Post-disturbance conifer tree-ring δ15N reflects openness of the nitrogen cycle across temperate coastal rainforests

1. Post-disturbance losses in nitrogen (N) may diminish forest productivity, and soils with inherently ‘open’ N cycles (high nitrification rates) are considered the most vulnerable to leaching losses of NO3-. Monitoring ongoing N depletion from soil profiles is challenging, but tree-ring δ15N of regenerating stands may offer an effective method for assessing site-specific, long-term soil N dynamics. Evidence to date is mixed, however, and includes increasing, unchanging, or decreasing tree-ring δ15N in young stands following stand-level disturbances, possibly because of contrasting soil fertility among study sites. In addition, a consensus on post-disturbance N trajectories is hampered by the sometimes inconsistent patterns in tree-ring δ15N found between tree species of differing mycorrhizal association. 2. We compared tree-ring δ15N of two conifer species (Picea sitchensis with ectomycorrhizal fungi and Thuja plicata with arbuscular mycorrhiza) from a replicated silviculture trial across temperate rainforests of Vancouver Island (Canada). A natural gradient in soil fertility across the six sites, driven largely by topography and parent materials, was confirmed by an in situ increase in N mineralization and nitrification rates with declining C:N ratios for both organic horizons and mineral soils. 3. Five decades after logging, the overall trend in tree-ring δ15N was positive, but among individual plots there was a wide range in δ15N slopes, ranging from nearly 0 to 0.13. We found the gains in tree-ring δ15N over time were consistent between mycorrhizal types and escalated sharply (up to 6‰) with increasing N mineralization rates, although less so on flat terrain with seasonal water tables. The most recent sapwood was also enriched in 15N as soil N mineralization rates increased, perhaps slightly more so for T. plicata than P. sitchensis. 4. Synthesis. The correspondence of tree-ring δ15N with soil fertility may be especially strong in regenerating forests because of tree ontogeny effects, including the expansion of rooting depth and differences in N resorption efficiency with stand age. Sharp increases in tree-ring d15N underscore the vulnerability of low C:N soils with open N cycles to post-disturbance N losses, and highlight how repeated, frequent harvest cycles may risk substantial N depletion from these productive rainforest ecosystems.

1. 扰动后氮（N）损失可能削弱森林生产力，而固有氮循环呈“开放”状态（即硝化速率（nitrification）较高）的土壤，被认为最易发生NO₃⁻淋溶损失（leaching losses）。针对土壤剖面中持续发生的氮素耗竭开展监测颇具挑战，但更新林分的树木年轮δ¹⁵N (tree-ring δ¹⁵N) 或可为评估特定样地的长期土壤氮动态提供有效手段。然而现有研究结论尚存分歧：林分尺度扰动后，幼龄林的树木年轮δ¹⁵N可呈现升高、不变或降低等多种变化趋势，这可能源于不同研究样地间土壤肥力的显著差异。此外，不同菌根共生类型（mycorrhizal association）的树种之间，树木年轮δ¹⁵N的变化模式有时并不一致，这也阻碍了学界就扰动后氮素变化轨迹达成共识。
2. 本研究依托加拿大温哥华岛温带雨林（temperate rainforests）区域的重复营林试验（silviculture trial），对比了两种针叶树种的树木年轮δ¹⁵N：一是与外生菌根真菌（ectomycorrhizal fungi）共生的西加云杉（*Picea sitchensis*），二是与丛枝菌根（arbuscular mycorrhiza）共生的北美乔柏（*Thuja plicata*）。6个样地存在自然形成的土壤肥力梯度，该梯度主要由地形与母质驱动，且通过原位测定的氮矿化（N mineralization）、硝化速率随有机质层（organic horizons）与矿质土壤（mineral soils）的碳氮比（C:N ratio）降低而升高得到验证。
3. 采伐发生50年后，树木年轮δ¹⁵N的整体变化趋势为正向，但各独立样地的δ¹⁵N斜率差异显著，范围介于接近0至0.13之间。研究发现，树木年轮δ¹⁵N随时间的增幅在不同菌根类型间保持一致，且随氮矿化速率升高呈显著上升趋势（最高可达6‰），但在具有季节性地下水位的平坦地形上，增幅相对较弱。随着土壤氮矿化速率提升，最新形成的边材（sapwood）δ¹⁵N也显著富集，北美乔柏的富集程度略高于西加云杉。
4. 综合与结论：由于树木个体发育效应（ontogeny effects，包括根系深度扩展以及林分年龄带来的氮回收效率（N resorption efficiency）差异），更新林分中树木年轮δ¹⁵N与土壤肥力的对应关系可能尤为显著。树木年轮δ¹⁵N的大幅升高，印证了碳氮比低且氮循环开放的土壤在扰动后更易发生氮素损失，同时也凸显了频繁重复的采伐周期可能对这类高生产力雨林生态系统造成严重氮素耗竭的风险。