Fertilizing the tundra
The claim is being made that the lemmings introduce N2 into soil rather than simply dispense the bacteria they digest (that would result in the zero or negative overall balance of N2, not helping the tundra at all). To do so the bacteria have to actively fix N2 in the gut. For that, at the very least, the lemmings should pump air into their intestines. First, I do not believe that they do. Second, if they do, then it would be mutualistic symbiosis, so we are back to the square one. Perhaps lemmings eat legumes and simply dispense the bacteria. Feces are already rich in nitrogen, so the contribution of the lemming-carried bacteria to fertilization of the tundra must be negligible. If that is the story, then we are such lemmings ourselves; there is nothing exciting about it. Correct?