The inhibition of N2 fixation by nitrogen is attenuated by the P supply, altering the plant metabolism

Soil nitrogen (N) acts as an inhibitor to the biological process of N2 fixation in legumes. Conversely, the phosphorus (P) supply fosters N2 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 N2 fixation in legumes. However, there is limited knowledge regarding whether P supply can mitigate the inhibition of N2 fixation. The objective of this work was to evaluate the effects of two levels of P and five N levels on N2 fixation, shoot, root, and nodule biomass produced, and metabolic changes of leaves and nodules of Pisum sativum and Vicia faba. N2 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 N2 fixation at the two P levels in both species. However, P supply reduced the inhibitory effect of N addition on N2 fixation due to higher plant biomass production, which resulted in an increased mass of nodules. Under P deficiency, N addition affected N2 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 N2 fixation to a high N supply indicates that P. sativum and V. faba downregulated N2 fixation but maintained viable nodules to resume the process if the N supply in the soil decreases.