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Fine roots (<1 mm dia.) of Dryas octopetala L., Carer rupestris All., Saxifraga oppositifolia L. and Salix polaris Wahlenb. were decomposed as single species, and in mixtures with fine roots of D. octopetala, under laboratory conditions. Decomposition was measured as carbon dioxide release and mass loss over 556 d at 6 degrees C. There was no relationship between cumulative CO2 release and mass loss, suggesting that leaching and loss as fine particulate matter are important in fine root decomposition. In single species comparisons of CO2 release, roots of C. rupestris decomposed to the greatest extent, perhaps because of their relatively high resource quality, although S. polaris roots showed greatest decay in comparisons using total mass loss. Differences between expected and measured values of root decomposition were evaluated, based on a comparison between the results from the single and mixed species. In C. rupestris + D. octopetala and S. oppositifolia + D. octopetala combinations, there was evidence of ’negative’ interactions which decreased the rate of CO2 release below that predicted. There were also ’positive’ interactions which increased the percentage loss of root mass in S. oppositifolia + D. octopetala and S. polaris + D. octopetala pairings. The results show that the decomposition of resources in mixtures cannot be predicted readily from the characteristics of the component resources decomposing in isolation. (C) 1999 Elsevier Science Ltd. All rights reserved.