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Soil nitrogen (N) acts as an inhibitor to the biological process of N 2 fixation in legumes. Conversely, the phosphorus (P) supply fosters N 2 fixation by promoting nodule formation, facilitating energy transfer, activating enzymes, and enhancing nitrogenase activity. This could help counteract the inhibitory effects that high soil N levels can have on N 2 fixation in legumes. However, there is limited knowledge regarding whether P supply can mitigate the inhibition of N 2 fixation. The objective of this work was to evaluate the effects of two levels of P and five N levels on N 2 fixation, shoot, root, and nodule biomass produced, and metabolic changes of leaves and nodules of Pisum sativum and Vicia faba. N 2 fixation was determined by measuring the natural abundance of 15N (%Ndfa) in shoots and roots, and metabolites were evaluated using GC-MS. The results showed that increasing N levels led to a reduction of N 2 fixation at the two P levels in both species. However, P supply reduced the inhibitory effect of N addition on N 2 fixation due to higher plant biomass production, which resulted in an increased mass of nodules. Under P deficiency, N addition affected N 2 fixation due to energy production in the nodules, which led to the accumulation of sugars and organic acids, especially in P. sativum. The response of N 2 fixation to a high N supply indicates that P. sativum and V. faba downregulated N 2 fixation but maintained viable nodules to resume the process if the N supply in the soil decreases. • Phosphorus increases biomass of Pisum sativum and Vicia faba fostering N 2 fixation. • P supply facilitates legumes in overcoming N 2 fixation inhibition induced by soil N by nodule resource allocation. • At low P supply, N 2 fixation inhibition is the result of sugar and organic acids accumulation in Pisum sativum. [ABSTRACT FROM AUTHOR]