Beyond Conventional Fertilizer: Tannin–Chlorella vulgaris Blends as Biostimulants for Growth and Yield Enhancement of Strawberry (Fragaria x ananassa Duch)

The increasing demand for sustainable agricultural practices has led to the exploration of natural biostimulants. This study investigates the effects of tannin extracts obtained via hydrodynamic cavitation and Chlorella vulgaris microalgae on the growth and physiological performance of strawberry (Fragaria x ananassa Duch) plants. A preliminary phytotoxicity test using Lepidium sativum L. confirmed the safety of the tannin water extract. Subsequently, two main experiments were conducted: the first identified the optimal tannin concentration, while the second assessed the individual and combined effects of tannins and C. vulgaris on strawberry plants. The results show that tannin water extract at double concentration of the commercial tannin (54% T.E.) significantly increased leaf dry biomass by 75% and doubled the number of main roots compared to the control. In the second experiment, C. vulgaris at 50% concentration (C1) enhanced fresh leaf biomass by 14% and fresh roots by 20%, while tannin extract (T) showed a declining effect on plant biomass as compared to the control. Positive effects were also observed for root growth in the combined treatment T+C1, with 32% fresh root biomass more than in the control. Regarding fruit, C1 maintained high fruit yield from the beginning of the experiment until September, while T+C1 showed a marked rising trend, reaching a comparable number of fruits to C1, about twofold more than the control. A chemical analysis of the main micro- and macro-elements in roots and leaves resulted in T+C1 having the highest content of Zn and Fe and C1 having the highest content of Fe and K (the latter only in the leaves) as compared to other treatments. In contrast, T+C1 showed about 50% less P and K in the leaves than in C. vulgaris treatments. In addition, in the tannin treatment, microelements such as Fe and Zn accumulated in the roots, evidencing absorption from the soil, but low translocation to the leaves. However, all treatments showed similar photosynthetic performance in terms of leaf gas exchange and chlorophyll fluorescence. These findings suggest that extracts of C. vulgaris and tannins or their blends represent a promising strategy for improving crop productivity and resilience in a sustainable manner.