Simulated acid rain (H2SO4) and microbial activity in soil

Soil mixtures containing 9% kaolinite, 9% montmorillonite, or no clay supplements were amended with 1% glucose and treated with H₂SO₄ to lower their bulk pH to levels ranging from 5.4 to 0.8. Acidification had little effect on soil respiration (CO₂ evolution) until the pH was lowered below 3. Glucose was not degraded at approximately pH 2 but was degraded once the soil pH was raised to non-inhibitory levels, i.e. pH 4.1–4.3. When the soil pH was reduced to 1.4 or below, it was necessary to reinoculate the soil and raise the pH to a non-inhibitory level to obtain CO₂ evolution. The addition of clay minerals, particularly montmorillonite, mitigated the toxic effect of H₂SO₄, especially at pH values below 3. The growth of Aspergillus niger, A. flavipes, Trichoderma viride and Penicillium brefeldianum was reduced or completely inhibited in soils acidified below pH 3.5. The addition of montmorillonite enhanced fungal growth under these acidic conditions, but kaolinite had no effect.