The Soil Food Web School taught me that the bacteria which form these nodules don't actually fix nitrogen until the nodules get big enough. At that point the center becomes anaerobic and the bacteria changes the way it functions to start fixing nitrogen. You can tell it's happening when the center of the nodule is red.
Discussion
Thanks for the info.
One of the cool parts of all this is that the red in the root nodules (leghaemoglobulin) a very similar structure to that which carries oxygen in our blood. Except in this instance oxygen slows the N fixation process and the leghaemoglobulin removes the oxygen thereby allowing other enzymes to work their magic.
