Characterization of arsenic availability in dry and flooded soils using sequential extraction and diffusive gradients in thin films (DGT) techniques.

Much attention has been paid to the availability of arsenic (As) in soils, while few studies were carried out on the comparison between the dry and flooded soils using different methods. In this study, chemical sequential extraction and diffusive gradients in thin films (DGT) techniques were employed to study the availability of As in soils amended with a range of exogenous As followed by one-year incubation under dry and flooded conditions, respectively. The results showed that the proportions of available solid As pools, including non-specifically adsorbed As (F1) and specifically adsorbed As (F2), had consistent increases with the increase of As amendment. The concentration of DGT-measured As (C DGT) and dissolved As in soil solution (C SOL) also increased up to 2573 (dry) and 1823 (flooded) times and 4067 (dry) and 3105 (flooded) times of the control, respectively, while their ratios (R) showing the extent of C SOL sustained from solid resupply decreased from 0.17 to 0.10 (dry) and 0.35 to 0.21 (flooded). Modelling with DGT-induced fluxes in soils (DIFS) further showed an increase of T c (the characteristic time to reach equilibrium from DGT perturbation) and decreases of desorption and adsorption rate constants (k 1 and k -1) with the increase of As amendment, reflecting a decrease in kinetic exchange rate of As between available solid As pool and soil solution. The flooded soils had greater values of R, k -1 and k 1 and lower value of T c in comparison with the dry soils, indicating a greater availability of As under the flooded condition.