Ageing effects of casts of Aporrectodea caliginosa on soil microbial community structure and activity

We studied the ageing of casts of the earthworm species Aporrectodea caliginosa in the field and their effects on soil microbial community structure and activity in a grassland soil. For this purpose, we sampled casts and the bulk soil (below and surrounding casts), 1 and 2 months after deposition of the casts. Discriminant and cluster analyses separated casts from soil samples, and revealed significant differences in microbial community structure in aged casts. There were no differences between soil below casts and surrounding soil samples. The overall microbial biomass and microbial activity were higher in casts than in soils. The same was true for the main groups of soil microbial communities, bacteria and fungi. Ageing of casts did not decrease the overall microbial biomass, and there was an unexpected increase in fungal biomass; however, ageing clearly decreased microbial activity. The effects of A. caliginosa on soil microbial communities in the grassland were strong, and contrary to our expectations, persisted over time. In fact, our data indicates that casts seem to function as isolated hot spots of microbial biomass and activity, with minimal interactions with soil.