Combining DGT with bioaccessibility methods as tool to estimate potential bioavailability and release of PTEs in the urban soil environment

Use of diffusive gradients in thin-film technique to predict the mobility and transfer of nutrients and toxic elements from agricultural soil to crops-an overview of recent studies

The purpose of this review was to survey the recent applications of the diffusive gradients in thin films (DGT) technique in the assessment of mobility and bioavailability of nutrients and potentially toxic elements (PTEs) in agricultural soil. Many studies compared the capabilities of the DGT technique with those of classical soil chemical extractants used in single or sequential procedures to predict nutrients and PTE bioavailability to crops. In most of the published works, the DGT technique was reported to be superior to the conventional chemical extraction and fractionation methods in obtaining significant correlations with the metals and metalloids accumulated in crops. In the domain of nutrient bioavailability assessment, DGT-based studies focused mainly on phosphorous and selenium labile fraction measurement, but potassium, manganese, and nitrogen were also studied using the DGT tool. Different DGT configurations are reported, using binding and diffusive layers specific for certain analytes (Hg, P, and Se) or gels with wider applicability, such as Chelex-based binding gels for metal cations and ferrihydrite-based hydrogels for oxyanions. Overall, the literature demonstrates that the DGT technique is relevant for the evaluation of metal and nutrient bioavailability to crops, due to its capacity to mimic the plant root uptake process, which justifies future improvement efforts.

Insight into the availability and desorption kinetics of Se and Cd in naturally-rich soils using diffusive gradients in thin-films technique

Cadmium (Cd) contamination of selenium (Se)-rich soils may jeopardize the nutritional benefits of Se-biofortified crops. This study employed diffusive gradients in thin-films (DGT) technique and DIFS (DGT-induced fluxes in soils) model to understand the interdependency and driving factors of Se and Cd distribution and desorption kinetics across 50 soils from south China with naturally elevated levels. DGT-labile Se was the highest (up to 2.66 μg L-1) in non-carbonate/shale-derived soils, while Cd was maximal (5.53 μg L-1) in carbonate-based soils, reflecting soil background concentrations and soil characteristics. Over one-third of the soils showed labile Se:Cd molar ratio below 0.7, suggesting Cd phytotoxicity risks. The DIFS-derived response times (Tc) and desorption rate constants (k-1) suggested that Se was resupplied to the soil solution faster than Cd in soils with higher pH and SOM level, but Se resupply was still restricted due to the rapid depletion of its labile pool. As the first study of Se and Cd release kinetics in soils, our results reveal dependence on soil parent materials, with low labile Se:Cd soils presenting greater Cd hazards. By elucidating Se and Cd lability and interactions in soils, our findings help to inform management strategies to balance reduced Cd risk with adequate Se availability.

Application of Diffusive Gradients in Thin-films (DGT) for assessing the heavy metals mobility in soil and prediction of their transfer to Russula virescens

Although edible mushrooms are considered a source of many beneficial nutrients for human, they can also represent a risk to health due to their capacity to accumulate heavy metals. In this study, the total dissolved in soil solution and labile concentrations of heavy metals (Cd, Pb, Cu, Zn, Co, Cr, Mn, Ni, and Fe) in soil were measured and correlated with their concentrations accumulated in Russula virescens wild mushrooms. The diffusive gradient in thin films (DGT) technique was used to measure the labile metals content in the soil (CDGT), and corroborated with the metals concentrations in soil solution (Csoln) was used to calculate an R-value, which can estimate the metals resupply from soil solid phase when they are uptake by mushroom. The DGT-labile metal concentrations decreased in the order Mn > Fe > Zn > Cu > Co > Ni > Cd ≅ Pb > Cr. The R-values, calculated as the ratio between CDGT and Csoln decreased in the order: Cd (0.50) > Zn (0.37) > Pb (0.33) > Cu (0.24) ≅ Ni (0.24) ≅ Co (0.23) > Mn (0.16) > Fe (0.12) > Cr (0.04). For the first time, we compared the R-values with the bioaccumulation factors (BAFs) in mushrooms, and it was observed that, a similar increasing trend of BAFs with the R-values exists, thus the capacity of the soil solid phase to fast re-supply metals to soil solution increases BAFs. Although the soil samples were not contaminated with heavy metals above the legislative limits, the concentrations of heavy metals accumulated in mushrooms were high enough to pose risks for humans, mainly for children, due to their Cu content.