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

A range of indicators including chemical, physical, morphological, and clay mineral properties were investigated to support
an integrated evaluation of the changes induced by a long established olive grove on a red Mediterranean soil. The study area was
located 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 matter
fractions were performed by horizon. Undisturbed soil samples were collected from aggregates for thin section and scanning electron
microscopy (SEM) analyses, complemented by clay mineralogy. Relative compaction was observed in tree topsoil due to the pressure
of the anchoring roots. Lower compaction, higher porosity, and less developed micro-aggregates were observed in deeper horizons
in relation with the loosening action induced by roots. Sharper vertical chemical gradients were determined under the tree. Stress
features were observed in thin sections from all the horizons. Traces of smectite were determined as indicator of past shrink–swell
processes suggesting that the present-day dominance of illite-like minerals might be explained by transformation of smectite induced
by 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.