Microbial biomass, biovolume and respiration in Lumbricus terrestris L. cast material of different age

Effects of the gut passage, the age of cast material and the type of ingested substrate on the microbial community in Lumbricus terrestris faeces were studied in laboratory microcosms containing four soil–litter combinations: lime forest soil+lime litter, beech forest soil+lime litter, beech forest soil+beech litter and beech forest soil without litter. Microbial biomass (SIR method), basal respiration and biovolume of bacteria and fungi were measured in earthworm casts and reference material after 1, 5, 10 and 100 d of incubation. To separate effects of mixing of litter and soil in the gut of L. terrestris from specific gut or cast associated processes, actual faecal properties were compared with ‘expected’ values, calculated on the basis of corresponding measurements in soil and litter ingested by L. terrestris. Microbial respiration, biomass and fungal volume in fresh (1 d old) faeces strongly depended on the type of soil and litter material consumed by L. terrestris. The comparison with ‘expected’ values indicated that microbial biomass and volume changed little during the passage through the gut of L. terrestris. In contrast, even a short incubation of casts caused marked changes in the microbial properties studied: microbial biomass sharply declined during the first 5 d of incubation; basal respiration which exceeded ‘expected’ values by 30–120% in fresh casts, decreased steadily and was significantly below ‘expected’ values by d 10 of the incubation; biovolume of bacteria and fungi in fresh casts was 20–60% higher than expected and then steadily declined in lime litter but not in beech litter treatments. Generally, the fungal-to-bacterial volume ratio in cast material was not significantly affected by the gut passage nor by the incubation of casts.