RATIONALE: The efficiency of extraction procedures for the determination of organic compounds in soil may be affected by the presence of the mineral phase. Our aim was to analyse the magnitude of such an effect on both total polysaccharide content and C-13-isotopic signature of the polysaccharides. METHODSAfter acid hydrolysis of C-13-labelled wheat, soil and a mixture of these, sugars were quantified and analysed isotopically. Measured values were compared with theoretical contents. RESULTSNo matrix effect was apparent for total sugar-C content of the mixture. However, a matrix effect was observed for the contribution of C-13-labelled wheat sugars. For the soil+plant mixture C-13-labelled wheat sugar contribution was overestimated. Soil-derived sugar-C contribution to the mixture was underestimated. CONCLUSIONSStudies using stable isotopes to follow the fate of added plant-derived compounds in soil need to take into account matrix effects. Further studies have to elaborate on correction procedures and/or the development of extraction procedures to overcome the influence of matrix effects and/or acid hydrolysis extraction on sugar-C contents.