Many aspects concerning the role of arbuscular mycorrhizal (AM) fungi in plant nutrient uptake from organic sources remain unclear. Here, we investigated the contribution of AM symbiosis to N and P uptake by durum wheat after the addition of a high C:N biomass to a P-limited soil. Plants were grown in pots in the presence or absence of a multispecies AM inoculum, with (Org) or without (Ctr) the addition of ¹⁵N-labelled organic matter (OM). A further treatment, in which ¹⁵N was applied in mineral form (Ctr+N) in the same amount as that supplied in the Org treatment, was also included. Inoculation with AM had positive effects on plant growth in both control treatments (Ctr and Ctr+N), mainly linked to an increase in plant P uptake. The addition of OM, increasing the P available in the soil for the plants, resulted in a marked decrease in the contribution of AM symbiosis to plant growth and nutrient uptake, although the percentage of mycorrhization was higher in the Org treatment than in the controls. In addition, mycorrhization drastically reduced the recovery of ¹⁵N from the OM added to the soil whereas it slightly increased the N recovery from the mineral fertiliser. This suggests that plants and AM fungi probably exert a differential competition for different sources of N available in the soil. On the whole, our results provide a contribution to a better understanding of the conditions under which AM fungi can play an effective role in mitigating the negative effects of nutritional stresses in plants.

关于丛枝菌根（AM）真菌在植物从有机来源吸收养分中的作用的许多方面仍不清楚。在这里，我们研究了在磷限制土壤中添加高 C:N 生物量后 AM 共生对硬粒小麦氮和磷吸收的贡献。植物在存在或不存在多物种AM接种物的情况下在盆中生长，添加(Org)或不添加(Ctr) ¹⁵ N-标记的有机物(OM)。还包括进一步的处理，其中以矿物质形式施用^{15 N (Ctr+N)，其量与 Org 处理中提供的量相同。}在两种对照处理（Ctr 和 Ctr+N）中，接种 AM 对植物生长都有积极影响，主要与植物磷吸收的增加有关。添加 OM，增加了土壤中植物可利用的磷，导致 AM 共生对植物生长和养分吸收的贡献显着下降，尽管 Org 处理中的菌根化百分比高于对照。此外，菌根化大大降低了添加到土壤中的 OM 中^{15 N 的回收率，而略微增加了矿物肥料中 N 的回收率。}这表明植物和 AM 真菌可能对土壤中不同的可用氮源产生不同的竞争。总的来说，我们的结果有助于更好地了解 AM 真菌在减轻植物营养胁迫负面影响方面发挥有效作用的条件。