Diversifying Grapevines With Aromatic Plants Changes the Soil Habitat, Microbial Community Composition and Functions Toward More Efficient Substrate Use and Nutrient Allocation

In vineyards facing soil degradation and biodiversity loss, crop diversification may improve sustainability, but its effects on the soil microbiome remain unclear. In a 3-year field study, we examined how diversifying the plant row under grapevine with aromatic plants affected topsoil properties (0–10 cm) in an organically farmed, steep-sloped vineyard. Specifically, we investigated the effects of diversification with oregano and thyme on microbial biomass, respiration, prokaryotic and fungal community compositions, enzyme activities, potential nitrification, and abiotic soil properties, including total and particulate organic carbon (TOC, POC), nutrient status, pH, and soil moisture. Grapevines alone with mechanical tillage served as control. The aromatic plants competed with grapevines by lowering soil nutrient contents and moisture. Aromatic plant litter had a small, mostly non-significant but consistent effect on POC contents, and POC stocks determined in the final year showed a slight increasing trend in the order control (10.9 ± 2.8 t POC ha⁻¹) < thyme (12.6 ± 3.1) < oregano (13.1 ± 4.1). Surprisingly, these changes coincided with a significant decrease in microbial biomass compared to control, indicating aromatic plant-microbe competition. Concomitant decreases in respiration and the activity of C-cycling enzymes but also the metabolic quotient, suggest lower carbon mineralisation but more efficient microbial carbon use. Multivariate statistics revealed that the prokaryotic community was primarily structured by abiotic soil properties, such as organic matter, nutrient and water availability. In contrast, the fungal community exhibited a stronger plant-specific response, with changes in composition likely driven by root-associated interactions, suggesting a more direct biotic influence. Especially, arbuscular mycorrhizal fungi and potential nitrification were promoted under both aromatic plants, which may benefit grapevine growth. Overall, we show that diversifying perennial agroecosystems such as vineyards with aromatic plants increases soil habitat heterogeneity with benefits for microbial diversity, carbon sequestration and nutrient cycling, demonstrating its positive impact on soil biodiversity and functioning.