Arbuscular mycorrhizal (AM) fungi lack efficient exoenzymes to access organic nutrients directly. Nevertheless, the fungi often obtain and further channel to their host plants a significant share of nitrogen (N) and/or phosphorus from such resources, presumably via cooperation with other soil microorganisms. Because it is challenging to disentangle individual microbial players and processes in complex soil, we took a synthetic approach here to study ¹⁵N-labelled chitin (an organic N source) recycling via microbial loop in AM fungal hyphosphere. To this end, we employed a compartmented in vitro cultivation system and monoxenic culture of Rhizophagus irregularis associated with Cichorium intybus roots, various soil bacteria, and the protist Polysphondylium pallidum. We showed that upon presence of Paenibacillus sp. in its hyphosphere, the AM fungus (and associated plant roots) obtained several-fold larger quantities of N from the chitin than it did with any other bacteria, whether chitinolytic or not. Moreover, we demonstrated that adding P. pallidum to the hyphosphere with Paenibacillus sp. further increased by at least 65% the gain of N from the chitin by the AM fungus compared to the hyphosphere without protists. We thus directly demonstrate microbial interplay possibly involved in efficient organic N utilisation by AM fungal hyphae.

丛枝菌根 (AM) 真菌缺乏有效的胞外酶来直接获取有机养分。然而，真菌通常从这些资源中获取并进一步向宿主植物输送大量的氮 (N) 和/或磷，大概是通过与其他土壤微生物的合作。由于在复杂土壤中解开单个微生物参与者和过程具有挑战性，我们在这里采用了一种合成方法来研究¹⁵ N 标记的几丁质（一种有机 N 源）通过 AM 真菌菌丝圈中的微生物循环进行循环。为此，我们采用了与菊苣根、各种土壤细菌和原生生物相关的隔室体外培养系统和不规则根瘤菌的一氧化氮培养苍白球藻。我们表明，在类芽孢杆菌属存在的情况下。在其菌丝层中，AM 真菌（和相关的植物根系）从几丁质中获得的 N 量是它从任何其他细菌中获得的 N 的数倍，无论是否具有几丁质溶解性。此外，我们证明了将P. pallidum添加到类芽孢杆菌的菌丝圈中。与没有原生生物的菌丝圈相比，AM 真菌从几丁质中获取的 N 进一步增加了至少 65%。因此，我们直接证明了 AM 真菌菌丝可能参与有效有机氮利用的微生物相互作用。