Original article C to N ratio strongly affects population structure of Eisenia fetida in vermicomposting systems M. Aira * , F. Monroy, J. Domínguez Departamento de Ecoloxía e Bioloxía Animal, Universidade de Vigo, Vigo 36310, Spain Available online 10 August 2006 Abstract Food quality influences not only the size of earthworm populations but also their growth and reproduction rates. Here we studied the effect of C to N ratio of pig slurry in microbial biomass and activity and in the growth and reproduction of the earthworm Eisenia fetida. We set up a batch of twelve vermireactors, six each for low (11) and high C to N ratio (19) of pig slurry applied; three of each without earthworms (control) and three containing 500 mature earthworms (E. fetida). After 36 weeks C to N ratio significantly affected earthworm numbers (sevenfold greater in high C to N ratio) and population structure. Thus, in the low C to N ratio treatment the population was composed mainly by mature earthworms (60%), with a higher mean weight than in the high C to N treatment. However, in the high C to N ratio treatment, the population was composed mainly by juvenile and hatchling earthworms (70%). A rapid depletion of dissolved organic C (DOC) content was observed in all treatments. Although earthworms produced an increase in microbial biomass and activity in young modules, finally a decrease in older modules was recorded. The decrease in available carbon did not seem to affect the relationships established between earthworms and microflora. © 2006 Elsevier Masson SAS. All rights reserved. Keywords: Vermicomposting; Pig slurry; Earthworms; Eisenia fetida; C to N ratio; Microbial biomass and activity; Resource limitation 1. Introduction Life history of an organism is the result of competi- tion for limited resources in time and energy between growth and reproduction. It has been reported for some vermicomposting earthworms species that different food diets can affect their growth, reproduction or both [3,4], and moreover earthworm’s growth is limited by carbon availability [11]. Therefore, it is possible that earthworms can allocate resources either to growth or reproduction depending on resource limitation and/or quality. This is important in vermicomposting, where high growth and reproduction rates are required in order to accelerate waste breakdown and stabilization. In this work we tested the hypothesis whether C to N ratio of pig manures are affecting growth and reproduc- tion of earthworms strongly; besides we questioned if the changes in earthworm population are also modify- ing the interactions that earthworms and microorgan- isms (microbial biomass and activity) establish during vermicomposting. 2. Materials and methods We used two pig slurries obtained from two different pig breeding farms placed near the University of Vigo, that differed mainly in its C to N ratio, that were 11 and 19 (low and high C to N ratio, respectively). We set up http://france.elsevier.com/direct/ejsobi European Journal of Soil Biology 42 (2006) S127–S131 * Corresponding author. Tel.: +34 986 81 2593; fax: +34 986 81 2556. E-mail address: aira@uvigo.es (M. Aira). 1164-5563/$ - see front matter © 2006 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2006.07.039