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Biological nitrogen fixation

Abbreviation
BNF
Also known as
nitrogen fixation, symbiotic nitrogen fixation, BNF

Biological nitrogen fixation (BNF) is the conversion of nitrogen gas from the air into ammonia by certain bacteria and archaea, using the enzyme nitrogenase. In agriculture, the most important example is rhizobia fixing nitrogen inside the root nodules of legumes.

What is biological nitrogen fixation?

Nitrogen gas makes up most of the air, but plants cannot use it: the two atoms in a nitrogen molecule are held by a very strong triple bond. Biological nitrogen fixation is the process by which some microorganisms break that bond and convert nitrogen gas into ammonia, a form that can be built into proteins and other molecules. Together with industrial fertilizer production and lightning, it is one of the main routes by which nitrogen enters living systems.

How it works

Nitrogen fixation is carried out by the enzyme nitrogenase, which only some bacteria and archaea possess. The reaction takes a great deal of energy, at least 16 molecules of ATP for each molecule of nitrogen gas fixed, so fixing organisms need a rich supply of carbon compounds to fuel it.

Nitrogenase is also destroyed by oxygen. Nitrogen-fixing organisms therefore need ways to keep oxygen away from the enzyme while still breathing. In legume root nodules, the plant produces leghemoglobin, a protein that binds oxygen and controls its concentration around the bacteria. It gives active nodules their pink color.

Types of biological nitrogen fixation

  • Symbiotic. Bacteria live inside a plant structure and exchange fixed nitrogen for sugars. The main agricultural example is rhizobia in legume root nodules. Some non-legume trees form similar partnerships with Frankia bacteria.
  • Associative. Bacteria live on or near roots, or inside plant tissue, and fix smaller amounts of nitrogen that the plant can partly use. Some endophytes and root-zone bacteria fall into this group.
  • Free-living. Bacteria such as Azotobacter and many cyanobacteria fix nitrogen independently in soil or water.

Symbiotic fixation in legumes supplies by far the most nitrogen to crops.

How much nitrogen do legumes fix?

The amount varies widely with crop, yield, soil nitrogen and growing conditions. For soybeans, reviews commonly estimate that fixation supplies about 50 to 60 percent of the crop’s nitrogen on average, with a reported range from well below half to nearly all, depending on how much nitrogen the soil supplies. High-yielding soybean crops take up very large amounts of nitrogen, so both fixation and soil nitrogen matter.

What limits fixation

  • Available soil nitrogen. When soil or fertilizer nitrogen is plentiful, legumes form fewer nodules and fix less, because using existing nitrogen costs the plant less energy.
  • Stress. Drought, waterlogging, heat and very acidic soils reduce nodule activity.
  • Poor nodulation. Too few effective rhizobia at planting, a problem addressed by a microbial inoculant.
  • Plant health. Anything that limits photosynthesis limits the energy available to nodules.

Why it matters

Biological nitrogen fixation reduces the nitrogen fertilizer legumes need, lowering input costs and the energy and emissions associated with fertilizer manufacture. Legumes in rotation can also leave nitrogen in residues for the following crop. It is central to nutrient use efficiency in legume systems and to reducing nitrogen loss from agriculture.

Nitrogen fixation at Locus

Locus’s soybean inoculants supply rhizobia and are positioned around nodulation and nitrogen fixation, including a multi-strain system that pairs rhizobia with other beneficial organisms.

Frequently asked questions

Nitrogenase is the enzyme that converts nitrogen gas into ammonia. Only certain bacteria and archaea make it. It needs a lot of energy and is damaged by oxygen, which is why nitrogen-fixing organisms have ways to protect it.

Well-nodulated soybeans obtain much of their nitrogen from fixation and the rest from soil. Applied nitrogen tends to reduce fixation, so it seldom pays as a routine practice; its value in specific situations is debated and depends on local conditions and yield levels.

Not by themselves. Some non-legumes benefit from associative or free-living nitrogen-fixing bacteria, and a few trees form nodules with Frankia, but these typically supply far less nitrogen than the legume-rhizobia symbiosis.

  • Rhizobia

    Rhizobia are soil bacteria that form nodules on the roots of legumes, such as soybeans, peas and alfalfa, and inside those nodules convert nitrogen from the air into a form the plant can use. Each legume partners with specific rhizobial species.

  • Nodulation

    Nodulation is the formation of root nodules on a legume, such as soybean, by nitrogen-fixing rhizobia bacteria. The nodules house the bacteria and are where atmospheric nitrogen is converted into a form the plant can use.

  • Nutrient use efficiency NUE

    Nutrient use efficiency (NUE) is a measure of how effectively a crop converts the nutrients available to it, particularly applied fertilizer, into yield. In practice NUE most often refers to nitrogen use efficiency, and it is expressed through several related ratios rather than one number.

  • Nitrogen loss

    Nitrogen loss is the escape of applied or soil nitrogen from the crop root zone before plants can use it. The main pathways are nitrate leaching, denitrification to gases, ammonia volatilization and runoff, and losses reduce yield, waste fertilizer and can harm water and air quality.

Sources

  1. Chapter 23: Nitrogen fixation, in iGrow Soybeans, South Dakota State University Extension
  2. Revisiting biological nitrogen fixation dynamics in soybeans, Peer-reviewed review, PMC8529188
  3. Nitrogen management for soybeans, University of Delaware Cooperative Extension