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 δ¹⁵N 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 ± 0.03 mg N/branch and 0.17 ± 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 δ¹⁵N data, which showed divergent δ¹⁵N 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.

中文翻译：

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成熟的花旗松的氮素吸收策略：以落基山脉北部为例

氮限制了温带生态系统中植物的生长，但是许多常绿植物显示出较低的光合氮利用效率，这在一定程度上可以解释为它们比在光合作用中储存更多的氮。但是，仍不确定成熟的针叶树在多大程度上迁移内部氮储量或从土壤吸收氮以支持新的生长。在这项研究中，我们通过将田间生长树木中的氮吸收与季节性土壤可用氮联系起来，探索了成熟花旗松（Pseudotsuga menziesii var。glauca）树中的氮动态。在多个针总N和茎队列和树干组织，以及用于叶面δ ¹⁵N评估土壤中N的转运/吸收。测量土壤树脂可交换的氮和净氮转化率，以评估土壤是否具有足够的氮来支持新的针叶生长。我们估计断芽后，花旗松树中的新针叶生物量在2016年和2017年分别平均积累0.20±0.03 mg N /枝和0.17±0.03 mg N /枝。尽管我们确实发现了一些证据，表明N在芽断裂前从较早的茎向芽转移，但在针扩张期间，我们没有发现N从往年的生长中显着下降。这表明用于新生长的大部分N不是从地上存储中重新分配，而是源自土壤。这一发现是由δ进一步支持¹⁵的N个数据，这表明发散δ ¹⁵叶片冲刷之前，较老的针和芽之间存在N型态（指示易位），但叶片膨胀后枯竭和随后富集的模式相似（指示N来自土壤）。总体而言，为了支持新的生长，我们的研究树木从土壤中获取了大部分N，这表明土壤中有效氮与生态系统中氮的吸收之间存在紧密联系。