Department of Land, Air, and Water Resources, University of California, Davis, 1 Shields Ave, Davis, CA, 95616, USA.
Oecologia. 2020 Mar;192(3):603-614. doi: 10.1007/s00442-020-04606-y. Epub 2020 Feb 6.
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.
植物与微生物（如菌根真菌或固氮菌）的根系联系使生态系统能够利用可能无法获得的氮（N）库，包括大气氮或土壤大分子中的氮。在生产力较低且土壤养分稀缺的后成演替逆行生态系统中，这种微生物辅助的 N 觅食策略可能尤为重要。在这里，我们使用天然 N 稳定同位素组成来限制不同植物功能群的 N 供应途径，这些植物功能群跨越了一个经过充分研究的自然土壤肥力梯度，其中包括一个高度逆行的阶段。我们证明，外生菌根真菌、石楠菌根真菌和固氮细菌支持生态系统演替的所有阶段的森林 N 供应，从相对年轻、富氮/富磷（P）的地点到古老的地点（约 500 千年），在这些地点中，N 和 P 的供应都极低。在土壤养分可用性极低的较老的生态系统中，微生物介导的 N 源最为重要，在最古老、最贫瘠的地点，占叶片 N 的 75-96%。这些基于同位素的发现指出了植物-微生物联系在塑造生态系统过程和功能方面的关键作用，特别是在逆行阶段的森林生态系统中。
