Tree-induced changes in a terra rossa soil under olive. Observations from an integrated field study

Abstract

A range of indicators including chemical, physical, morphological, and clay mineral properties were investigated to supportan integrated evaluation of the changes induced by a long established olive grove on a red Mediterranean soil. The study area waslocated in Sardinia (Italy). Two soil profiles were studied in a site where land use had remained unchanged for the previous 150 years:one under the canopy and around the trunk of an olive tree and one between the trees. Chemical analyses including organic matterfractions were performed by horizon. Undisturbed soil samples were collected from aggregates for thin section and scanning electronmicroscopy (SEM) analyses, complemented by clay mineralogy. Relative compaction was observed in tree topsoil due to the pressureof the anchoring roots. Lower compaction, higher porosity, and less developed micro-aggregates were observed in deeper horizonsin relation with the loosening action induced by roots. Sharper vertical chemical gradients were determined under the tree. Stressfeatures were observed in thin sections from all the horizons. Traces of smectite were determined as indicator of past shrink–swellprocesses suggesting that the present-day dominance of illite-like minerals might be explained by transformation of smectite inducedby pedogenesis and by the long-lasting K-enriching action of the plants. Our findings indicate the need to undertake more systematic,integrated, and comparative future research on this subject.