Pelagic microbial communities are a key component of the Arctic Ocean when evaluating the ecological impact of the thinner Arctic icescape, as they constitute the basis of the marine food web and biogeochemical cycles. During the Norwegian young sea ICE expedition (N-ICE2015), that took place in drifting pack ice north of Svalbard between January-June 2015, seawater was collected at 5, 20 or 50, and 250 m depth on 9 March, 27 April and 16 June, together with environmental data. Illumina MiSeq paired-end reads from SSU rRNA amplicon and metagenomes were sequenced to study the composition, diversity and key nitrogen-cycling functions of the Arctic’s microbiome through the winter-spring transition. Results show that nitrifiers, aerobic ammonia oxidizers and nitrite oxidizers, mostly affiliated with Thaumarchaeota and Nitrospinae, are abundant in subsurface waters below the pack ice during winter-early spring (6.3-27.4%) but nearly absent in late spring (0.1-2.0%). Urease and ammonia monooxygenase encoding genes are positively correlated with total dissolved nitrogen (urea included) suggesting the coupling of ureolysis and ammonia oxidation. Urease encoding gene increases along depth meaning that distinct species of thaumarchaeotes, i.e., ‘Ca. Nitrosopumilus’ and Marine Group I, found at different depths in the water column have different potential to carry out ureolysis. These results provide new knowledge about the nitrogen-cycling communities and pathways in the Arctic Ocean.