Given below are two statements: Assertion (A) and Reason (R). Assertion (A): Organic matter is the portion of the soil that includes animal and plant residues in various stages of decay. Grassland soils have highest organic matter content. Reason (R): The organic matter content of a soil varies greatly depending on the climate. Soil microbes rapidly digest compounds such as starch, cellulose, sugars and amino acids.

Correct option is A

The correct answer is (A) Both (A) and (R) are correct and (R) is the correct explanation of (A)

Explanation:
• Assertion (A) is correct: Soil organic matter (SOM) consists of plant and animal remains at various decomposition levels. Grassland soils (Mollisols) are known for having high organic matter because of the deep, fibrous root systems of grasses that decay in situ.
• Reason (R) is correct: Climate (temperature and rainfall) is the primary driver of organic matter accumulation and decomposition. In warm, moist climates, microbial activity is high, leading to rapid digestion of simple compounds like sugars and starch.
• Relationship: The reason explains why organic matter varies (climate/microbes) and implies why certain ecosystems like grasslands can accumulate high amounts based on the balance between additions and microbial decomposition rates.

Information Booster:
• Humus is the stable, dark-colored end product of organic matter decomposition that improves soil structure and water-holding capacity.

Additional Knowledge:
• Starch and Sugars: These are 'labile' organic fractions, meaning they are easily and quickly broken down by bacteria.
• Lignin and Tannins: These are 'recalcitrant' fractions, meaning they take a long time for microbes to decompose, contributing more to stable humus.