Solid fraction of digestate from olive pomace modulates abiotic and biotic processes in soil: Retention of agrochemicals and inhibition of fungal pathogens

A new type of solid digestate (DG) obtained exclusively from two-phase olive pomace was characterised and evaluated for physicochemical and biological properties. In slurry-type experiments, the adsorption of the fungicide boscalid and the herbicide oxyfluorfen on non-amended loam soil (SOIL) and on soil amended with DG at doses of 1 % (SOIL-DG1), 3 % (SOIL-DG3) and 6 % (SOIL-DG6) (w/w) was quantified and modelled. The DG showed a remarkable and stable capacity to adsorb the compounds over a temperature range of 5–40 °C. Based on the data of the adsorption isotherms conducted at 20 °C, the distribution coefficients of SOIL, SOIL-DG1, SOIL-DG3 and SOIL-DG6 were, respectively, 1.3, 2.2, 2.2 and 3.4 mg kg-1 for boscalid and 2.0, 2.1, 2.2 and 5.9 mg kg-1 for oxyfluorfen, which suggested a significant increase in soil retention capacity after the addition of DG, especially at the highest dose. The desorption of both compounds from all treatments, especially from SOIL-DG6, was slower than adsorption and incomplete (hysteresis coefficient < 1), thus indicating a prolonged permanence of the molecules on the soil. In lab-scale experiments, the phytopathogenic fungi Armillaria mellea, Fusarium culmorum and Verticillium dahliae were exposed to 0.02, 0.1, 0.5 and 1 % (w/w) DG alone, and 0.02 and 0.1 % DG preliminary interacted with 100 mg L-1 of soil humic acid (HA-DG). Fungal response was clearly influenced by the species, the treatment and the dosage adopted. In general, all treatments did not significantly modify the growth rate of A. mellea and V. dahliae, whereas all DG treatments, especially HA-DG, caused evident suppressive effects on F. culmorum. Based on the results of this study, it can be concluded that the addition of DG to the soil can regulate the bioavailability of agrochemicals in pore water and exert an inhibitory or irrelevant action depending on the phytopathogenic fungus.