The influence of plant litter-compost of the hot rotten-phase as additional inoculum for anaerobic batch digestion of sugar beet silage (SBS) was studied. Four simultaneously driven batch-fermenters were inoculated with sewage sludge. Two of the fermenters were inoculated additionally with the same amount of organics by compost of the hot rotten-phase. Two of the fermenters were mesophilic (40 degrees C) and the other two were thermophilic (60 degrees C). The impact on the gas production rate and gas yield was observed to be boosted for thermophilic (60 degrees C) and only a minor effect of 6-13% for mesophilic (40 degrees C) digestion. The gas yield increased considerably up to 26.5% at 60 degrees C (batch). Also the methane content increased from 57.4% to 62.3% by adding compost (continuously run mesophilic digestion). Fluorescence In Situ Hybridization (FISH) indicated that a microbial effect was responsible for the observed stimulation of gas production rates, but not simply by increasing the bacterial counts. By analysing each fermenter for its mineral and trace element content a mineralic effect could be excluded. However, the bacterial counts by FISH of 10 different groups were somewhat ambiguous. But an effect on the presence of Chloroflexi could be demonstrated. They nearly doubled to 15-16% by supplementation with compost. Furthermore, under thermophilic conditions, the added compost induced a significant shift in the microbial composition towards hydrogenotrophic Methanobacteriales. The suggestive conclusion drawn is that this explicitly increase in hydrogenotrophic activity could alone or in combination with accompanying fermentative bacteria forces the microbial food chain towards stimulation of methane generation. (C) 2013 Elsevier Ltd. All rights reserved.