Foliar application of fertilizers and biostimulant has a strong impact on the olive (Olea europaea) rhizosphere microbial community profile and the abundance of arbuscular mycorrhizal fungi

Many studies on biostimulants and nutrient-based fertilizers of olive trees have been focused on the ability of these products to increase olive productivity and ameliorate oil quality. However, little information is available on their effects on rhizosphere microbial communities; whereas it is well known that microorganisms associated with plants play a key role on the productivity and the health of their host. The aim of this study was to evaluate the effects of foliar application of a biostimulant and nutrient fertilizers on photosynthesis, carbohydrate profile of the roots and soil microbial community structure. Specific attention was paid to arbuscular mycorrhizal (AM) fungi. The phospholipid fatty acid (PLFA) 16:1co5 and the neutral lipid fatty acid (NLFA)16:1co5 in the roots (intraradical mycelium) and in the soil (extraradical mycelium) were used as biomarkers for AM fungi. Experimentation consists of the annual foliar application of six nutrient-based treatments during two successive growing seasons: TC (untreated trees), T1 (nitrogen-based fertilizer), T2 (biostimulant rich in boron, magnesium, sulfur, manganese and amino acids), T3 (products rich in phosphorus, potassium and boron), T4 (product rich in phosphorus, calcium and zinc), T12 (application of T1 and T2) and T1234 (application of T1, T2, T3 and T4). At the end of the experiment, statistical analysis revealed a significant modification of macronutrient and micronutrient profiles of olive leaves, which caused a significant increase of photosynthesis and induced quantitative changes in the most of individual root sugars, essentially the translocated forms of sugars (sucrose and mannitol). Foliar application of fertilizers and biostimulant increased the PLFA biomarkers indicative of Gram-positive bacteria (i15:0, i16:0), Gram-negative bacteria (16:1co7, 16:1co9, cy17:0, 18:1co7 and cy19:0) and actinobacteria (10Me16:0). The level of NLFA 16:1co5 in both the rhizosphere and roots, increased significantly under foliar fertilization, however the level of PLFA 16:1co5 did not appear to be significantly influenced, suggesting that spore and vesicle formation was more sensitive to foliar fertilization than the hyphae development. The NLFA 16:1co5/PLFA 16:1co5 ratio, in the roots of olive trees, showed a significant increase, indicating that AM fungi allocated high carbon to storage structures following foliar fertilization. Our study provides evidence that foliar fertilization induces rapid changes in specific microbial groups, which in turn increase the rhizosphere carbon metabolism.