This article throws light upon the two main mechanisms by which the exchange of gases between the soil and the atmosphere is facilitated. The mechanisms are: 1. Mass Flow 2. Diffusion.
Mechanism # 1. Mass Flow:
Mass flow of air is apparently due to pressure difference between the atmosphere and the soil air. With every rain or irrigation a part of the soil air moves out into the atmosphere as it is displaced by the incoming water. As and when moisture is lost by evaporation and transpiration, the atmospheric air enters the soil pores.
The variations in soil temperature cause changes in temperature of soil air. As the soil air gets heated during the day, it expands, and the expanded air moves out into the atmosphere, when the soil begins to cool the soil air contracts and the atmospheric air drawn in.
Mechanism # 2. Diffusion:
Most of the gaseous interchange in soils occurs by diffusion. Atmospheric and soil air contains a number of gases such as nitrogen, oxygen, carbon dioxide etc., each of which exerts its own partial pressure in proportion to its concentration. The movement of each gas is regulated by the partial pressure under which it exists.
If the partial pressure of one of the gases (i.e., carbon dioxide) is greater in the soil air than in the atmospheric air, it (CO2) moves out into the atmosphere. Hence,the concentration of CO2 is more in said air. On the other hand, partial pressure of oxygen is low in the soil air.
As oxygen present in soil air is consumed as a result of biological activities, the oxygen present in the atmospheric air (partial pressure of O2 is greater), therefore, diffuses into the soil air till an equilibrium is established, thus, diffusion allows extensive movement and continual change of gases between the soil air and the atmosphere. Oxygen and carbon dioxide are the two important gases that take in diffusion (Fig. 3.1).
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