Organic-inorganic interactions in an aridisoil of Oman along vertical and lateral trends of the soluble major, trace and rare-earth elements

Major, trace and rare-earth elements "(REEs)" dissolved by successive water and acetic-acid leaching of soil samples were quantified along a vertical and a lateral trend relative to a reference aridisoil covered by palm trees. The total content of the organic carbon ranges from 0.07 to 2.7% with the highest values in the topsoil closely below vegetation, confirming a higher organic activity. The water-removed elements decrease irregularly with depth and with increasing distance to the vegetated area, the highest concentrations being observed at 20 cm depth and 18 m away from vegetation. The nutrients removed by acetic-acid decrease with depth and until 22 m from vegetation. Maximum leaching was observed in the surface sample and 32 m away from vegetation. In fact, no straight trends were obtained in both the vertical and horizontal samplings; it looks that the elemental contents of each soil layer are only representative for themselves. The metallic trace elements are more abundant in the topsoil than in the subsoil, suggesting some anthropogenic supply. Mainly controlled by solid organic exudates, the REEs are the only ones with general vertical and horizontal trends: they decrease at depth, together with an increase laterally away from vegetation. The Ce and Eu positive anomalies increase deeper below the vegetation and away: an oxidation-reduction change is visible for the former anomaly, probably due to decreasing organic activity. The latter anomaly away from vegetation could reflect a larger contribution of soluble minerals to the leachates.