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Nitrogen bearing species are common tracers of the physical conditions in awide variety of objects, and most remarkably in dark clouds. The reservoir ofgaseous nitrogen is expected to be atomic or molecular, but none of the twospecies are observable in the dark gas. Their abundances therefore deriveindirectly from those of N-bearing species through chemical modelling. Therecent years have accumulated data which stress our incomplete understanding ofthe nitrogen chemistry in dark cloud conditions. To tackle this problem of thenitrogen chemistry in cold gas, we have revised the formation of nitrogenhydrides, which is initiated by the key reaction \ce{N+ + H2 -> NH+ + H}. Wepropose a new rate for this reaction which depends on the ortho-to-para ratioof H2. This new rate allows to reproduce the abundance ratios of the threenitrogen hydrides, NH, \ce{NH2}, and \ce{NH3}, observed towards IRAS16293-2422,provided that the channel leading to NH from the dissociative recombination of\ce{N2H+} is not closed at low temperature. The ortho-to-para ratio of H2 isconstrained to O/P=$10^{-3}$ by the abundance ratio NH:NH2, which provides anew method to measure O/P. This work stresses the need for reaction rates atthe low temperatures of dark clouds, and for branching ratios of criticaldissociative recombination reactions.

Nitrogen hydrides and the H2 ortho-to-para ratio in dark clouds