Purpose of Review

We provide an overview of the main processes occurring during the interactions between atmospheric nitrogen and forest canopies, by bringing together what we have learned in recent decades, identifying knowledge gaps, and how they can be addressed with future research thanks to new technologies and approaches.

Recent Findings

There is mounting evidence that tree canopies retain a significant percentage of incoming atmospheric nitrogen, a process involving not only foliage, but also branches, microbes, and epiphytes (and their associated micro-environments). A number of studies have demonstrated that some of the retained nitrogen can be assimilated by foliage, but more studies are needed to better quantify its contribution to plant metabolism and how these fluxes vary across different forest types. By merging different approaches (e.g., next-generation sequence analyzes and stable isotopes, particularly oxygen isotope ratios) it is now possible to unveil the highly diverse microbial communities hidden in forest canopies and their ability to process atmospheric nitrogen through processes such as nitrification and nitrogen fixation. Future work should address the contribution of both foliar nitrogen uptake and biological transformations within forest canopies to whole ecosystem nitrogen cycling budgets.

Summary

Scientists have studied for decades the role of forest canopies in altering nitrogen derived from atmospheric inputs before they reach the forest floor, showing that tree canopies are not just passive filters for precipitation water and dissolved nutrients. We now have the technological capability to go beyond an understanding of tree canopy itself to better elucidate its role as sink or source of nutrients, as well as the epiphytes and microbial communities hidden within them.

中文翻译：

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森林生态系统中氮通量的冠层交换和改变

审查目的

我们总结了大气氮和森林冠层相互作用期间发生的主要过程，将我们近几十年来所学的知识结合起来，确定知识差距，以及如何通过新技术和方法在未来的研究中解决这些问题。

最近的发现

越来越多的证据表明，树冠保留了很大比例的大气氮，这一过程不仅涉及树叶，还涉及树枝、微生物和附生植物（及其相关的微环境）。许多研究表明，一些保留的氮可以被树叶同化，但需要更多的研究来更好地量化其对植物代谢的贡献以及这些通量如何在不同森林类型中变化。通过合并不同的方法（例如，下一代序列分析和稳定同位素，特别是氧同位素比率），现在可以揭示隐藏在森林冠层中的高度多样化的微生物群落及其通过硝化和氮等过程处理大气氮的能力固定。

概括

几十年来，科学家们一直在研究森林冠层在改变来自大气输入的氮到达森林地面之前的作用，表明冠层不仅仅是降水和溶解养分的被动过滤器。我们现在拥有的技术能力超越了对树冠本身的理解，可以更好地阐明其作为营养物质的汇或来源的作用，以及隐藏在其中的附生植物和微生物群落。