Linking microbial functional diversity of olive rhizosphere soil to management systems in commercial orchards in southern Spain

A comparative study of organic and conventional commercial olive farming systems was conducted in a wide-area of Andalusia, southern Spain to determine the effect of management practices on chemical and biological soil properties. Soils from 41 accredited organic farms, 49 conventionally managed neighboring farms, and three sites containing wild or feral forms of olive were analyzed. A polyphasic approach was used combining traditional soil physicochemical analysis, culture-dependent microbiological analyses, sole-carbon-source utilization profiles using the Biolog EcoPlate assay and enzymatic activities using the API ZYM assay. Different multivariate statistical analyses clearly demonstrated that olive orchard soils could be differentiated by farm management system. Discriminant analysis differentiated among three well defined soil groups that correlated with the farm management system (conventional, organic or wild olives). Thus, enzymatic activities and sole-carbon-source utilization profiles correctly classified 75, 77 and 100% of soil samples from orchards under organic and conventional management or wild olives, respectively. Overall, all diversity and functional indexes estimated from the Biolog EcoPlate and API ZYM assays were positively and significantly correlated with SOM, organic C and N content and the C:N ratio, and negatively and significantly correlated with clay content. In general olive orchards under organic management showed significantly higher SOM, organic C and N content, and C:N ratio, as well as higher microbial diversity as measured by catabolic capability and functional indexes of the Biolog EcoPlate and API ZYM assays as compared to conventionally managed orchards. Given the crucial importance of maintaining or increasing soil health in agricultural ecosystems, this is the first demonstration that commercial olive orchards under organic management in southern Spain are agricultural systems that may contribute to promote and conserve soil quality and health.