Soluble ions dynamics in Mediterranean coastal pinewood forest soils interested by saline groundwater

- The dynamics of soluble ions in soils of a coastal pinewood forest were studied.
- Marked freshening and salinization phenomena were observed along the soil profiles.
- The major variations of soluble salts content occurred in the vadose zone.
- Na+ and Cl− had a sea origin, Ca2 + and SO42 − where partially released from the soil.
- A better understanding of plant presence and disease symptoms was allowed.

The aim of the study was to evaluate the dynamics of soluble ions in the soils of a pinewood forest of the Mediterranean coastal area in the Province of Ravenna, Italy. A one year monitoring was carried out in four Entisols of the San Vitale pinewood forest, a vulnerable natural area due to subsidence and salt water intrusion. Ionic soluble forms were examined in saturated paste extracts (SPEs) and compared to groundwater soluble forms. Due to the local thermo-pluviometric conditions, both the water table depth and groundwater electrical conductivity (ECw) varied widely ranging respectively from 0 to − 162 cm in depth and from 2 to 28 dS·m− 1 in salinity content. As expected, this greatly influenced soluble ions concentrations in soil horizons, leading to variations in salt's speciation.In soils with a deep water table (Typic Ustipsamments), superficial horizons showed a prevalence of Ca2 + and HCO3- ion forms during the whole year, while in the deep horizons, Na+ and Cl− were the dominant ions. During summertime in soils with a shallow water table (Sodic Psammaquents), superficial ions enrichment was detected with Na+ and Cl− ions being prevalent. Autumn and winter rainfall provoked the leaching of both Na+ and especially of Cl− ions, leaving HCO3- as the main anion. In the soils with intermediate water table depth and moderate salinity (Aquic Ustipsamments), less relevant variations were observed. During winter the Ca2 + ion was the most present at the soil surface together with HCO3−. During summer Ca2 + and Cl− were the most present ions at the soil surface. At deeper horizons Na+ and Cl− ions always prevailed. The sodium adsorption ratio (SAR) reached the greatest values (> 40) during summertime in Sodic Psammaquents superficial horizons and in Typic Ustipsamments deep horizons. On the basis of mixing curves, Na+ and Cl− displayed a conservative behavior and a likely sea origin. Both the Ca2 + and SO42 − ions appeared to be partially released from the solid soil fraction, while the Mg2 + ion seemed to be partially absorbed by the solid matrix. On the basis of the hydrochemical characterization of both groundwater and soil SPEs, it was possible to identify different refreshing and salinization processes along the soil profiles. The one year monitoring of both the whole soil profile and groundwater, never performed before, allows a better understanding of the presence of distinct plant species and disease symptoms.