ACIDITY CHARACTERISTICS OF SOLUBLE ORGANIC-SUBSTANCES IN SPRUCE-FIR FOREST FLOOR LEACHATES

Dissolved organic carbon (DOC) leached from the forest floor is an imp ortant source of acidity In forest soils. The study of DOC and its aci dic properties has been particularly difficult because of uncertaintie s about the use of isolated fractions of DOG. To evaluate the acidic p roperties of soluble organic substances in time forest floor we collec ted and examined leachates from a spruce-fir Spodosol in east-central Maine. Filtered, lyophilized, bulk leachates and DOC fractions were an alyzed by potentiometric titration in constant ionic media (0.01 M KCl ) to determine the charge density, exchange acidity, and apparent acid ity constants (pK(a)) 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 ac ids, and hydrophilic acids had mean (+/- SE) exchange acidities of 0.1 31 +/- 0.002, 0.127 +/- 0.002, and 0.198 +/- 0.003 mu eq mu mol(-1) C, respectively, and pK(a)s of 3.71 +/- 0.05, 4.10 +/- 0.03, and 3.70 +/ - 0.03. From the titration of bulk leachates, we calculated organic an ions contributions of 267 to 858 mu eq L(-1). An organic charge contri bution model was also used to evaluate the titration data, along with DOC fractionation percentages and total DOC in the forest floor leacha tes. The acidic properties calculated from this model and those of the bulk leachate samples agreed closely (r(2) = 0.93), suggesting that a cidic properties of DOC were not altered by isolation procedures. Fore st floor organic anion contribution to zero-tension lysimeter solution s was also estimated by three methods (a charge contribution model, an ion deficit calculations, and chemical equilibrium model simulations) that yielded similar results. For the best estimation of organic anion s in soil solutions, empirically derived constants from large samples of bulk leachates (isolation not needed) and use of a chemical equilib rium model are recommended.