EFFECTS OF ATMOSPHERIC SULFUR-DIOXIDE ON MICROBIAL ACTIVITY IN DECOMPOSING FOREST LITTER

At extremely high concentrations, relative to those occurring in the atmosphere, sulphur dioxide (SO2) has known antimicrobial properties. There is also circumstantial evidence, based on field surveys, to indicate that the occurrence and activities of a number of phylloplane fungi and soil microorganisms are correlated with atmospheric concentrations of SO2 occurring in parts of Europe and North America. The results of these studies need to be corroborated by controlled fumigation experiments applying realistic concentrations of SO2. Unfortunately such experiments have been rare.

The suggestion that SO2 may be affecting soil microorganisms merits serious consideration because of the fundamental role of these organisms in maintaining soil fertility, especially in forests. Events in the forest litter layer are considered to be particularly important because it forms an interface between the atmosphere and the soil system.

The research described in this paper involved exposing leaf litter (from a Pinus sylvestris L. stand and a mixed deciduous woodland) to arithmetic mean concentrations of SO2 of less-than-or-equal-to 0.050-mu-l l-1 in controlled field-based experiments lasting up to 215 days. Fungal cultures, isolated from the pine litter, were also fumigated with less-than-or-equal-to 0.053-mu-l l-1 SO2 in laboratory-based studies.

Results showed that arithmetic mean concentrations of SO2 as low as 0.015-mu-l l-1 significantly reduced microbial activity (respiration) in both pine and deciduous litter in the open-air fumigation experiment. Results should also be interpreted in relation to the peak SO2 concentrations (often considerably higher than arithmetic means) to which the litter was exposed. Pure cultures of Cladosporium cladosporiodes (Fres.) de Vries and Coniothyrium olivaceum Bonord, isolated from the litter, were shown to be sensitive to SO2 concentrations of less-than-or-equal-to 0.053-mu-l l-1 in laboratory-based fumigations.

It is concluded that the dry deposition of SO2 to forest soils may have important implications for nutrient cycling processes and therefore forest productivity and community structure.