What they are
Air is 78 percent nitrogen. Almost nothing can use it.
The triple bond is one of the strongest in chemistry. Breaking it takes lightning, the Haber-Bosch process (1 to 2 percent of global energy), or one enzyme. That enzyme is nitrogenase. Only certain bacteria and archaea carry it.
Three groups matter for land work.
Rhizobium and relatives. Bradyrhizobium, Sinorhizobium, Mesorhizobium. They partner with legumes: clovers, beans, peas, acacias, locusts.
Frankia. Filamentous actinobacteria. They partner with non-legume trees: alders, bayberry, casuarina, Elaeagnus.
Free-livers. Azotobacter, Azospirillum, Clostridium, cyanobacteria. No plant host required.
Together they fix 100 to 200 million tonnes of nitrogen a year. Industry fixes about 120 million. The bacteria win.
How the nodule forms
The Rhizobium-legume handshake starts as a chemical conversation. Roots leak flavonoids. Compatible bacteria reply with Nod factors, lipochitooligosaccharides that tell the plant to build a nodule.
A root hair curls around the bacteria. The plant builds an infection thread, a tunnel into the root cortex. There it constructs the nodule itself: a small organ with vascular plumbing, low oxygen inside, fed sugar from the canopy.
The bacteria swell into bacteroids and start fixing.
Cut a healthy nodule open and you see pink. That is leghemoglobin, a cousin of the haemoglobin in your blood. It rations oxygen to the bacteroids. Enough to respire, not enough to wreck the nitrogenase. The enzyme is destroyed irreversibly by oxygen, so the plant runs the breathing for it.
Frankia does the same trick on alder and casuarina. The nodules come out lobed and coral-shaped instead of round. The deal is identical: carbon down, fixed nitrogen up.
How much they fix
Numbers vary with host, soil, and weather. Rough ranges from field trials:
- White clover stand: 150 to 250 kg N per hectare per year.
- Lucerne (alfalfa): 150 to 300 kg.
- Leucaena or Gliricidia in the tropics: 100 to 500 kg.
- Alder stands: 40 to 300 kg, most around 100 to 150.
- Casuarina: 40 to 200 kg.
- Bayberry, sweetfern: 10 to 50 kg.
Free-livers add less but matter. Azotobacter delivers 1 to 30 kg per hectare per year. Cyanobacteria in arid soil crusts add 5 to 30 kg and carry whole dryland ecosystems. In rice paddies, the Azolla-Anabaena partnership has fed crops for thousands of years at 20 to 80 kg per cycle.
For the plant side of this story, see nitrogen-fixing trees and shrubs.
Inoculating a degraded site
Compacted ground, mine spoil, scraped subsoil. The native bacterial community is usually gone or crippled.
Inoculate at planting.
For agricultural legumes, buy a commercial Rhizobium inoculant matched to the species. Wrong strain, no nodules. Apply as a seed coat or a drench in the hole.
For alder and other actinorhizal species, commercial Frankia is rare. Use native inoculum instead. Dig two handfuls of soil from under a healthy alder grove and drop it in each planting hole. You get Frankia plus mycorrhizae plus the wider microbial community.
This trick has revegetated mine sites across Europe and North America. On dead ground, nodulation begins within weeks instead of years. That head start decides whether the seedling lives.
Standard practice in serious reforestation and assisted regeneration work.
Why the tree matters as much as the bacterium
The bacteria fix. The tree delivers.
A nitrogen-fixing tree is a solar-powered nitrogen factory. Leaves catch light. Sugars travel down to the nodules. Bacteria spend that sugar to crack N2. The fixed nitrogen moves back up, then out through leaf drop, fine root turnover, and the mycorrhizal network.
Without the tree, the bacteria barely fix. Without the bacteria, the tree cannot fix at all. The unit is the partnership.
This is why one alder, black locust, or tagasaste is worth more than it looks. You are installing a fertility engine for everything growing within reach. In fruit tree guild design, fixers around the productive trees replace the bag of fertiliser.
Healthy soil biology supports better nodulation, faster fixation, faster recovery. Skip the till. Keep cover. Add organic matter. Cut chemical inputs. Masanobu Fukuoka's natural farming runs on this same logic, just stated more quietly.
See also
- Nitrogen Fixers the host trees and shrubs
- Alder the Frankia partner for wet ground
- Soil Food Web the community fixers feed into
- Mycorrhizal Fungi the other half of the root partnership
- Masanobu Fukuoka biological fertility over chemical