Plant root associations with microbes such as mycorrhizal fungi or N-fixing bacteria enable ecosystems to tap pools of nitrogen (N) that might otherwise be inaccessible, including atmospheric N or N in large soil organic molecules. Such microbially assisted N-foraging strategies may be particularly important in late-successional retrogressive ecosystems where productivity is low and soil nutrients are scarce. Here, we use natural N-stable isotopic composition to constrain pathways of N supplies to different plant functional groups across a well-studied natural soil fertility gradient that includes a highly retrogressive stage. We demonstrate that ectomycorrhizal fungi, ericoid mycorrhizal fungi, and N-fixing bacteria support forest N supplies at all stages of ecosystem succession, from relatively young, N-rich/phosphorus (P)-rich sites, to ancient sites (ca. 500 ky) where both N supplies and P supplies are exceedingly low. Microbially mediated N sources are most important in older ecosystems with very low soil nutrient availability, accounting for 75-96% of foliar N at the oldest, least fertile sites. These isotopically ground findings point to the key role of plant-microbe associations in shaping ecosystem processes and functioning, particularly in retrogressive-phase forest ecosystems.

中文翻译：

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生态系统退化期间植物氮获取策略的同位素约束。

植物根系与诸如菌根真菌或固氮菌等微生物的结合，使生态系统能够利用否则可能无法获得的氮（N）库，包括大土壤有机分子中的大气氮或氮。这种微生物辅助的N觅食策略在生产力低，土壤养分稀缺的后继退化生态系统中尤其重要。在这里，我们使用天然的N稳定同位素组成，在经过充分研究的天然土壤肥力梯度（包括高度退化的阶段）中，将N供给途径限制到不同植物功能组。我们证明外生菌根真菌，类固醇菌根真菌和固氮细菌在生态系统演替的各个阶段都支持森林N的供应，这些阶段来自相对年轻的，富氮/富磷的地方，到N和P都极低的古代遗址（大约500 ky）。微生物介导的氮源在土壤养分利用率非常低的较老的生态系统中最为重要，在最古老，肥沃的地区占叶面氮的75-96％。这些同位素研究结果表明，植物-微生物协会在塑造生态系统过程和功能，特别是在退行期森林生态系统中起着关键作用。