Abstract Increases in UV-B radiation have been shown to slow the rate of litter decomposition in ecosystems. However, it is unclear if this is a result of direct UV-B effects on saprobic microorganisms, or a result of UV-B-induced changes in litter quality that indirectly affect decay by saprobes. In this study, we evaluated the magnitude of direct and indirect effects on litter decomposition of Brassica napus by soil fungi, under growth chamber conditions. We found that, both, direct and indirect UV-B negatively influenced litter decomposition, however, direct effects were much more pronounced. We then tested whether UV-B radiation would have species-specific effects on fungal colonization and competitive ability, rather than influencing all fungal species equally. We predicted that darkly pigmented fungi would increase their relative competitive ability under high UV-B. The test fungi were all isolated from field soil under Brassica napus. Two fungi were hyaline (Aspergillus terreus, Trichoderma koningii), two were darkly-pigmented (Cladosporium sphaerospermum, Epicoccum purpurascens) and one had a hyaline mycelium but darkly-pigmented conidia (Aspergillus niger). Elevated UV-B radiation had differential direct and indirect effects on fungal growth, and caused shifts in the competitive balances between pigmented and non-pigmented fungi. However, in only two of six pair-wise challenges did the pigmented species increase their relative competitive ability under UV-B conditions. It is clear that UV-B profoundly influence fungal community structure in soil, but the direction of such effects remains unpredictable
