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We experimentally show that the reaction between ground state nitrogen atoms N(^4S) and acetonitrile CH_3CN can lead to two distinct chemical pathways that are both thermally activated at very low temperatures. First is CH_3CN isomerization which produces CH_3NC and H_2CCNH. Second is CH_3CN decomposition which produces HNC and CH_3CNH^+CN- fragments, with the possible release of H_2. Our results reveal that the mobility of N(^4S)-atoms is stimulated in the 3-11 K temperature range, and that its subsequent encounter with one acetonitrile molecule is sufficient for the aforementioned reactions to occur without the need for additional energy to be supplied to the CH_3CN + N(^4S) system. These findings shed more light on the nitrogen chemistry that can possibly take place in dense molecular clouds, which until now was thought to only involve high-energy processes and therefore be unlikely to occur in such cold and dark interstellar regions. The reaction pathways we propose in this study have very important astrochemical implications, as it was shown recently that the atomic nitrogen might be more abundant, in many interstellar icy grain mantles, than previously thought. Also, these reaction pathways can now be considered within dense molecular clouds, and possibly affect the branching ratios for N-bearing molecules computed in astrochemical modelling.