Agronomic examples: Which crop groups are well known for symbiotic nitrogen fixation via root nodules with Rhizobium (legume–Rhizobium associations)?

Introduction / Context:
Legumes form symbioses with Rhizobium/Bradyrhizobium, producing root nodules where nitrogenase fixes N2 to ammonia. Identifying legume examples helps link microbiology with farming practices and crop rotations.

Given Data / Assumptions:

• Examples listed include classic legumes used worldwide.
• Question asks which are involved in nitrogen fixation via symbiosis.

Concept / Approach:
All listed crops are legumes that commonly host rhizobial symbionts: alfalfa and clover (forage legumes), soybean (major oilseed legume), beans and lupine (grain and forage legumes). Each forms nodules that support nitrogen fixation, reducing synthetic fertilizer needs.

Step-by-Step Solution:
Recognize each crop as a legume. Recall legume–Rhizobium symbiosis and nodule formation. Choose the inclusive option since all listed crops fix nitrogen symbiotically.

Verification / Alternative check:
Agricultural manuals recommend inoculation with appropriate rhizobial strains for these crops to maximize nodulation and yields.

Why Other Options Are Wrong:

• Any single subset is incomplete because all listed examples fix nitrogen.
• “None of these” contradicts well-known agronomy.

Common Pitfalls:
Confusing legumes with non-legumes; cereals like wheat or maize do not form these nodules.

Final Answer:
All of these crops participate in symbiotic nitrogen fixation.