My understanding is that the roots with the bacterial nodules gets released back to the soil upon death of the plant. So for example, sea buckthorn will spread

My understanding is that the roots with the bacterial nodules gets released back to the soil upon death of the plant. So for example, sea buckthorn will spread out roots and propogate. You’d add the nitrogen back by either severing the roots between the mother tree and its offshots, or you chop one down so that it can release those nodules back into the soil.