Carbon Compounds Differ in Their Effects on Soil pH and Microbial Respiration

The mechanisms of soil pH change after the addition of organic matter to soil are not fully understood. The aim of this study was to investigate changes in pH after addition of carbon compounds over a 60-d incubation period. Seven organic compounds commonly found in plant residues (acetic acid, malic acid, citric acid, benzoic acid, ferulic acid, glucosamine hydrochloride and glucose) were selected based on the number and type of functional groups, and added at 0.5 mg C·g

−1

soil to two soils differing in initial pH. Addition of organic acids (R-COOH) immediately decreased pH. The magnitude of the pH decrease depended on dissociation constant of the acid and the initial soil pH. In subsequent incubation, pH was slowly returned to original levels as organic anions were mineralized, consuming H

+

ions. Glucose which contains hydroxyl (R-OH) group did not alter soil pH. However, carboxyl (R-COOH) and amine (R-NH

2

) groups changed pH significantly. Soil respiration was also increased by the addition of C compounds. Cumulative respiration was higher in soil with malic acid, citric acid, ferulic acid and glucose than with other compounds. The addition of glucose, citric acid and malic acid resulted in priming as cumulative respiration was greater than the actual amount of C added.