Toby Hemenway forwarded a notion that water cycles are sufficient to transport nitrogen from nitrogen-rich plants to their neighbors as a continuous basis instead of on an annual one. The model he builds from research he had read goes something like this:
After a rain, as the soil goes from waterlogged to dry, there are phases that are hostile to root hairs and ones that are conducive. So the plant roots pulse with these cycles, advancing and retreating, sometimes arriving at places vacated by other plants. By this mechanism, any plant can scavenge some of the nitrogen left in the decaying root hairs of their neighbors.
I would also warn that thinking nitrogen=legume is a potentially limiting strategic error. While something like 7% of all plants fall into the Fabaceae (legume) family, some other pockets of the Fabales order also have root nodules, in particular, alder, and up into the Rosids clade (buckthorn, ceanothus), and then a few odd stragglers like borage, which makes me wonder if it's a taxonomic problem, parallel evolution, or what.
And then there's mondo grass (asparagaceae), which nobody expects to have root nodules. I knew about that plant for about two decades before someone pointed out it's nitrogen fixing, and that's not even in the wikipedia article. And in fact I'm having a dickens of a time finding a citation for that.
After a rain, as the soil goes from waterlogged to dry, there are phases that are hostile to root hairs and ones that are conducive. So the plant roots pulse with these cycles, advancing and retreating, sometimes arriving at places vacated by other plants. By this mechanism, any plant can scavenge some of the nitrogen left in the decaying root hairs of their neighbors.
I would also warn that thinking nitrogen=legume is a potentially limiting strategic error. While something like 7% of all plants fall into the Fabaceae (legume) family, some other pockets of the Fabales order also have root nodules, in particular, alder, and up into the Rosids clade (buckthorn, ceanothus), and then a few odd stragglers like borage, which makes me wonder if it's a taxonomic problem, parallel evolution, or what.
And then there's mondo grass (asparagaceae), which nobody expects to have root nodules. I knew about that plant for about two decades before someone pointed out it's nitrogen fixing, and that's not even in the wikipedia article. And in fact I'm having a dickens of a time finding a citation for that.