Responses of enchytraeids to increased temperature, drought and atmospheric CO2: Results of an eight-year field experiment in dry heathland

•Effects of climate change on enchytraeids were studied in a long-term experiment.•CO2 fumigation increased enchytraeid populations after 1–2 years, but not after 7–8 years.•Abundance of enchytraeids was highly influenced by soil water content.•Enchytraeid species richness was reduced by low soil water content.

In a long-term field trial we investigated the responses of enchytraeids to simulated future climatic conditions predicted for Denmark. At a semi-natural Danish heathland site we exposed 9.1 m2 plots to elevated atmospheric CO2 concentration (510 ppm), extended summer drought and passive night-time warming. Treatments and all possible combinations of treatments were replicated 6 times. The enchytraeid community was assessed after 8 years of treatment. Atmospheric CO2 did not have any significant effect on the enchytraeid community even though root biomass was increased in plots with elevated CO2. The warming treatment had a modest effect on soil temperatures (0.3 °C at 5 cm depth) and did not have significant effect on abundance or biovolume of enchytraeids. However, the individual body size of Chamaedrilus chlorophilus (= Cognettia sphagnetorum partim.) was negatively correlated with soil temperature in spring 2013, perhaps indicating that warming stimulates fragmentation (reproduction) rates at this time of the year. Increased drought in May–June 2012 did not have lasting effects on abundance or biomass 3 months after the termination of drought treatment. However, comparison with earlier assessments of enchytraeids in the CLIMAITE experiment shows that the severity of drought and the time elapsed since the last drought is the best predictor of the biovolume (or biomass) of enchytraeids. Moreover, species richness was significantly impacted by the average soil water content experienced by enchytraeids during the 8-year study. It seems, therefore, that the most important factor for enchytraeid abundance and species diversity in the projected future climate conditions is soil water content.