Bradyrhizobia are common members of soil microbiomes and known as N₂‐fixing symbionts of economically important legumes. Many are also denitrifiers, which can act as sinks or sources for N₂O. Inoculation with compatible rhizobia is often needed for optimal N₂‐fixation, but the choice of inoculant may have consequences for N₂O emission. Here, we determined the phylogeny and denitrification capacity of Bradyrhizobium strains, most of them isolated from peanut‐nodules. Analyses of genomes and denitrification end‐points showed that all were denitrifiers, but only ~1/3 could reduce N₂O. The N₂O‐reducing isolates had strong preference for N₂O‐ over NO₃⁻‐reduction. Such preference was also observed in a study of other bradyrhizobia and tentatively ascribed to competition between the electron pathways to Nap (periplasmic NO₃⁻ reductase) and Nos (N₂O reductase). Another possible explanation is lower abundance of Nap than Nos. Here, proteomics revealed that Nap was instead more abundant than Nos, supporting the hypothesis that the electron pathway to Nos outcompetes that to Nap. In contrast, Paracoccus denitrificans, which has membrane‐bond NO₃⁻ reductase (Nar), reduced N₂O and NO₃⁻ simultaneously. We propose that the control at the metabolic level, favouring N₂O reduction over NO₃⁻ reduction, applies also to other denitrifiers carrying Nos and Nap but lacking Nar.

中文翻译：

---

电子竞争有利于在反硝化慢生根瘤菌分离物中减少 N2O

慢生根瘤菌是土壤微生物组的常见成员，被称为具有重要经济意义的豆科植物的N ₂固定共生体。许多也是反硝化剂，可作为 N ₂ O 的汇或源。通常需要接种兼容的根瘤菌以实现最佳 N ₂固定，但接种剂的选择可能会对 N ₂ O 排放产生影响。在这里，我们确定了慢生根瘤菌菌株的系统发育和反硝化能力，其中大部分菌株是从花生根瘤中分离出来的。基因组和反硝化终点的分析表明，所有这些都是反硝化菌，但只有约 1/3 可以减少 N ₂ O。N ₂ O 减少分离株对 N ₂有强烈的偏好O- 超过 NO ₃ ^-还原。在对其他慢根瘤菌的研究中也观察到了这种偏好，并暂时将其归因于通向 Nap（周质 NO ₃ ^-还原酶）和 Nos（N ₂ O 还原酶）的电子途径之间的竞争。另一个可能的解释是 Nap 的丰度低于 Nos。在这里，蛋白质组学显示 Nap 比 Nos 更丰富，支持了通往 Nos 的电子通路胜过 Nap 的假设。相反，Paracoccus denitrificans具有膜结合 NO ₃ ^-还原酶 (Nar)，可减少 N ₂ O 和 NO ₃ ^-同时。我们建议在代谢水平上的控制，有利于 N ₂ O 减少而不是 NO ₃ ^-减少，也适用于其他携带 Nos 和 Nap 但缺少 Nar 的反硝化剂。