Nitrogen fixation pathways — Which groups of microorganisms participate in biological nitrogen fixation (conversion of N2 to ammonia) in soils and plant associations?

Introduction / Context:
Biological nitrogen fixation is catalyzed by nitrogenase and supplies “new” reactive nitrogen to ecosystems. It occurs both in free-living microbes and in tight symbioses with plants, greatly influencing soil fertility and crop productivity.

Given Data / Assumptions:

• We consider biological, not industrial (Haber–Bosch), fixation.
• Symbioses include legume–Rhizobium and actinorhizal partnerships; free-living fixers include Azotobacter, Clostridium, some cyanobacteria.

Concept / Approach:
Fixation requires nitrogenase and a low-oxygen microenvironment with abundant reducing power. These conditions are met in nodules (symbiosis) and in specialized niches of free-living microbes. Therefore, both symbiotic and nonsymbiotic microorganisms are involved; restricting to either group is incorrect.

Step-by-Step Solution:

Verification / Alternative check:
Field N budgets allocate significant inputs to legume symbiosis, with smaller but real contributions from free-living fixation in natural systems.

Why Other Options Are Wrong:

• Non-symbiotic only / Symbiotic only: each omits a major pathway.
• None of the above: incorrect because both are correct together.

Common Pitfalls:
Equating “most important” with “only”; both pathways exist even if symbiosis often dominates in agriculture.

Final Answer:
Non-symbiotic and symbiotic microorganisms only