What happens to oxygen levels when soils become saturated?

• A. Oxygen increases
• B. Oxygen remains the same
• C. Oxygen decreases
• D. Oxygen becomes highly variable

Answer: (C)

When soils become saturated, the pore spaces fill with water, not air. Oxygen must diffuse from the atmosphere into the soil, but diffusion through water is vastly slower than through air, so the supply of oxygen to roots and soil microbes drops sharply. At the same time, those organisms keep using oxygen for respiration. If the incoming supply can’t meet the demand, oxygen levels decline, creating reducing conditions. With prolonged saturation, microbes switch to using other electron acceptors (like nitrate, iron, or sulfate) and anaerobic processes become more common, potentially producing reduced compounds such as Fe2+, Mn2+, sulfide, and even methane.