Earthworm inoculation in degraded soils: A meta-analysis

Earthworms are key soil organisms involved in several soil functions. Earthworm inoculation can be used in the context of the agroecological transition and of Nature-based solutions to produce food while restoring soil fertility. In this meta-analysis we wanted to identify the earthworm inoculation parameters (density of the inoculated earthworms, time between inoculation and sampling, presence/absence of physical barriers, and number of species inoculated) that lead to a successful intervention (earthworm abundance and biomass increase).

For this quantitative analysis of earthworm inoculation success, we exhaustively reviewed the literature using the ISI Web of Knowledge with the “All Databases” option. We selected articles on field experiments that compared earthworm abundance or biomass in the inoculated area and in a clearly defined control area (same sampling period). We excluded articles on laboratory studies, vermicomposting, and non-intentional earthworm introduction.

Based on the 25 studies included in the corpus, the meta-analysis showed an increase in the abundance of inoculated earthworm species (4.48-fold) and of the whole earthworm community (1.55-fold). When >150 individuals m⁻² were inoculated, the abundance of the inoculated earthworm species increased by a factor of 48.0 compared with the control plot. By monitoring the efficiency after at least 2 years, the increase in inoculated species abundance reached a factor of 37.0. Conversely, in short-term studies (<1 year), this increase factor was not significant and was reduced to 1.9. Multi-species inoculations (i.e., different ecological categories of earthworms) could be more efficient (inoculated species abundance and total community biomass increases by a factor of 30.3 and 3.6, respectively). In a soil restoration perspective, an efficient inoculation must be designed in function of the targeted soil functions.

To perform a successful inoculation, we recommend to inoculate >150 individuals m⁻², preferably of different species, and to assess their establishment after a relatively long interval (>2 years after inoculation).