Abstract：
Tree-ring δ15N may depict site-specific, long-term patterns in nitrogen (N) dynamics under N2-fixing species, but field trials with N2-fixing tree species are lacking and the relationship of temporal patterns in tree-ring δ15N to soil N dynamics is controversial. We examined whether the tree-ring δ15N of N2-fixing red alder (Alnus rubra Bong.) would mirror N accretion rates and δ15N of soils and whether the influence of alder-fixed N could be observed in the wood of a neighboring conifer. We sampled a 27-year-old replacement series trial on south-eastern Vancouver Island, with red alder and coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) planted in five proportions (0/100, 11/89, 25/75, 50/50 and 100/0) at a uniform stem density. An escalation in forest floor N content was evident with an increasing proportion of red alder, equivalent to a difference of ~750 kg N ha-1 between 100% Douglas-fir versus 100% alder. The forest floor horizon also had high δ15N values in treatments with more red alder. Red alder had a consistent quadratic fit in tree-ring δ15N over time, with a net increase of $\\sim$1.5‰, on average, from initial values, followed by a plateau or slight decline. Douglas-fir tree-ring δ15N, in contrast, was largely unchanged over time (in three of four plots) but was significantly higher in the 50/50 mix. The minor differences in current leaf litter N content and δ15N between alder and Douglas-fir, coupled with declining growth in red alder, suggests the plateau or declining trend in alder tree-ring δ15N could coincide with lower N2-fixation rates, potentially by loss in alder vigor at canopy closure, or down-regulation via nitrate availability.
摘要：
树木年轮δ15N可以描绘特定地点，固氮物种下氮（N）动力学的长期模式，但是缺乏固定N2树种的田间试验，并且年轮δ15N的时间模式与土壤N动力学的关系存在争议。我们检查了固定N2的红al木（Alnusrubra）的年轮δ15N是否会反映土壤的氮吸积率和δ15N，以及是否可以在邻近针叶树的木材中观察到the木固定的N的影响。我们在温哥华岛东南部进行了27年的替代系列试验，红木和沿海道格拉斯冷杉（Pseudotsugamenziesii）以5种比例（分别为0/100、11/89、25/75、50/50和100/0）以均匀的茎密度种植。随着红al木比例的增加，森林地面N含量明显上升，相当于100％道格拉斯冷杉与之间的差异约为750kgNha-1100%Alder在使用更多的红色al木的处理中，森林地面水平也具有很高的δ15N值。随着时间的推移，红木在树轮δ15N中具有一致的二次拟合，净增加$\\sim$1.5‰，平均而言,从初始值，其次是平稳或轻微下降。道格拉斯冷杉年轮δ15N,相比之下,随着时间的推移（在4个地块中的3个中）基本没有变化，但在50/50组合中明显更高。Alder木和道格拉斯冷杉之间当前凋落叶N含量和δ15N的微小差异，再加上红木的生长下降，表明the树年轮δ15N的平稳或下降趋势可能与较低的N2固定率相吻合，可能是由于树冠关闭时al木活力丧失，或通过硝酸盐的可用性下调。
