Holm oak (Quercus ilex L.) rhizosphere affects limestone-derived soil under a multi-centennial forest

Background and Aims - The activity of roots and associated microorganisms plays a key-role in soil formation and evolution, but we lack of knowledge on the quality and extent of the “rhizosphere effect” in the different soil horizons. The aim of this study was to assess the interactions between rhizosphere processes and genetic horizons in a forest (Quercus ilex L.) soil developed from limestone. Specifically, we tested (a) if the rhizosphere effect was significant in all the horizons of the soil profiles, and (b) if the intensity of the rhizosphere effect was associated to structure, composition and activity of the microbial community. Methods - Bulk and rhizosphere soils were characterized by physical, mineralogical, chemical and biological (microbial activity and community structure) analyses. Results - Throughout the soil profile, the rhizosphere processes affected properties like particle-size distribution and soil structure, mineralogy, pH, and organic C and total P content. Conversely, amounts of exchangeable Ca, Mg and K, iron oxides, available P, and total nitrogen showed no significant change. As for the microbial community, its structure and metabolic activity differed between rhizosphere and bulk only in the core of the solum (2Bwb and 3Bwb horizons). Conclusions - The main processes controlling the intensity of the rhizosphere effect on the soil horizons were root activity, soil faunal perturbation and slope dynamics. While root activity impacted the whole soil profile, although to a lesser extent at depth, the influence of fauna and slope was confined atop the profile. It follows that long-term changes due to root activity and associated microbial community were more strongly expressed in the core of the solum, not at the surface, of this limestone-derived soil.