“…This helps conserve resources and energy when nitrogen fixation is not needed, thus maintaining the optimal balance of nitrogen with carbon and other resources (Voisin, Munier‐Jolain, & Salon, ). Low levels of exogenously applied nitrogen (<2 mM) enhance nodulation, likely by promoting plant health without being sufficient to make nitrogen fixation redundant, whereas nodule numbers gradually decrease with the application of increasing nitrogen concentrations (Barbulova, Rogato, D'Apuzzo, Omrane, & Chiurazzi, ; Carroll, McNeil, & Gresshoff, ; Day, Carroll, Delves, & Gresshoff, ; Hussain, Jiang, Broughton, & Gresshoff, ; van Noorden et al, ). Interestingly, the response seems to be contingent on the source of nitrogen, as nitrate and ammonia have a strong inhibitory effect on nodule development, while urea generally does not, with some species‐specific exceptions reported (e.g., cowpea; Dart & Wildon, ; Vigue, Harper, Hageman, & Peters, ).…”