The flows of nitrogen, bacteria and viruses from the soil to water compartments are influenced by earthworm activity and organic fertilization (compost vs. vermicompost)

The amendment of vermicompost is a management practice that may contribute to sustainable agroecosystems by making them less dependent on inorganic fertilizers. However, little is known about the impact of this practice on soil biota and the flow of microbes to the water system. Using a 30-days laboratory experiment, we investigated the development of the peregrine earthworm species, Dichogaster bolaui, in presence of compost or vermicompost, and assessed its impact on the flow of bacteria and viruses to the water system. The dynamics of soil bacterial diversity (assessed by DGGE) and concentration in water together with their viral parasites were also assessed through an incubation of solution during 5 days (comparison between T0 and T5). tudy highlights the rapid development of D. bolaui after compost amendment. However, the low quality of vermicompost and the absence of organic amendment in the control treatment allowed the survival but not the development of D. bolaui. Higher bacterial and viral abundances in compost and vermicompost substrates led to more important transfer of these communities from the soil to the water system in comparison with the untreated soil, but no difference was observed between compost and vermicompost treatments. In terms of abundance, the bacterial to virus ratio was rather stable in the soil solution but no such a relation was observed in the soil. A reduction of bacterial diversity (OTU) was measured at the end of the incubation period for all the treatments. However, higher number of OTU at T5 for the compost treatment suggested a better adaptation and/or resistance of soil bacteria to the aquatic system, in comparison with the control treatment. Vermicompost treatment led to intermediate conclusions. The presence of D. bolaui significantly reduced bacterial abundance in the soil organic layer (both compost and vermicompost treatments) but it did not influence bacterial and viral abundance in water, suggesting independent processes. Earthworms buffered bacterial DGGE patterns after five days of incubation, probably through a facilitation of soil bacterial groups more able to resist in solution.