Here's a link to the post
https://www.reddit.com/r/Permaculture/comments/yd3iab/perennial_nitrogen_fixers/?utm_source=share&utm_medium=android_app&utm_name=androidcss&utm_term=1&utm_content=share_buttonAnd here's what a person, u/SaintUlvemann, replied about using them. I think it's very insightful and interesting and it cuts through a lot of beliefs that these plants are pumping nitrogen into the soil like crazy.
I'm a crop geneticist who studies legumes and knows nodulation down to many of the individual protein-protein interactions. We say that form follows function; it is typically necessary to understand what a thing is, in terms of things like shape and molecular structure, before one can understand its ecological function.
Root nodules are a novel organ that legumes (and actinorhizal relatives) develop. The organ exists in order to provide an enclosed anaerobic environment for the symbiotes to live in and for the plant to eat from. It is an internal microhabitat within the plant, that plants grow for their symbiotes.
There is a lengthy and tightly-controlled process for how plants get these symbiotes inside themselves without ending up infected with pathogens. Developing a nodule is a risky and energetically intense process; plants extract as much benefit from it as they can.
They invest large amounts of photosynthate into tasks such as feeding these symbiotes. To give another example; they regulate the oxygen content of this anaerobic environment through production of leghemoglobins. Those leghemoglobins, oxygen-binders like heme, are the reason why legume root extracts can be used to make plant-based meats taste meaty. This oxygen regulation is also energetically expensive.
Given the amount of energy plants put into developing these nodules safely, they can't just secrete nitrogenous compounds out of those roots: that would defeat their purpose. The nitrogen is produced inside of their bodies; it must be brought out of their bodies in order to make it to the environment.
The life cycle of the nodule itself provides little opportunity for nitrogen to make its way into the soil.
For those legumes with determinate nodules (meaning that the nodules are developed for a set, determinate, amount of time, and then discarded), a small amount of nitrogen may be expected to return directly to the soil at the programmatic end of the nodule's lifespan; but like the extraction of nutrients from leaves in fall, the plant avoids this waste where possible, and many legumes don't even have determinate nodules in the first place: their nodules are indeterminate in lifespan, organs that die only when the roots to which they're attached do.
Thus the route by which the nitrogen fixed by nodulating species ends up in the rest of the environment, has to be through the decomposition of the dead body parts of that nodulating plant; because the nitrogen was fixed inside the plant's body.
Mulches would be one way of doing that. The dead root systems of legumes would contain nitrogen too.
However, for many species, the majority of the nitrogen fixed by the plant is not found in the roots, leaves, and stems; it is packaged up by the plant into its seeds. Nitrogen is a core atomic building block of protein; nitrogen fixation is why beans are protein-rich. We may find that protein delicious, but from the plant's perspective, it is meant as a bequest to the next generation of the species. Annual plants' reproductive strategy is to deliberately self-sacrifice (timed to match what would be their seasonal death anyway) in order to produce higher-quality seed with the nutrients required for the developing embryo to have a higher chance of survival.
The survival strategy for perennial nitrogen fixers is not to completely kill themselves off each year; so they will reserve more nutrients for themselves. But plants that produce protein-rich seed do so for the sake of increasing the survival rate of their offspring, and perennials may adopt this strategy as surely as other plants will. This is part of why nuts and seeds are such sought-after food for animals.
The precise proportion of nitrogen that remains in the leaf, root, and seed material of a perennial nitrogen-fixing species, is likely to vary by species, depending on life strategy. I might imagine that perhaps the roots of an asexually-propagating rhizomal nitrogen fixer such as the potato bean maintained a higher nitrogen content within said roots, than a nitrogen fixer such as alder that maintains heavier seasonal investments into its reproductive structures.
Some plants that do not really fix nitrogen per se, are called nitrogen fixers due to casual associations.
The term for nitrogen fixation that is done on the outside of plants, by microorganisms that are only in casual association with them, is associative nitrogen fixation. It is harder to study, and so not as well-studied. Plants that participate in such casual relationships need not necessarily have any nodule organs; grasses lack nodules, yet have been found to participate in associative nitrogen-fixing symbioses.
Nitrogen fixed by these organisms would enter the environment via usual aqueous routes, having been fixed in the environment, not inside the body of a plant.