Data for stable C and N isotope natural abundances of arbuscular mycorrhizal (AM) fungi are currently sparse, as fungal material is difficult to access for analysis. So far, isotope analyses have been limited to lipid compounds associated with fungal membranes or storage structures (biomarkers), fungal spores and soil hyphae. However, it remains unclear whether any of these components are an ideal substitute for intraradical AM hyphae as the functional nutrient trading organ. Thus, we isolated intraradical hyphae of the AM fungus Rhizophagus irregularis from roots of the grass Festuca ovina and the legume Medicago sativa via an enzymatic and a mechanical approach. In addition, extraradical hyphae were isolated from a sand-soil mix associated with each plant. All three approaches revealed comparable isotope signatures of R. irregularis hyphae. The hyphae were ¹³C- and ¹⁵N-enriched relative to leaves and roots irrespective of the plant partner, while they were enriched only in ¹⁵N compared with soil. The ¹³C enrichment of AM hyphae implies a plant carbohydrate source, whereby the enrichment was likely reduced by an additional plant lipid source. The ¹⁵N enrichment indicates the potential of AM fungi to gain nitrogen from an organic source. Our isotope signatures of the investigated AM fungus support recent findings for mycoheterotrophic plants which are suggested to mirror the associated AM fungi isotope composition. Stable isotope natural abundances of intraradical AM hyphae as the functional trading organ for bi-directional carbon-for-mineral nutrient exchanges complement data on spores and membrane biomarkers.

丛枝菌根（AM）真菌的稳定C和N同位素天然丰度数据目前很少，因为难以获取真菌材料进行分析。到目前为止，同位素分析仅限于与真菌膜或存储结构（生物标记），真菌孢子和土壤菌丝相关的脂质化合物。但是，尚不清楚这些成分中的任何一种是否可以作为功能性营养物交易器官替代自由基内AM菌丝。因此，我们从草Festuca卵子和豆科植物紫花苜蓿的根中分离出了AM真菌不规则根瘤菌的根内菌丝。通过酶促和机械方法。另外，从与每种植物相关的沙土混合物中分离了自由基外的菌丝。所有这三种方法都揭示了不规则红杆菌菌丝的同位素特征。相对于叶和根，菌丝相对于叶和根均富集¹³ C和¹⁵ N，而与土壤相比，它们仅富集^15N。AM菌丝的¹³ C富集意味着植物碳水化合物的来源，因此这种富集很可能会因其他植物脂质来源而减少。在¹⁵氮富集表明AM真菌有可能从有机源获取氮。我们研究的AM真菌的同位素特征支持了真菌异养植物的最新发现，该现象被认为可以反映相关的AM真菌同位素组成。自由基AM菌丝的稳定同位素自然丰度作为双向碳对矿物质营养交换的功能交易器官，可补充孢子和膜生物标记物的数据。