Legumes don’t fix nitrogen.
Only bacteria do.
And there are many more bacteria capable of fixing nitrogen than those associated with legumes.
We just need to stop killing them, begin encouraging and feeding them, and our soils and crops can be supplied with 100% of the nitrogen requirements at the highest yield levels.
A new report describes some of the rhizobia and other organisms found in the rhizosphere of plants other than legumes1.
The important part, of course, is that these biology need an abundant energy source to be able to fix nitrogen. The more energy they have available, the more N will be fixed. A large part of their energy during the growing period is supplied by plant root exudates. When we have plants with optimum photosynthesis, producing large volumes of exudates, much more N can be sequestered, which leads to higher yields.
1. Yoneyama, T., Terakado-Tonooka, J., Bao, Z. & Minamisawa, K. Molecular Analyses of the Distribution and Function of Diazotrophic Rhizobia and Methanotrophs in the Tissues and Rhizosphere of Non-Leguminous Plants. Plants 8, (2019)