Evaluating microarthropod indices to assess the impact of mycorrhizae inoculants across European cropping systems

The use of bacterial and fungal rhizospheric inoculants and their consortia is pivotal for sustainable crop systems when their use allows a reduction of chemical fertilizers and plant protection products, and/or enhance soil biodiversity supporting soil health. Therefore, the application of suitable ecological indices to assess soil health is crucial for monitoring the effects of these inoculants on resident soil communities. In this work we evaluated two inoculants i) endophytic arbuscular mycorrhizal fungi (AMF, n=14) and ii) the AMF with the addition of bio-effectors (i.e., organic products showing biostimulating effects, AMF+be, n=12) in 17 sites across Europe. Three-year treatment effects on soil-dwelling microarthropod communities were compared to untreated control plots (UTC, n=17) on tomato, strawberry, or apple cropping systems, managed with integrated or organic methods. The microarthropod community was classified in biological forms (BFs) by their functional adaptation to the soil life, and diversity of the community was measured using the QBS-ar_BF (biological soil quality based on all microarthropods biological form) index method. Inoculants generally enhanced biodiversity, and apple orchards showed higher frequency of increase compared to other crops. Unexpectedly, the increase rate was less frequent when the bioeffector was added (AMF: 71%, AMF+be: 42%). The treatments were more effective under integrated than organic management. In conclusion, application of bioinoculants induced changes in microarthropod communities, and the QBS-ar_BF index proved to be a useful tool to monitor these changes. In addition, the results suggest that organically managed soils, which have a more complex and stable community, may benefit less from AMF treatment. Nevertheless, AMF inoculation was beneficial for microarthropod diversity in most cases and offers new perspectives to enhance soil health.