Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, Michigan, USA
W. K. Kellogg Biological Station, Department of Integrative Biology, Michigan State University, Hickory Corners, Michigan, USA.
Appl Environ Microbiol. 2019 Mar 6;85(6). doi: 10.1128/AEM.02546-18. Print 2019 Mar 15.
Free-living nitrogen fixation (FLNF) in the rhizosphere, or N fixation by heterotrophic bacteria living on/near root surfaces, is ubiquitous and a significant source of N in some terrestrial systems. FLNF is also of interest in crop production as an alternative to chemical fertilizer, potentially reducing production costs and ameliorating negative environmental impacts of fertilizer N additions. Despite this interest, a mechanistic understanding of controls (e.g., carbon, oxygen, nitrogen, and nutrient availability) on FLNF in the rhizosphere is lacking but necessary. FLNF is distinct from and occurs under more diverse and dynamic conditions than symbiotic N fixation; therefore, predicting FLNF rates and understanding controls on FLNF has proven difficult. This has led to large gaps in our understanding of FLNF, and studies aimed at identifying controls on FLNF are needed. Here, we provide a mechanistic overview of FLNF, including how various controls may influence FLNF in the rhizosphere in comparison with symbiotic N fixation occurring in plant nodules where environmental conditions are moderated by the plant. We apply this knowledge to a real-world example, the bioenergy crop switchgrass (), to provide context of how FLNF may function in a managed system. We also highlight future challenges to assessing FLNF and understanding how FLNF functions in the environment and significantly contributes to plant N availability and productivity.
土壤中自由生活固氮（FLNF），即生活在根表面或附近的异养细菌固氮，在一些陆地系统中普遍存在，是氮的重要来源。FLNF 作为化肥的替代物，在作物生产中也具有吸引力，它可能降低生产成本并减轻化肥氮添加对环境的负面影响。尽管对此很感兴趣，但对根际 FLNF 的控制因素（例如碳、氧、氮和养分供应）的机制理解仍很缺乏，但这是必要的。FLNF 与共生固氮不同，它发生在更具多样性和动态性的条件下；因此，预测 FLNF 速率和理解 FLNF 的控制因素一直很困难。这导致我们对 FLNF 的理解存在很大差距，需要进行旨在确定 FLNF 控制因素的研究。在这里，我们提供了 FLNF 的机制概述，包括各种控制因素如何与共生固氮（发生在植物根瘤中的固氮作用，其环境条件受植物调节）相比，可能影响根际中的 FLNF。我们将这一知识应用于现实世界的例子——生物能源作物柳枝稷（），以提供在管理系统中 FLNF 可能发挥作用的背景。我们还强调了评估 FLNF 和理解 FLNF 在环境中的功能以及如何显著促进植物氮素供应和生产力的未来挑战。
