In the northern parts of the Frankenwald, a mountain area in Northeast Bavaria, soils are acidified mainly due to deposition of atmospheric acids. Typically, spring water in this area does reflect the low pH and high sulfate and aluminium concentrations of the soil solution. However, some forested watersheds covered by acidic soils are drained by springs with near neutral solution pH and increased alkalinity. Because such springs are located within peat bogs, where anaerobic processes take place, one hypothesis would be that internal processes in these bogs might neutralize the acidic inputs from the soils. Sulfate and Fe(III)-reduction and subsequent accumulation of FeS or FeS2 are assumed to generate alkalinity and subsequently neutralize soil solution acidity. Until now, only little attention was paid to redox reactions as relevant processes in forest ecosystems.
It is the object of this study to identify the alkalinity generating processes and to quantify the degree of acidity neutralisation within a peat bog in an acidified small catchment (<0,06 km2) using a mass balance approach. Therefore the amount and chemistry of precipitation, throughfall, soil water and runoff was monitored weekly from June 1994 to November 1995. Suction cups were installed at different depths along a catena from the forest soils to the spring peat bog to analyse the soil pore water. Additionally, sulfate reduction rates were determined using the 35SO4-radio-labelled isotope technique and d34S values were measured in different compartments of the system.
Alkalinity was low or almost zero in the upper forest soil and increased up to about 250 µmol/l within the peat bog. Despite the high alkalinity in the peat bog, alkalinity of the spring water was always less than 50 µmol/l. However, an alkalinity mass balance for the whole catchment, comparing total deposition and runoff, shows an alkalinity gain of about 1100 mol.ha-1a-1.
In this presentation we will discuss some hypothesis and models about reaction pathways which could be responsible for the observed net alkalinity gain within the system.