Effect of low static nitrate concentrations on mineral nitrogen uptake, nodulation, and nitrogen fixation in field pea

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Abstract
Combined nitrogen [nitrate (NO3‐), ammonium (NH4+), and urea] will inhibit all components of symbiotic nitrogen (N2) fixation if present in sufficient concentrations. It is generally accepted that nitrate is particularly inhibitory to nodule growth and nitrogenase activity, and somewhat less inhibitory to the infection process. This project examined whether providing low (0.1 ‐ 0.5 mM) static concentrations of NO3‐ to pea (Pisum sativum L. cv. Express), seedlings could avoid the period of N hunger experienced prior to the establishment of N2 fixation, without delaying or reducing the symbiotic N2 fixation. All concentrations of NO3 − tested significantly inhibited all measured components of N2 fixation. The nodulation process as measured by nodule number was inhibited to a similar degree as the other parameters. A concentration dependent response was evident, with 0.1 mM NO3 − causing less inhibition than the 0.2 or 0.5 mM concentrations. Our results indicate the within the concentrations of 0.1 mM and 0.5 mM NO3 −, it is not possible to stimulate the growth of pea plants without inhibiting nodulation and N2 fixation.