Mineralization and microbial assimilation of C-14-labeled straw in soils of organic and conventional agricultural systems

An incubation experiment on straw decomposition was carried out with soils from a long-term held trial at Therwil, Switzerland. Two conventional agric ultural systems, one with (CONFYM) and one without manure, an organic syste m managed according to bio-dynamic farming practice (BIODYN) and an unferti lized control were compared. CONFYM received stacked manure and an addition al mineral fertilizer. BIODYN received composted farmyard manure and no min eral fertilizers. Both systems received the same amount of manure based on 1.4 livestock units ha(-1). The aim of the investigation was to explain the large differences in soil microbial biomass and activity between the syste ms, especially between the manured soils. Differences in microbial C-utiliz ation efficiency were suggested to be the main reason. We followed the deco mposition of C-14-labeled plant material over a period of 177 days under co ntroled incubation conditions. Prior to incubation, microbial biomass was 7 5% higher and qCO(2) up to 43% lower in the BIODYN soil than in the convent ional 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 o ther soils. This difference became significant 2 weeks after application of plant material and is suggested to be due to decomposition of more recalci trant compounds. After addition of plant material, the increase of microbia l biomass in the unmanured systems was higher than in the manured systems, but with a higher loss rate thereafter. The amount of C-14 incorporated int o C-mic, as related to (CO2)-C-14 evolved was markedly higher in the BIODYN soil. The results support the hypothesis that agricultural measures applie d to the BIODYN system invoke a higher efficiency of the soil microbial com munity with respect to substrate use for growth. (C) 2000 Elsevier Science Ltd. All rights reserved.