While many studies have focussed on nodule organogenesis at the molecular, cellular and organ scales, little information is available concerning the establishment and growth of nodules at the whole plant level. Our aim was to identify specific patterns and quantitative determinants of nodule number and biomass in peas grown under contrasting environmental conditions (variation of light intensity, nitrate supply and inoculation date). Using mutants impaired in the autoregulation of nodulation (AON) together with an integrated analysis of plant N nutrition, we revealed the respective roles of the AON and N signalling pathways in the regulation of nodulation. Nodulation was first repressed by a seed-borne AON independent signal. Then, nodulation was initiated under limiting N conditions through plant-N signalling and AON-dependent pathways. The number of nodules formed during the first nodulation wave was linearly related to plant growth, through both AON dependent and N-signalling pathways. Nodule biomass and number were shown to be co-regulated through an AON-independent mechanism. As a tight adaptation to the environment, our results highlight the finely-tuned regulation of nodule number as the main component of the whole plant regulation of N(2) fixation, adapting precisely to environmental conditions and thus meeting plant N needs with minimal C costs.