Which groups of bacteria are capable of fixing atmospheric nitrogen into soil nitrogen pools?

Introduction:
A range of bacteria fix atmospheric nitrogen, either in association with plants or as free-living organisms. This question distinguishes between symbiotic and nonsymbiotic diazotrophs and asks which groups contribute to soil nitrogen input.

Given Data / Assumptions:

• Symbiotic fixers form specialized associations (for example, nodules on legumes).
• Nonsymbiotic fixers operate independently in soil or water.
• Both pathways ultimately increase available nitrogen for ecosystems.

Concept / Approach:
Symbiotic bacteria such as Rhizobium, Bradyrhizobium, and Frankia fix nitrogen in intimate associations with plant hosts. Nonsymbiotic or free-living fixers include Azotobacter, Beijerinckia, and anaerobic Clostridium. Diazotroph diversity ensures nitrogen input across habitats and seasons, independent of host plant presence.

Step-by-Step Solution:
List representative symbiotic and nonsymbiotic diazotrophs. Recognize both perform the same core reaction: N2 to NH3 via nitrogenase. Acknowledge that both contribute to soil nitrogen accumulation. Select the comprehensive option covering both groups.

Verification / Alternative check:
Agronomic studies show enhanced soil nitrogen in legume rotations (symbiotic) and measurable fixation by free-living diazotrophs in organic-rich soils.

Why Other Options Are Wrong:

• Symbiotic or nonsymbiotic alone: Each is incomplete; both groups fix nitrogen.
• None of these: Incorrect because nitrogen fixation is widely distributed among bacteria.

Common Pitfalls:
Overemphasizing legumes and forgetting significant free-living contributions under high carbon availability.

Final Answer:
both (a) and (b) — symbiotic and nonsymbiotic bacteria can fix nitrogen.