Legumes have the capacity to fix large amounts of atmospheric N2 into the biosphere through their symbiosis with soil rhizobia. However this legume contribution to the N bio-geochemical cycle varies with the nodulated-root rhizospheric environment, in particular P availability. In order to assess the environmental constrains that might limit this symbiosis, a nodular diagnosis was performed in field-sites chosen with farmers of the Mediterranean basin, with common bean as a model grain-legume and major source of plant proteins for world human nutrition. With this methodology a large partial and temporal variation in the legume nodulation was found in cerealcropping and horticultural systems. In various reference areas of bean production, the nodular diagnosis confirmed that low P availability of soils is a major limiting factor of the rhizobial symbiosis. The relation with legume improvement was further addressed by participatory assessment of bean recombinant inbred lines contrasting for their efficiency in use of phosphorous for symbiotic nitrogen fixation. In some organic farming fields, soil P bio-availability was found to be the highest in the rhizosphere of the most efficient symbioses. The interactions between nitrogen fixation and phosphorous availability involve various mechanisms that are investigated in hydroaeroponic and rhizotron cultures under controlled environment, with emphasis on phosphatases, including phytase recently found in nodules. It is concluded that by increasing the phosphorous use efficiency for symbiotic nitrogen fixation, a virtuous cycle of fertility is activated within legume rhizosphere, which can contribute to the sustainability of agricultures through the use of appropriate legumes and cultural systems