Absence of air in soil limits the activity of

Correct option is D

The correct answer is (D) Microorganisms and affects oxidation process

Explanation:
• Soil air is essential for the respiration of aerobic microorganisms and plant roots. When soil is waterlogged or compacted, oxygen ($O_2$) is displaced.
• In the absence of air, aerobic activity stops and is replaced by anaerobic microorganisms. This shift significantly affects the chemical state of the soil by halting oxidation and promoting reduction processes.
• For example, in aerated soil, nitrogen is oxidized to nitrate ($NO_3^-$); in anaerobic soil, it is reduced to nitrogen gas ($N_2$) or nitrous oxide ($N_2O$) via denitrification.
• Iron and manganese also shift from oxidized forms (rust/red color) to reduced forms (grey/bluish color), a process called gleying.

Information Booster:
• Soil air typically contains the same amount of Nitrogen ($78%$) as atmospheric air, but much less Oxygen ($<21%$) and much more Carbon Dioxide ($>0.03%$).
• The process of gas exchange between soil and atmosphere is called soil aeration.
• Lack of oxygen leads to the accumulation of toxic substances like methane, hydrogen sulfide, and organic acids.

Additional Knowledge:
• Oxidation: Loss of electrons/gain of oxygen (occurs in well-aerated soils).
• Reduction: Gain of electrons/loss of oxygen (occurs in waterlogged soils).
• Macroorganisms: Like earthworms, also require air and will migrate or die in anaerobic, waterlogged conditions.