COMBINED USE OF OPEN-AIR AND INDOOR FUMIGATION SYSTEMS TO STUDY EFFECTS OF SO-2 ON LEACHING PROCESSES IN SCOTS PINE LITTER

Both an open-air fumigation system and a laboratory-based system were used to expose decomposing Scots pine (Pinus sylvestris L.) needles to controlled concentrations of SO2 (arithmetric mean less-than-or-equal-to 48 nl litre-1) during a period, in total, of 301 days. The experimental design involved reciprocal litter transplants from 'clean' to 'polluted' air and vice versa, using the two fumigation systems. The objectives were (1) to observe the effects of SO2 on leachate and litter chemistry, (2) to assess whether pollution-induced changes are reversible in clean air, and (3) to test the suitability of small-scale fumigation chambers (litter microcosms) compared with open-air systems in soil studies.Through the formation of SO4(2-) ions, dry-deposited SO2 exhibited a marked capacity to remove 'base' cations (Ca2+, Mg2+ and K+) from decomposing pine needles, and also to acidify litter leachates (as indicated by proton fluxes from the litter). When litter was transferred from polluted air (48 nl litre-1 SO2, in the open-air system) to either clean or polluted air in the laboratory, the effects of prior exposure to SO2 on leachate composition were still evident even after 86 days: the role of base cation depletion within the litter, caused by SO42- -induced leaching, is discussed.Data for SO42- fluxes in leachates collected from the small-scale chambers indicated that dry deposition velocities for SO2 were not anomalously high within this fumigation system. It is therefore concluded that microcosm studies can provide information complementary to the open-air fumigation approach in soils research.</p

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 &le;0.050 &mu;l 1-1 in controlled field-based experiments lasting up to 215 days. Fungal cultures, isolated from the pine litter, were also fumigated with &le;0.053 &mu;l 1-1 SO2 in laboratory-based studies. Results showed that arithmetic mean concentrations of SO2 as low as 0.015 &mu;l 1-1 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 cladosporioides (Fres.) de Vries and Coniothyrium olivaceum Bonord, isolated from the litter, were shown to be sensitive to SO2 concentrations of &le;0.053 &mu;l 1t-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. &copy; 1991