Hey Richard, thanks for posting this! I’m a very amateur gardener, and I’ve been puzzling over “nitrogen fixing.” From just reading the abstract, it explains a little to me, but I have a couple of questions. I understand that a cover crop of legumes fixes nitrogen in the soil, but from that abstract it seems that the legumes do this in order to use the nitrogen themselves–that they are trading sugars to the bacteria for nitrogen in return. How, then, does this help the tree that one plants the cover crop around? Does enough nitrogen escape being used by the legumes that there is extra in the soil for the tree? Or is it that the legumes “fix” nitrogen into their own plant matter, and then only by tilling them or mulching the legumes into the soil around the plant do you actually provide the tree with nitrogen? In which case the legumes are just pretty rich with nitrogen so they make good compost, but not that they actually change the soil–in other words they don’t so much fix nitrogen in the soil as they get the bacteria to produce nitrogen and then suck it up themselves…
Yes, they trade sugar to the bacteria in exchange for nitrogen, and the net effect is what you describe.
Lizzy - you’re right, only once the cover crop decomposes do you get the benefit of the nitrogen fixing.
And as the above research points out, only in some species - the distribution of which is still under study.