The quality of organic matter incorporated in the soil spans a diverse range in forest ecosystems, from highly labile dissolved organic carbon to recalcitrant coarse woody debris. At the same time, how variation in carbon quality, rather than quantity, affects ecosystem functioning has been little explored in natural ecosystems. Evidence from laboratory incubations suggest that the lability of carbon substrates can have important effects on soil biotic activity, but demonstrations in field conditions are lacking.  We conducted a manipulative experiment in a southern beech forest in Patagonia, Argentina, examining the effects of carbon additions of sugar (labile) and sawdust (recalcitrant) on soil enzymatic activity and litter decomposition. We hypothesized that soil C additions of would alter soil enzymatic activity with recalcitrant C additions stimulating beta-glucosidase and phenol oxidase activity and labile C additions reducing enzyme activity. Forest floor plots of 1m² were established in mid-summer with the following treatments: 1) control (no addition); 2) sugar; 3) sawdust and 4) sugar+sawdust.  In addition, we placed litterbags of Nothofagus pumilio and twigs in all plots.  We collected soil samples in nine dates during one year, and collected litterbags and twigs at three dates (4, 8, and 12 months).  We found that C additions had marginally significant effects on beta-glucosidase and phenol oxidase activity in the fall sampling (P<0.1), with sawdust stimulating enzyme activity and sugar without a significant effect.  Litter decomposition was significantly different (P<0.05), but in contrasting ways: recalcitrant C additions stimulated decomposition of litter while labile C additions reduced mass loss of twigs. These results suggest that soil microbial communities were affected in both the short- and medium-term by modification of detritus quality, which could have altered their physiological capacities, and this effect had an important consequence for carbon turnover in this forest ecosystem.