CHEMICAL-CHANGES IN DECOMPOSING FOREST LITTER IN RESPONSE TO ATMOSPHERIC SULFUR-DIOXIDE

Decomposing needle and leaf litter, from a pine (Pinus sylvestris L.) stand and a mixed deciduous woodland, respectively, were exposed to arithmetic mean SO2 concentrations of up to 50 nl l-1 (mm3 m-3) in controlled field-based experiments lasting up to 215 d. The objectives of the study were: (1) to evaluate whether SO2 concentrations, known to occur in parts of Europe, could alter the chemical composition of forest litter and leachates, and (2) to use such information to complement results obtained during microbiological studies (Wookey et al., 1991).

Dry deposition of SO2 on the litter resulted in the production of sulphate during damp conditions or when the litter was moist. The formation of SO4(2-) and associated H+ ions led to cation exchange processes whereby Mg2+ and Ca2+ in particular were leached from the litter. This resulted in significant (P <0.01) depletion of magnesium and calcium in both litter types, even at the lowest SO2 concentrations tested (15 nl l-1). Incomplete buffering by base cations resulted in acidification of leachates.

The magnitude of cation leaching and depletion within the litter was not always linearly related to SO2 or leachate SO4(2-) concentrations, and the role of microbial processes and litter quality as modifiers of such responses is considered.