Soil pH increase due to biological decarboxylation of organic anions

Organic matter incorporated into soils may influence soil pH. Until now the detailed processes responsible for a change in soil pH after the incorporation of organic matter were not completely understood. We studied the effects of organic anions, glycine as a representative of amino acids, and glucose as a representative of carbohydrates on soil pH in aerobic incubation experiments. Addition of malate and citrate resulted in an immediate soil pH increase associated with additional CO2 release. Addition of acetate had no major effect on soil pH but if applied at higher rates decreased soil CO2 release. Malate added together with ammonium resulted in an immediate soil pH increase, which was followed by a pH decrease associated by an increase in nitrate after 10 days. Also glycine gave an immediate pH increase which was associated with an equivalent decrease of soil organic N extractable with CaCl2 solution. Addition of glucose gave a slight pH decrease in the first days of incubation followed by an increase to the initial pH value. The results are interpreted in terms of decarboxylation of organic anions, a process which requires one proton per carboxylic group decarboxylated. Under aerobic soil conditions decarboxylation is a major process in organic matter decomposition. The decomposition of carbohydrates in the glycolytic pathway produces carboxylic groups which, after dissociation, may decrease soil pH. As soon as these groups are decarboxylated in the citrate cycle an equivalent amount of protons is required inducing a rise in soil pH. For this reason addition of glucose brought about only a transient soil pH decrease.