Increased solubility of (heavy) metals in soil during microbial transformations of sucrose and casein amendments

Unsterile soil samples pH 6.8 were amended with 10% sucrose or 0.1% NH4NO3/2% casein to influence solubility of metal cations. Microbial formation of aliphatic carboxylic acids reduced pH of sucrose soil to 4.3 and increased the solubility of heavy metals by 60-fold due to the increased presence of H+, and the metal-chelating and humic-molecule fragmenting properties of carboxylic acids. Soil pH increased finally to 7.8 upon the degradation of the carboxylic acids. In casein soil, ammonia production increased pH initially to 8.5 and promoted solubility of metal-containing humic compounds. Subsequent nitrification reduced pH to 5.6 to increase concentations of Ca and Mg in the soil solution, which prevented a comparable increase in the solubility of heavy metals competitively. It is concluded that amendment with sugar and nitrogen occasionally increases the concentrations of (hazardous) elements in the soil solution to facilitate their uptake by metal hyperaccumulating plants.