Which gas becomes scarcer in soil airspaces when the soil is waterlogged?

• A. Carbon dioxide
• B. Oxygen
• C. Nitrogen
• D. Water vapor

Answer: (B)

Oxygen is the gas that becomes scarcer when soil is waterlogged. When soils are saturated, pore spaces fill with water, so gas exchange between the soil and the atmosphere is greatly slowed. Oxygen must diffuse through water-filled pores to reach roots and microbes, but diffusion in water is orders of magnitude slower than in air. As these organisms respire, they use up the limited O2 that can reach the pore spaces, and since replenishment is so slow, the oxygen concentration in soil airspaces declines, creating low-oxygen (hypoxic) or anaerobic conditions.

In contrast, carbon dioxide is produced by respiration, so its levels in saturated soils tend to rise rather than fall. Nitrogen gas is abundant and relatively inert in this context, so its concentration doesn’t drop in the same way oxygen does. Water vapor relates to humidity and isn’t the limiting gas affected by the saturation of pores in the same way.