How do legumes fix nitrogen?

They do not — bacteria do it, inside nodules on the plant's roots, in one of the most consequential symbioses in agriculture. The plant supplies energy and a protected environment; the bacteria supply nitrogen.

Why it is difficult. The atmosphere is 78% nitrogen, and plants cannot use it. Atmospheric nitrogen is N₂, held together by a triple bond that is among the strongest in chemistry. Breaking it industrially requires high temperature and pressure — the Haber-Bosch process. Certain bacteria do it at ambient temperature using an enzyme called nitrogenase.

How the partnership forms. Legume roots release chemical signals; compatible rhizobia bacteria in the soil respond with their own signals; the root hair curls around them and admits them; and the plant grows a nodule to house them. Cutting a healthy nodule open reveals pink or red tissue — leghaemoglobin, chemically related to the haemoglobin in your blood and serving a comparable purpose: managing oxygen. This matters because nitrogenase is destroyed by oxygen, so the nodule must supply the bacteria with enough oxygen to respire while keeping free oxygen away from the enzyme.

What follows for gardeners:

The nitrogen is mostly in the plant, not the soil, during growth. It becomes available when the plant material decomposes — which is why the benefit comes from leaving the roots in the ground and composting or incorporating the tops, rather than from simply having grown peas.

Pulling up the whole plant removes much of the benefit. Cut at ground level instead.

Green manures like clover, vetch and field beans are grown specifically to be incorporated.

Inoculation matters where a legume is new to a site, since the right rhizobia may not be present — which is why bought inoculants exist for some crops.

Rotation logic follows from this: legumes precede hungry crops.

Other nitrogen fixers exist beyond legumes, including alder and some free-living soil bacteria.