“Leguminous plants enrich soil by fixing atmospheric nitrogen into forms that are available for plant uptake.”

Legumes can fix atmospheric nitrogen (N) and facilitate N availability to their companion plants in crop mixtures. However, biological nitrogen fixation (BNF) of legumes in intercrops varies largely with the identity of the legume species. We found beans to exhibit low levels of BNF, and to potentially compete with other species for available soil N in crop mixtures.

On the one hand, soil N enrichment increases the availability of N for legumes, enhancing functions related to nutrient supply and carbon (C) cycling and potentially boosting legume biomass. On the other hand, soil N enrichment may inhibit rhizobacterial symbiosis, thereby weakening N2 fixation and negatively affecting legume growth. Supplementation with externally available N enhances soil N content, which may lead legumes to adjust their N utilization strategies and reduce their reliance on SNF.

By conducting a global meta-analysis, we revealed that legumes increased the soil net N mineralization rate (Rmin) by 67%, which was greater than the recently reported increase associated with N deposition (25%). Leguminous plants are an important component of terrestrial ecosystems and significantly increase soil nitrogen (N) cycling and availability, which affects productivity in most ecosystems.

The highest wheat agronomic parameters observed in the legume-wheat rotations compared to the continuous wheat-wheat cropping were attributed to availability and increased soil N concentration due to BNF. This is because legumes contribute to soil N and provide available N for subsequent wheat crop. The findings indicated that legumes improved yield and N uptake of the subsequent wheat crop.

Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. *Rhizobium*-legume symbioses are the primary source of fixed nitrogen in land-based systems and can provide well over half of the biological source of fixed nitrogen. Atmospheric N2 fixed symbiotically by the association between *Rhizobium* species and legumes represents a renewable source of N for agriculture. Values estimated for various legume crops and pasture species are often impressive, commonly falling in the range of 200 to 300 kg of N ha−1 year−1.

Garden crops, such as peas and beans, are unique plants that can establish a nitrogen fertilizer factory in their roots. Members of the legume family develop a symbiotic relationship with Rhizobia bacteria that operate the nitrogen factory. The bacteria fix or capture atmospheric nitrogen gas, convert it to ammonia and make it available to the plant.

Legumes convert atmospheric nitrogen into usable forms, enriching soil and reducing fertilizer needs for subsequent crops. When legumes die, their residue is easily broken down by microorganisms that release nitrogen back into the soil. The result is a net increase of nitrogen in the soil system, because much of the nitrogen released from the decaying plant was not obtained from existing nitrogen in the soil.

Most legume species form a symbiosis with rhizobia bacteria, which fix nitrogen from the atmosphere. Root nodules are associated with the presence of nitrogen-fixing bacteria called rhizobia. Many legume species develop these nodules and fix nitrogen from the atmosphere. This nitrogen becomes available to other crops after the cover crop is incorporated in the soil and residues mineralise.

During nitrification, chemosynthetic bacteria oxidize ammonia compounds to produce nitrates and nitrites. Nitrates can also enter the soil from other sources.

Planting legumes as a cover crop can increase soil nitrogen and perhaps even reduce reliance on fertilizers. Plant species in the legume family, however, have a unique ability to collaborate with a specific species of bacteria (Rhizobium) that can transform this atmospheric nitrogen into forms that plants can utilize.

Forage legumes provide many benefits to livestock operations through increased forage quality, added plant diversity to pasture systems, and biological nitrogen fixation (BNF). Legumes have a mutually beneficial relationship with soil bacteria *Rhizobium* that can colonize legume root hairs to form small nodules. These bacteria help remove nitrogen gas from the atmosphere and convert it to a plant-usable form. This process is referred to as *biological nitrogen fixation*. While the legume plant receives nitrogen sourced by the bacteria, the bacteria receive sugars from the host legume.

Even though legumes can fix nitrogen from the atmosphere, they can take up large quantities of soil nitrogen if it is available. Inoculated with the proper strain of Rhizobia bacteria, legumes can supply up to 90% of their own nitrogen (N) needs. Shortly after a legume seed germinates in the presence of Rhizobia bacteria in the soil, the bacteria penetrate the root hairs and move into the root itself.

Nitrogen fixation by legumes is a partnership between a bacterium and a plant. Biological nitrogen fixation can take many forms in nature. In fact, they usually don’t respond to nitrogen fertilizer as long as they are capable of fixing nitrogen. When an excessive amount of nitrogen is applied, the legume literally slows or shuts down the nitrogen fixation process.

Legumes tend to be higher in nitrogen than other plant species, which can improve soil nitrogen availability, reduce (often eliminate) the need for nitrogen fertilizers. As a result of this symbiotic process, legumes tend to be higher in nitrogen than other plant species, which can improve soil nitrogen availability.

Using leguminous cover crops provides multiple below ground ecosystem services that support crop production, restore/maintain soil health. The first part of the presentation will focus on understanding soil ecological processes that enable leguminous cover crops to serve as a nitrogen source for subsequent cash crops. Join to learn how to reduce nitrogen need by incorporating legumes into your rotation.

Legume plants can form a symbiotic relationship with rhizobia, a type of soil bacteria, that can fix nitrogen (pull nitrogen out of the air ...). The nitrogen fixation process takes atmospheric nitrogen (N2), which plants cannot use, and transforms into ammonium (NH4), a soil-bound form that plants can use. Nitrogen fixation takes place in nodules that form on legume roots. The nitrogen fixed by the bacteria is in the same form as the N in ammonium nitrate (34-0-0) and ammonium sulfate (21-0-0) fertilizer.

Legumes can fix substantial amounts of N2 into usable N. An alfalfa crop, for example, has the potential to fix several hundred pounds of N per acre per year.

Legumes (peas, vetches, clovers, beans and others) grow in a symbiotic relationship with soil-dwelling bacteria. The bacteria take gaseous nitrogen from the air in the soil and feed this nitrogen to the legumes; in exchange the plant provides carbohydrates to the bacteria. This is why legume cover crops are said to “fix” or provide a certain amount of nitrogen when they are turned under for the next crop or used for compost.

“From an agricultural perspective, continued nitrogen fixation could be a beneficial trait that increases nitrogen availability, both for the legume and for future crops that rely on the nitrogen left behind in the soil after legumes are grown,” said Dr. Dugald Reid, lead author and researcher with ENSA.

In general, the more fertilizer, manure, or legume N available in the soil above the optimal amount, the greater the amount of nitrate-N that will not be taken up by the crop and therefore subject to loss.

The relationship between legumes and bacteria is at the core of biological nitrogen fixation: Bacteria form small growths known as nodules on the plant roots, converting atmospheric nitrogen into forms usable by plants, thereby enriching the soil.

Legumes form symbiotic relationships with Rhizobia bacteria in root nodules, where nitrogenase enzyme converts atmospheric N2 into ammonia (NH3), which is then assimilated into amino acids and other nitrogen compounds available for the plant and, upon decomposition, for soil microbes and other plants.