Effects of earthworms and mycorrhizal fungi on the growth of the medicinal herb Calendula officinalis (Asteraceae)

Both earthworms and symbiotic arbuscular mycorrhizal fungi (AMF) often co-occurr in ecosystems, however very little is known on their interrelationships. Here we tested to what extent earthworms (Annelida) or AMF (Glomales) separately or in combination affect the growth of the pharmaceutical plant species, pot marigold (Calendula officinalis, Asteraceae). We conducted a greenhouse experiment using non-sterilized field soil where we manipulated the factors earthworms (addition/no addition of the vertical burrowing species Lumbricus terrestris) and AMF (addition/no addition of a mix of the four Glomus taxa G. geosporum, G. mosseae, G. intraradices, G. claroideum). Leaf length and flower stem length was significantly increased by earthworms but remained unaffected by additional AMF. The longest leaves and flower stems were observed in pots containing earthworms but no additional AMF. The number of flower buds was unaffected by earthworms but marginally significantly increased by AMF. Plant shoot biomass production was significantly higher when earthworms were present; AMF inoculation had no effect on biomass production. Root biomass production and total plant biomass production remained unaffected by earthworms or AMF. These results indicate that in soil already containing AMF earthworm addition primarily affects vegetative growth while additional AMF inoculation tended to affect reproductive plant parts.