Acidity characteristics of soluble organic substances in spruce-fir forest floor leachates

Dissolved organic carbon (DOC) leached from the forest floor is an important source of acidity in forest soils. The study of DOC and its acidic properties has been particularly difficult because of uncertainties about the use of isolated fractions of DOC. To evaluate the acidic properties of soluble organic substances in the forest floor we collected and examined leachates from a spruce-fir Spodosol in east-central Maine. Filtered, lyophilized, bulk leachates and DOC fractions were analyzed by potentiometric titration in constant ionic media (0.01 M KCl) to determine the charge density, exchange acidity, and apparent acidity constants (pKa) of each organic material. Based on the titration data, hydrophilic acids had the greatest content of acidic functional groups (e.g., carboxylic acid groups). Bulk leachates, hydrophobic acids, and hydrophilic acids had mean (± SE) exchange acidities of 0.131 ± 0.002, 0.127 ± 0.002, and 0.198 ± 0.003 µeq µmol C, respectively, and pKas of 3.71 ± 0.05, 4.10 ± 0.03, and 3.70 ± 0.03. From the titration of bulk leachates, we calculated organic anions contributions of 267 to 858 µeq L. An organic charge contribution model was also used to evaluate the titration data, along with DOC fractionation percentages and total DOC in the forest floor leachates. The acidic properties calculated from this model and those of the bulk leachate samples agreed closely (r = 0.93), suggesting that acidic properties of DOC were not altered by isolation procedures. Forest floor organic anion contribution to zero-tension lysimeter solutions was also estimated by three methods (a charge contribution model, anion deficit calculations, and chemical equilibrium model simulations) that yielded similar results. For the best estimation of organic anions in soil solutions, empirically derived constants from large samples of bulk leachates (isolation not needed) and use of a chemical equilibrium model are recommended.