a sustainable supply of nitrogen to support plant growth and productivity. Nitrate, the end product of nitrification, can be readily taken up by plant roots, serving as a vital nutrient. However, excessive nitrification can lead to nitrate leaching, a form of nitrogen pollution, which can contaminate groundwater and surface water, posing environmental challenges. Proper nitrogen management practices are essential to striking a balance between providing enough nitrogen for plant growth and minimizing the risk of environmental degradation.
Immobilization
Immobilization is the penultimate stage of the nitrogen cycle. In this stage, nitrogen in the form of ammonium (NH4+) is temporarily incorporated into microbial biomass and organic matter in the soil, making it unavailable for plant uptake. Microbes and decomposers in the soil require nitrogen for their own growth and metabolism. When an influx of organic material with a high carbon-to-nitrogen ratio, like dead plant material, is added to the soil, microbes consume nitrogen to break down the carbon-rich organic matter. This consumption of ammonium by soil microbes can lead to a short-term reduction in the available nitrogen pool for plants.
While immobilization temporarily ties up nitrogen, it ultimately contributes to nutrient cycling. As microbes die and decompose, the nitrogen they contain is released back into the soil in the form of ammonium, making it available for plant uptake once again. Immobilization acts as a buffer in the nitrogen cycle, regulating nitrogen availability and ensuring its efficient utilization by plants and soil organisms, contributing to the overall fertility of the ecosystem.
Denitrification
The final stage of the nitrogen cycle is denitrification. In this stage, nitrogen is returned to the atmosphere as nitrogen gas (N2). his microbial-driven process occurs in anaerobic or oxygen-depleted environments within the soil, where specialized denitrifying bacteria convert nitrate (NO3-) and nitrite (NO2-) into nitrogen gas or, in some cases, nitrous oxide (N2O). This step helps maintain the balance of nitrogen in ecosystems and prevents an excessive buildup of nitrates.
By converting nitrates into inert nitrogen gas, denitrification helps mitigate water pollution and greenhouse gas emissions. However, excessive denitrification can also result in nitrogen loss from agricultural fields, impacting crop productivity. Because of this, managing the denitrification process is important for environmental conservation and preserving the delicate equilibrium of the nitrogen cycle.
For additional information on nitrogen’s role in plants and what you can do to manage the nitrogen cycle, visit the Verdesian website to Connect with a Specialist.