The predominant mechanism of gaseous exchange in soil is

Correct option is D

The correct answer is (D) Diffusion

Explanation:
• Gaseous exchange in soil (soil aeration) is the process by which $O_2$ enters the soil and $CO_2$ is expelled to maintain a healthy environment for roots and microbes.
• Diffusion is the movement of individual gas molecules along a concentration gradient (from higher partial pressure to lower partial pressure).
• It is the most important and continuous mechanism, accounting for over 90-95% of total soil air exchange.
• This process follows Fick's Law, which states that the flux of a gas is proportional to the concentration gradient.
• Oxygen diffuses into the soil because it is consumed by root respiration, creating a lower concentration inside, while $CO_2$ produced by respiration diffuses out.

Information Booster:
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• Soil porosity and moisture content significantly affect the diffusion rate; water-filled pores slow down diffusion by about 10,000 times compared to air-filled pores.
• Mass flow (convection) is a secondary mechanism caused by barometric pressure changes, wind, or rainfall, but its contribution is minimal.

Additional Knowledge:
• Mass flow (Option A): Movement of the entire air mass due to pressure differences; much less significant than diffusion.
• Contact exchange and Ion exchange (Options B & C): These refer to the movement and exchange of cations and anions between soil colloids and plant roots, not gases.