Earthworms promote greater richness and abundance in the emergence of plant species across a grassland-forest ecotone

Aims Chalk grasslands are subject to vegetation dynamics that range from species-rich open grasslands to tall and encroached grasslands,and woods and forests.In grasslands,earthworms impact plant communities and ecosystem functioning through the modification of soil physical,chemical and microbiological properties,but also through their selective ingestion and vertical transportation of seeds from the soil seed bank.Laboratory experiments showed that seed-earthworm interactions are species specific,but little is known on the impact of seed-earthworm interactions in the field.The overall aim of this study was to better understand seed-earthworm interactions and their impact on the plant community.First we analyzed the composition of seedlings emerging from casts after earthworm ingestion.Then we compared seedling composition in casts to the plant composition of emerging seedlings from the soil and of the aboveground vegetation along four stages of the secondary succession of chalk grasslands.Methods Four stages of the secondary succession of a chalk grassland—from open sward to woods—were sampled in Upper Normandy,France,in February 2010.Within each successional stage(×3 replicates),we sampled the standing vegetation,soil seed bank at three soil depths(0-2,2-5 and 5-10 cm)and earthworm surface casts along transects.Soil and cast samples were water sieved before samples were spread onto trays and placed into a greenhouse.Emerging seedlings were counted and identified.Effect of successional stage and origin of samples on mean and variability of abundance and species richness of seedlings emerging from casts and soil seed banks were analyzed.Plant compositions were compared between all sample types.We used generalized mixed-effect models and a distance-based redundancy multivariate analysis.Important Findings Seedling abundance was always higher in earthworm casts than in the soil seed bank and increased up to 5-fold,4-fold and 3.5-fold,respectively,in the tall grassland,woods and encroached grassland compared to the soil surface layer.Species richness was also higher in earthworm casts than in the soil seed bank in all successional stages,with a 4-fold increase in the encroached grassland.The plant composition of the standing vegetation was more similar to that of seedlings from casts than to that of seedlings from the soil seed bank.Seedlings diversity emerging from casts in the tall and encroached grasslands tended toward the diversity found in woods.Our results indicate that earthworms may promote the emergence of seedlings.We also suggest that the loss of some plant species in the seed bank and the tall grass vegetation in intermediary successional stages modify the local conditions and prevent the further establishment of early-successional plant species.

Impacts of wetting-drying cycles on short-term carbon and nitrogen dynamics in Amynthas earthworm casts

Effects of earthworm casts on soil nutrient dynamics and their responses to changing moisture availability in subtropical ecosystems remain poorly understood.This study aimed to examine short-term carbon(C)and nitrogen(N)dynamics and their interactions with wetting-drying cycles in three different structural forms(i.e.,granular,globular,and heap-like)of Amynthas earthworm casts.The rates of C and N mineralization in the earthworm casts were examined under two different wetting-drying cycles(i.e.,2-d and 4-d wetting intervals)using a rainfall simulation experiment.After three simulated rainfall events,subsamples of the earthworm casts were further incubated for 4 d for the determination of CO2 and N2O fluxes.The results of this study indicated that the impacts of wetting-drying cycles on the short-term C and N dynamics were highly variable among the three cast forms,but wetting-drying cycles significantly reduced the cumulative CO2 and N2O fluxes by 62%-83%and 57%-85%,respectively,when compared to the control without being subjected to any rainfall events.The C mineralization rates in different cast forms were affected by the amount of organic substrates and N content in casts,which were associated with the food preference and selection of earthworms.Meanwhile,the cumulative N2O fluxes did not differ among the three cast forms.Repeated wetting and drying of casts not only enhanced aggregate stability by promoting bonds between the cast particles,but also inhibited microbial survival and growth during the prolonged drying period,which together hindered decomposition and denitrification.Our findings demonstrated that the interactions between the structural forms,aggregate dynamics,and C and N cycling in the earthworm casts were highly complex.