Nitrogen (N) limits plant growth in temperate ecosystems, yet many evergreens exhibit low photosynthetic N use efficiency, which can be explained in part by their tendency to store more N than to use it in photosynthesis. However, it remains uncertain to what extent mature conifers translocate internal N reserves or take up N from soils to support new growth. In this study, we explored N dynamics within mature Douglas-fir (Pseudotsuga menziesii var. glauca) trees by linking N uptake in field-grown trees with seasonal soil available N. We used a branch-level mass balance approach to infer seasonal changes in total N among multiple needle and stem cohorts and bole tissue, and used foliar d 15 N to evaluate N translocation/uptake from soils. Soil resin-exchangeable N and net N transformation rates were measured to assess whether soils had sufficient N to support new needle growth. We estimated that after bud break, new needle biomass in Douglas-fir trees accumulated an average of 0.20 AE 0.03 mg N/branch and 0.17 AE 0.03 mg N/branch in 2016 and 2017, respectively. While we did find some evidence of translocation of N from older stems to buds prior to bud break, we did not detect a significant drawdown of N from previous years' growth during needle expansion. This suggests that the majority of N used for new growth was not reallocated from aboveground storage, but originated from the soils. This finding was further supported by the d 15 N data, which showed divergent d 15 N patterns between older needles and buds prior to leaf flushing (indicative of translocation), but similar patterns of depletion and subsequent enrichment following leaf expansion (indicative of N originating from soils). Overall, in order to support new growth, our study trees obtained the majority of N from the soils, suggesting tight coupling between soil available N and N uptake in the ecosystem.

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Declines in winter snowpack have increased the severity of summer droughts in western U.S. forests, with the potential to also impact soil available nitrogen (N). To understand how snowpack controls spatiotemporal N availability, we examined seasonal N dynamics across elevation, aspect, and topographic position (hollow vs. slope) in a forested watershed in the northern Rocky Mountains. As expected, peak snow‐water equivalent (SWE) was generally greater at higher elevations and on north‐facing aspects. However, the effects of topographic position and snowdrift led to variability in snow accumulation at smaller spatial scales. Spatial patterns of the snowpack, in turn, influenced soil moisture and temperature, with greater SWE leading to generally higher soil moisture levels during the summer and smaller temperature fluctuations throughout the year. Wetter conditions in spring or fall generally supported greater inorganic N pools, but at the driest locations (low‐elevation slope), pulses of N mineralization in summer may have played important roles in overall N dynamics. More importantly, soil moisture during the summer appeared to be more influenced by antecedent snowpack from the previous year than by current‐year summer rain. Subsequently, N mineralization under snowpack may be strongly influenced by soil moisture and temperature conditions from the previous fall, before snowpack accumulation. Together, our results indicate that snowpack strongly influences N dynamics beyond the current growing season in western coniferous forests through mediation of soil moisture and temperature, and suggest that further decline in winter snowpack may affect these forests through constraints in both water and N availability.

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