Long-term multifactorial climate change impacts on mesofaunal biomass and nitrogen content

• Mite biomasses and N content increased after 8 years of elevated CO2 treatment.
• Enchytraeid N content decreased at elevated CO2.
• Mesofauna did not adapt to drought after 8 years of repeated drought exposure.
• Warming exacerbated drought effects.
• Collembola and Enchytraeida were more drought-sensitive than mites.

Rising atmospheric CO2 concentration accompanied by temperature increases and altered precipitation patterns calls for assessment of long-term effects of these climatic changes on soil organisms that are essential for ecosystem functioning.In a long-term, full-factorial climate change field experiment, with factors elevated atmospheric CO2 concentration, warming and prolonged summer drought, we assessed the responses of Collembola, oribatid and mesostigmatic mites and enchytraeid worms after 8 years of treatment. Both the biomasses and N content of oribatid and mesostigmatic mites increased at elevated CO2, or tended do so. In contrast, enchytraeid N content decreased at elevated CO2. Soil microbial biomass N pool and litter C:N ratio also increased with elevated CO2, which suggests that mite biomasses are more coupled to microbial biomass, whereas enchytraeid biomass to a larger extent is governed by litter nitrogen concentration, i.e. litter quality. Structural equation modelling confirmed the positive coupling between soil microbial N content and oribatid biomass and further between oribatid and mesostigmatic biomass. The SEM also revealed a negative relationship between microbial N content and enchytraeid biomass.The biomass of all mesofaunal groups was reduced by spring drought, especially when combined with warming. Enchytraeid and especially collembolan biomass suffered greater drought declines than mite biomasses.We conclude that under long-term elevated CO2 exposure, energy and elements will to a larger extent pass through decomposer organisms such as oribatid mites, which are based on food sources with relatively high nitrogen content.After eight years of repeated spring drought events, soil mesofauna did not show signs of adaptation to acute stress effects imposed by drought. However, Collembola and enchytraeids were more drought-sensitive than mites, and although the soil temperature increase in warmed treatments was very modest, warming exacerbated the drying of soil and thus also the negative drought impact on soil mesofauna.