PROTON BUFFERING IN ORGANIC HORIZONS OF ACID FOREST SOILS

We studied the reactions of proton consumption in the organic O(i) and O(a + e) horizons of strongly acid forest soils (Dystrochrepts) on granite, phyllite and basalt in NE Bavaria, where spruce shows various degrees of decline. To this purpose we applied pH-stat titrations, batch and percolation experiments. In pH 3 stat titrations, the O horizons of the granitic and phyllitic soils reached an end-point of proton consumption within about 20 min. Only the basaltic soil showed a minor and slower second part of the reaction, which could be fitted with a pseudo-first-order equation. The total acid neutralizing capacity (ANC) of the soils was between 10 and 100 mmol(c) kg-1. The amount of protons consumed is fully accounted for by the amount of Ca2+, Mg2+ and Al3+ released into solution. The apparent average acid strength of an O(a) material obtained from potentiometric titration was pK(a) = 4.5 in the H form and pK(a) 5.0 in the Al form. Batch titration (300 days, pH 3) and 10(-3) M H2SO4 percolation experiments showed two additional H+-consuming processes: (1) reductive formation of Fe2+ and Mn2+ under limited O2 supply by which 2 mol H+ mol-1 metal were consumed; and (2) decomposition of water-soluble organic salts (releasing mainly Ca2+) and ammonification (releasing NH4+). This shows that the organic horizons of acid soils are an important barrier against acidification of underlying mineral horizons by acid rain.