Functional attributes: Compacting vs decompacting earthworms and influence on soil structure

Abstract A short term field mesocosm experiment was performed in semi-deciduous forest areas of Ivory Coast to assess the impact of a decompacting (Hyperiodrilus africanus, Eudrilidae) and two compacting (Millsonia omodeoi and Dichogaster terraenigrae, Acanthodrilidae) earthworm species on soil properties. These species have been selected for their predominance in the region and their contrasting impact on soil structure. The experimental design consisted of a treatment without worms (control), and treatments with one, two or three species of earthworms. Both compacting and decompacting earthworms increased water infiltration rate in all treatments, with marked impact in H. africanus and M. omodeoi+D. terraenigrae treatments. Interactions between compacting and decompacting species resulted in more large aggregates in comparison to when the compacting species D. terraenigrae was alone. This may be accounted for by their compacting attribute as compacting earthworms are responsible for producing the highest number of large aggregates. The low values of mean weight diameter in treatments combining decompacting and compacting earthworms compared with compacting “M. omodeoi” one also confirmed the trend of decline in soil compaction in the presence of the decompacting species. These results showed positive impact of species richness on soil structure regulation, which is crucial in ecosystem productivity and support consequently the insurance hypothesis. In fact, this study showed that the preservation of earthworm species belonging to these two contrasting functional groups is essential for the maintenance of stable soil structure regulation in agro-tropical ecosystems.