Soil quality and fertility in an olive orchard managed for 20 years with differential agronomic systems

Climate change, in terms of increased temperature and extreme precipitation regimes, will have agricultural consequences due to the interrelations between climate, land and water use, soil degradation and landscape changes. Conservation agriculture offers new chances to mitigate the effects of climate change. In sustainable agro-forestry systems, management practices are able to increase carbon (C) inputs into the soil and possibly reduce GHGs emissions due to some revised field operations (e.g., irrigation techniques, use of recycled water, pest and disease management, fertilization, soil and plant farming systems). Carbon enrichment increases biological activities by improving soil structure, as well as the soil moisture and nutrient contents, that are beneficial to plant growth and production. This study reports results on the effects of changed soil practices of an experimental olive orchard from a conventional management (Cmng: soil tillage, mineral fertilizers, burning of pruning residues) to a sustainable management (Smng: no-tillage, pruning residues, cover crop retention, and compost application) on soil quality/fertility and soil/plant microbiota. Results show that a 20-year period of Smng (including C inputs at a mean rate of 8-9 t C ha-1 year-1) caused increases in soil organic carbon (SOC) from 1.0-1.3% w/w up to 1.7-2.0% in the topsoil, and in soil water retention (up to 40% more) and permeability (from 13 to 160 mm H2O day-1). The adoption of a correct irrigation management had a key role in the potential role of orchards in C sequestration (soils become from C sources to C sinks) and in the consequent greenhouse effect mitigation. Indeed, compared to dry areas, wetted soils generally had a higher microbial respiration and SOC mineralization, and a faster bacterial C and N turnover. Finally, the Smng brought beneficial effects on plant yield, that was improved by 30-50%, compared to Cmng, and on the levels of a wide range of plant protective secundary metabolites in the xylem sap. The endogenous C additions and the different irrigation systems also affected the reserves of soil nutrients (N, P, K, Ca, Mg) and CO2 soil emission. Promoting cost-effective sustainable land use strategies can avoid SOC decline, soil erosion and soil degradation, with consequent benefits in terms of soil quality parameters. This is essential for sustaining and improving yield and quality of olive plants.