Symbiotic nitrogen fixation

1. The differences in the amount of N fixation for different legumes could be due to many reasons. Major reason is the host specificity of bacteria (i.e. the interaction of a particular bacterium with a plant species), which means that some bacterium may fix considerable amounts of N in a particular legume crop whereas with another legume crop it couldn't fix high amounts of N.
2. Yes, the symbiotic N-fixing bacteria on certain plant roots supply the plants directly with useable N (i.e., in the form of NH₃).
3. Almost all of the nitrogen fixed by a bacteria goes directly into the legume plant. However, some nitrogen can be transferred into the soil (when the legume plant dies and decomposes) and from that soil, neighboring non-legume plants can take up that fixed N. Fixed N may also be transferred to a non-legume plant through fungal hyphae since fungi provide the plant with many major nutrients including N from the soil.