Response of Soil Nematode Communities and Trophic Structure to Trichoderma atroviride P. Karst., in Olive Groves of Mediterranean Croatia

Regenerative agriculture is oriented around restoring soil health through natural processes. In this context, soil biota plays a central role, and bioinoculation represents a potentially effective approach for targeted modification of microbial communities. Among beneficial microorganisms, Trichoderma atroviride is prominent for its biocontrol agent (BCA) activity against plant-parasitic nematodes (PPNs), whereas its effects on free-living nematodes (FLNs) under in vivo conditions remain insufficiently explored. The aim of this study was to assess the response of nematode communities to bioinoculation with T. atroviride as an indicator of soil functional status. A three-year field study was conducted in organic olive orchards at Vodnjan and Nadin on four autochthonous olive cultivars, applying two inoculum doses of T. atroviride: 1 × 106 spores mL−1 (LD) and 1 × 108 spores mL−1 (HD). Bioinoculation increased the diversity of the soil nematode communities at both locations. However, the responses differed between the two inoculum doses. Both doses were associated with an increased abundance of FLNs and a reduced abundance of herbivorous nematodes relative to the control, with LD showing a more consistent and ecologically favourable effect. In combination with biotic and abiotic factors, the LD dose was associated with greater trophic diversity and a more structured soil food web, whereas increasing the inoculum concentration (HD) did not result in additional functional improvement.