In boreal forests, soil nitrate has been considered a negligible source of nitrogen, since conventional means to evaluate nitrate availability generally detect low nitrate concentration in soils during growing seasons. On the other hand, the capacity by boreal coniferous species to use nitrate has almost never been investigated, especially during non-growing seasons. We investigated nitrate use by black spruce in interior Alaska, USA, taking non-growing seasons into account. Nitrate reductase activity (NRA) of black spruce was measured in different phases in winter, as it is an essential enzyme that catalyzes the rate-limiting step in nitrate assimilation processes, and compared with NRA in summer. In addition, nitrate concentrations were assessed in current needles, current twigs and roots both in summer and winter. The highest NRA in needles was detected in October, while no significant difference was observed in other seasons including summer. Current needles showed higher nitrate concentrations than the other tissues in both summer and winter. In winter, nitrate concentrations declined in twigs and roots more than in needles. The results suggested that nitrate remained in needles in winter, while nitrate in twigs and roots was consumed or transported to other tissues, and NR in winter was supplied with nitrate from internal storage.