Abstract
Soils in Mediterranean agroecosystems are highly vulnerable to degradation by erosion and increasing climatic variability. In this study, we characterised soil microbial structure and functioning in a long-term Mediterranean olive orchard in southern Spain, examining variation across management zones, seasons and slope positions. Soils from conventionally tilled (CT) and mixed natural cover (MC) interrows, and from herbicide-treated tree-lines (TL), were collected during seasonal sampling campaigns, with the upper and lower parts of each plot sampled separately. Soil physical and chemical properties were analysed together with microbial biomass, extracellular enzyme activities, and bacterial community composition determined by 16S rRNA gene sequencing. Microbial biomass and enzymatic activity varied across seasons and slope positions, with MC soils generally showing higher values than CT and TL, particularly in spring and autumn. Soil organic carbon and pH were important environmental gradients associated with bacterial community composition, although their influence was embedded within broader spatial and temporal variation. Notably, the greater microbial biomass in winter was not accompanied by similarly high potential enzymatic activity, suggesting that microbial community structure and functioning followed distinct seasonal trajectories. Overall, our findings demonstrate that soil microbial structure and functioning in Mediterranean olive orchards are jointly shaped by seasonal, topographic and edaphic gradients. They also highlight the potential of vegetation cover to promote microbial biomass and enzymatic activity when seasonal conditions are favourable.