Rainfed olive farming in south-eastern Spain: Long-term effect of soil management on biological indicators of soil quality

Deteriorating soil quality in arid areas is related primarily to inappropriate farming techniques. The use of environmentally friendly agriculture practices has proven to be effective in restoring or improving soil quality and health in these areas. In this study, four long-term approaches to olive–orchard management, categorized by the presence/absence of two main factors – cover crops and chemical weed control – were evaluated using biological indicators of soil quality. Soil bacterial population size and community structure were estimated by real-time PCR assays targeting 16S rRNA genes and denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA fragments (PCR–DGGE), respectively, and the activity of six enzymes (dehydrogenase, o-diphenoloxidase, β-glucosidase, phosphatase, urease and arylsulphatase) representative of the C, P, N, and S cycles were determined. Covered soils exhibited greater bacterial biomass and diversity, as well as higher microbial functional diversity than non-covered soils. The elimination of weeds with herbicides reduced the microbial functional diversity in covered soil but did not affect the other microbiological parameters, revealing that the effect of the cover crop predominated. When non-covered soil was considered, the lowest values of microbial activity and diversity were found when weeds were controlled with chemical methods. The results of this study reveal that covered soils are the best option when olive orchards are managed under rainfed conditions.