An incubation experiment on straw decomposition was carried out with soils from a long-term field trial at Therwil, Switzerland. Two conventional agricultural systems, one with (CONFYM) and one without manure, an organic system managed according to bio-dynamic farming practice (BIODYN) and an unfertilized control were compared. CONFYM received stacked manure and an additional mineral fertilizer. BIODYN received composted farmyard manure and no mineral fertilizers. Both systems received the same amount of manure based on 1.4 livestock units haˇ1. The aim of the investigation was to explain the large differences in soil microbial biomass and activity between the systems, especially between the manured soils. Di€erences in microbial C-utilization effciency were suggested to be the main reason. We followed the decomposition of 14C-labeled plant material over a period of 177 days under controled incubation conditions. Prior to incubation, microbial biomass was 75% higher and qCO2 up to 43% lower in the BIODYN soil than in the conventional soils. At the end of the incubation period, 58% of the applied plant material was mineralized to CO2 in the BIODYN soil compared to 50% in the other soils. This difference became signifcant 2 weeks after application of plant material and is suggested to be due to decomposition of more recalcitrant compounds. After addition of plant material, the increase of microbial biomass in the unmanured systems was higher than in the manured systems, but with a higher loss rate thereafter. The amount of 14C incorporated into Cmic as related to 14CO2 evolved was markedly higher in the BIODYN soil. The results support the hypothesis that agricultural measures applied to the BIODYN system invoke a higher efciency of the soil microbial community with respect to substrate use for growth.