Acidic properties of dissolved organic matter leached from organic layers in temperate forests

The production of organic acids in forest organic layers is one of the major acid producing processes. In the organic layer leachates collected in Central Japan, the negative charge deficit between cations and inorganic anions ranged from 10 to 92% of the total positive charge of cations. The anion deficit was well correlated with the concentration of dissolved organic carbon (DOC). Carboxyl group contents in DOC and their pKa values were statistically optimized so that the anion deficit could be evaluated by the measurement of the DOC concentration and pH value. Optimized values for carboxylic group contents were in therange of 4.9 to 5.9 mu-molc mg(-1) C, which were lower than the amounts determined by titration, with the average values ranging from 5.7 to 7.8 mu-molc mg-1 C. The optimized values were rather close to those with pKa less than 3.2 determined by the titration. The dissolved organic matter was fractionated into four groups according to hydrophobicity and acid-base properties. Hydrophobic acid fraction was the largest and accounted for 52 to 71% of the whole DOM. It contained about 10 mu-molc mg(-1) C of carboxylic groups, and 68+-9% of them showed a pKa value of less than 3.2. Such a low pKa value of carboxylic groups is considered to have been caused by paired carboxylic groups with much higher pKa values, and hydroxyl groups. Hence the carboxylic groups were considered to exhibit a large range of pKa values. The carboxylic groups which were actually dissociated in the analyzed leachates were considered to be those with low pKa values. The estimation assuming a single pKa value did not give reliable results, and was biased in that it reflected the properties of the low pKa carboxylic groups. The proportion of the hydrophilic acid fraction was lower than that reported by other authors in cooler regions