Gas exchange between the soil and the atmosphere
Two processes help the flow of gases between the atmosphere and soil air.
1. Mass Flow:
The pressure differential between the atmosphere and soil air is thought to be the cause of mass air movement. Every time there is rain or irrigation, some of the soil’s air is forced out of the ground and into the atmosphere. The ambient air seeps into the soil pores as and when moisture is lost through evaporation and transpiration.
The temperature of the soil air varies as a result of fluctuations in soil temperature. When the soil starts to chill, the soil air contracts and the atmospheric air is sucked in. As the soil air warms up during the day, it expands and the expanded air travels out into the atmosphere.
Diffusion is the primary mechanism by which gases exchange in soils. Numerous gases, including nitrogen, oxygen, carbon dioxide, and others, are present in atmospheric and soil air. Each of these gases exerts a distinct partial pressure that is inversely proportional to its concentration. The partial pressure that each gas lives under controls how it moves.
One of the gases, such as carbon dioxide, will escape into the atmosphere if its partial pressure in soil air is higher than that in atmospheric air. Therefore, the air in question has a higher CO2 content. On the other hand, the soil air has a low partial pressure of oxygen.
Diffusion enables extensive movement and continuous exchange of gases between the soil air and the atmosphere. As oxygen present in soil air is consumed as a result of biological activities, the oxygen present in atmospheric air (where the partial pressure of O2 is greater) diffuses into the soil air until an equilibrium is established. The two main gases that take in diffusion are oxygen and carbon dioxide (Fig. 3.1).