Reciprocal interactions between roots and earthworms in the presence of vermicompost and bokashi

Recycling of biowaste from the food industry as soil amendment is a solution to compensate the overall decline of organic matter in agricultural soils. Some low-cost and low-tech recycling methods, such as vermicomposting and bokashi (lactic fermentation) are currently understudied in Europe. When applied to the soil their potential advantages are two-fold: increasing plant growth (fertilizing effect) and promoting soil biology (amending effect), and these effects can be linked since the organic matter is also transformed into nutrients by the action of earthworms.To characterize and disentangle these effects, we carried out a laboratory experiment using repacked soil cores to test four products (two vermicomposts and two bokashis with different bulking agents) in the presence or absence of plants (winter barley) and earthworms (Aporrectodea nocturna and Allolobophora chlorotica). Products (equivalent to 8 t of organic C per ha) were incorporated into the 10 upper cm and compared to a control without organic matter. After two months, we measured shoot and root biomasses, and quantified root allocation and morphological traits (root mass density, root length diameter, mean root diameter, specific root length). The cores were analyzed by X-ray tomography to characterize the earthworm burrow systems (volume, diameter, and barycenter).There was no difference between the two vermicomposts or the two bokashis, which indicates that the process was more important than the nature of the bulking agent. Compared to the control, only the vermicomposts had a significant effect on the plant, increasing shoot and root biomass by 25% and 33%, respectively, and modifying root traits (root mass density, root length density, mean root diameter). Only the bokashis significantly influenced the earthworm burrowing with a higher burrow volume (+76%) in the shallowest soil layers, where the products were mixed, and a lower barycenter of the burrow system (-17%). In general, earthworm presence significantly increased the shoot biomass (+13%), but decreased the root biomass (-16%). Unexpectedly, we also observed that plant presence significantly decreased the burrow volume (-25%) and the barycenter of the burrow system (-12%).In conclusion, we found that vermicompost and bokashi are both suitable amending products for field application. However, they have distinct properties: vermicompost has higher fertilizing properties, while bokashi is more palatable to soil macrofauna. Furthermore, while our study confirms the positive effect of earthworms on above-ground plant growth, it also reveals an often-overlooked finding: roots reciprocally influence earthworm behavior.