Nitrogen fixation — Identify the nonsymbiotic (free-living) nitrogen-fixing bacteria that live independently in soil, and comment on their overall relative importance compared with symbiotic fixers.

Introduction / Context:
Nitrogen fixation converts atmospheric N2 into ammonia, making nitrogen bioavailable. In soils, both symbiotic and nonsymbiotic (free-living) microbes contribute. Recognizing key genera and their relative contribution guides agronomic interventions and expectations from biofertilizers.

Given Data / Assumptions:

• Nonsymbiotic fixers operate without plant nodules.
• Azotobacter is aerobic; Clostridium is anaerobic.
• Symbiotic systems (e.g., Rhizobium-legume) typically contribute larger net N inputs per unit area.

Concept / Approach:
Free-living diazotrophs fix nitrogen at the expense of their own energy and are often limited by carbon availability. Symbiotic fixers receive photosynthate and protective niches in nodules, enabling higher sustained fixation. Therefore, both named genera are valid examples of nonsymbiotic fixers, and it is accurate that their aggregate contribution is generally less than that of symbiotic systems in agricultural soils.

Step-by-Step Solution:

Verification / Alternative check:
Field N-balance studies show greater N inputs from legumes via symbiosis, while free-living fixation occurs but is modest per hectare under typical conditions.

Why Other Options Are Wrong:

• Azotobacter only or Clostridium only: incomplete; both are valid examples.
• Less important alone: true statement but does not list the organisms; the comprehensive correct choice is “all of the above.”

Common Pitfalls:
Assuming “less important” means negligible; free-living fixation can be ecologically significant, especially in undisturbed ecosystems.

Final Answer:
all of the above