Symbiotic nitrogen (N) fixation (SNF) is critical to satisfying the nutritional need of soybean (Glycine max (L.) Merr.) and maintaining productivity and high seed protein concentration. Due to its low environmental impact, a key factor for increasing the sustainability of soybean systems is to enhance SNF. Seed inoculation with the free-living Azospirillum brasilense alone or with Bradyrhizobium japonicum (herein called co-inoculation) are plausible strategies that have been explored in tropical environments but lack information in temperate regions. Following this rationale, this study aimed to evaluate the impact of seed inoculation with Azospirillum brasilense (herein called Azospirillum) alone or combined with Bradyrhizobium japonicum (herein called Bradyrhizobium) in a range of environments in the United States (US) for: (i) seed yield, (ii) relative abundance of ureides (RAU) as a proxy of SNF, and (iii) seed protein concentration. Twenty-five field studies across the US states with the same experimental design were performed during the 2019 and 2020 growing seasons. The primary outcomes of this research were: (i) yield responses to co-inoculation were considered significant in only 2 out of 25 site-years, (ii) RAU was not increased by Azospirillum inoculation or co-inoculation, and lastly, (iii) seed protein concentration was marginally associated with the inoculation strategies. Although Azospirillum did not impose remarkable gain in any observed plant traits, future studies should focus on mechanistically understanding whether Azospirillum can naturalize in temperate region soils. Still, strategies for enhancing SNF are required for sustainably improving productivity and quality for US soybean systems.

共生固氮 (N) (SNF) 对于满足大豆 ( Glycine max (L.) Merr.) 的营养需求和保持生产力和高种子蛋白浓度至关重要。由于其对环境的影响低，提高大豆系统可持续性的一个关键因素是提高 SNF。单独使用自由生活的巴西固氮螺菌或日本慢生根瘤菌进行种子接种（本文称为联合接种）是在热带环境中探索过但在温带地区缺乏信息的可行策略。根据这一基本原理，本研究旨在评估接种巴西固氮螺菌（本文称为固氮螺菌)在美国 (US) 的一系列环境中单独或与日本慢生根瘤菌（本文称为慢生根瘤菌）组合用于：(i) 种子产量，(ii) 作为 SNF 代表的脲类化合物 (RAU) 的相对丰度，以及 ( iii) 种子蛋白质浓度。在 2019 年和 2020 年的生长季节，在美国各州进行了 25 项具有相同实验设计的田间研究。这项研究的主要结果是：（i）在 25 个地点年中，只有 2 个对共同接种的产量反应被认为是显着的，（ii）RAU 没有因固氮螺菌接种或共同接种而增加，最后，（iii ) 种子蛋白浓度与接种策略相关。虽然固氮螺菌没有对任何观察到的植物性状产生显着的影响，未来的研究应该集中在机械地理解固氮螺菌是否可以在温带土壤中归化。尽管如此，仍需要提高 SNF 的策略来可持续地提高美国大豆系统的生产力和质量。