Can the composition of plant’s guttation fluids be used to assess the effectiveness of soil remediation?

When dealing with soils affected by metal(loid)s contamination, especially in the case of low soil organic matter content and poor-fertility conditions (e.g., mine tailings), phytostabilization can be an effective remediation technology, also contrasting soil degradation. In fact, covering polluted sites with herbaceous plants limits the mobility of contaminants by rhizostabilization within the soil, reduces aeolian transport, enhances the microbial activity, increases the organic matter content and decreases soil erosion processes. These effects may lead not only to the reduction of potential risks from hazardous chemical contaminants, but also to an overall improvement of soil chemical, physical and biological properties. In phytostabilization, stress-resistant plants tolerant to high levels of contaminants are commonly used, often in combination with soil amendments that ameliorate and speed up the beneficial effects of the plant cover. In this study, phytostabilization experiments have been carried out growing Lolium rigidum plants in laboratory mesocosms prepared with tailings from a dismissed silver mining site (Argentiera, Sassari, Italy) and a soil from a former shooting range (Acerra, Napoli, Italy), both characterized by high concentrations of Pb, As, Cd, Sb and Zn. For both soils, the effect of different amendments (compost, biochar and plant growth promoting rhizobacteria-PGPR) on the mobility of the contaminants has been studied through sequential extraction procedures (SEP) and by assessing hazardous elements' uptake in plants' biomass (roots and shoots). Beside these quite conventional analytical approaches, a novel approach has been developed, based on the analysis of polluting and mineral elements in plants' guttation fluids via total reflection X-ray spectrometry (TXRF). Indeed, through guttation many plants exudate droplets of xylem sap by means of specialized foliar structures called hydathodes, located at the tips and at the edges of the leaves. Since the xylem sap is directly translocated from roots to shoots, its analysis allow an indirect measure of the uptake from soil and translocation to the aerial parts of polluting and mineral elements, without the need of uprooting the plant from soil and subsequent sample preparation and analysis. Our first results show that the mineral elements composition of guttation fluids is influenced by soil properties and by the applied amendment; for instance, no Pb is detected in fluids from mesocosms of the shooting range soil (where Pb is mostly stabilized as insoluble phosphates) while it is more concentrated in the sap of plants grown in the soil where Pb is less stabilized (mining tails), especially in PGPR-treated mesocosms with respect to other treatments. These early evidences suggest that this TXRF-based method can be a non-invasive, reliable and expeditious method to assess hazardous metal(loid)s mobilization from polluted soils and translocation to plants’ shoots, useful to evaluate not only the efficiency of phytoremediation experiments as in the present study but also other remediation technologies, where plants are used as bioindicators of soil pollution