Plant roots are shaping microbial communities that are distinct from the surrounding soil. These root-associated microbial communities can have both positive and negative effects on the host nutrient acquisition and thereby growth, yet how loss of soil microbial diversity will constrain the plant microbiome selection is relatively unknown. In this study, we manipulated the soil microbial community using a removal-by-dilution approach to examine how microbial diversity modulates microbiome selection in barley, including microbial guilds involved in nitrogen (N) cycling processes causing N loss, and its consequences for plant performance. We found that microbial diversity loss reduced the barley's ability to recruit specific microorganisms from the soil and only members of the Alphaproteobacteria and Bacteroidetes were enriched in both rhizosphere and root-associated compartments irrespective of dilution level. Loss in soil microbial diversity and the presence of plants affected the N-cycling communities, with the abundance of nitrous oxide reducers being 2–4 times higher in both barley compartments in the lower diversity soils. In these soils, the low abundance of bacterial ammonia oxidizers (close or below detection level in the barley compartments) was concomitant with an increase in leaf greenness (ca. 12%), an indicator of the plant N status. The reduction in soil microbial diversity was thus coupled to a change in functional traits of rhizosphere and root-associated communities, with consequences for plant performance. This work contributes to our understanding of plant-microbe interactions, which is needed to steer the crop microbiome towards increased N-use efficiency while minimizing negative environmental impact.

植物根系正在塑造与周围土壤不同的微生物群落。这些与根相关的微生物群落可以对宿主养分的获取和生长产生积极和消极的影响，但土壤微生物多样性的丧失将如何限制植物微生物群落的选择还相对未知。在这项研究中，我们使用稀释去除方法操纵土壤微生物群落，以检查微生物多样性如何调节大麦中的微生物组选择，包括参与导致 N 损失的氮 (N) 循环过程的微生物群及其对植物性能的影响. 我们发现微生物多样性的丧失减少了大麦 s 从土壤中招募特定微生物的能力，并且无论稀释水平如何，只有 Alphaproteobacteria 和 Bacteroidetes 的成员在根际和与根相关的隔室中都富集。土壤微生物多样性的丧失和植物的存在影响了氮循环群落，在多样性较低的土壤中，两个大麦区室中一氧化二氮还原剂的丰度高出 2-4 倍。在这些土壤中，细菌氨氧化剂的低丰度（接近或低于大麦隔间的检测水平）伴随着叶子绿度的增加（约 12%），这是植物 N 状态的指标。因此，土壤微生物多样性的减少与根际和根相关群落功能性状的变化相结合，从而对植物性能产生影响。