Using two different guided ion beam machines the cross section for the slightly endothermic title reaction has been measured in a collision energy range which includes the threshold onset. High-energy resolution has been achieved by combining a slow guided ion beam with a supersonic deuterium beam, in one case by crossing the two beams, in the other case by merging them. In a brief discussion the kinematic conditions are compared for the two geometries with those of a beam scattering cell arrangement. In contrast to previous low-resolution measurements the present data clearly indicate a threshold, but, despite the high experimental resolution, it is not straightforward to extract a precise value of the reaction endothermicity or barrier since both reactants can contribute internal energy. Variation of the D-2 rotational temperature proves that kinetic and rotational energy are equivalent in promoting the reaction. In contrast to that, the two beam experiments give some hints that energy from excited N+ fine structure states is inefficient. This is also corroborated by a low-temperature ion trap study. With this assumption new phase space theory calculations have been performed to estimate threshold values. First evaluations indicate 11 meV for the reaction with H-2 and 29 meV for the reaction with D-2.