The first stable product of fixation of atmospheric nitrogen in leguminous plants is

The most prominent symbiotic biological nitrogen fixing association is the legume-bacteria relationship. Species of rod-shaped Rhizobium has such relationship with the roots of several legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.  The most common association on roots is as nodules. These nodules are small outgrowths on the roots. Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghemoglobin. The enzyme nitrogenase is a Mo-Fe protein and catalyzes the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.

${\mathrm{N}}_2+ 8{\mathrm{e}}^{-}+8{\mathrm{H}}^{+}+16\mathrm{ATP}\to 2{\mathrm{NH}}_3+{\mathrm{H}}_2+ 16\mathrm{ADP}+16{\mathrm{P}}_{\mathrm{i}}$