Air‐Filled Porosity as a Key to Reducing Dissolved Arsenic and Cadmium Concentrations in Paddy Soils

Abstract

Simultaneous suppression of rice ( L.) uptake of As and Cd is challenging because these toxic elements are immobilized under contrasting redox conditions. Given the notion that oxygen diffusion governs redox conditions in temporarily drained paddy soil, we assume that the key to simultaneous suppression of dissolved As and Cd concentrations is air-filled porosity (AFP) of soil. The objectives of this study were to reveal relationships between AFP and dissolved As and Cd concentrations in paddy soils and to identify the optimum water management, in terms of AFP, for simultaneous reduction of As and Cd. Dissolved As and Cd concentrations were determined in soil cores collected at different depths and times during rice growing seasons. Dissolved As concentrations were appreciable (>3 µg L) only when AFP was below a threshold value of 0.04 to 0.10 m m, suggesting that dissolved As was rapidly immobilized once AFP exceeded the threshold value on drainage. Dissolved Cd concentrations were roughly proportional to AFP, with higher concentrations associated with lower soil pH. Although dissolved As concentrations tended to be low in soil samples with high dissolved Cd concentrations and vice versa, both concentrations were low when AFP was slightly above the threshold value for As immobilization. The results suggest that dissolved As and Cd can be simultaneously kept at low levels by appropriate water management practices that produce AFP slightly above the threshold value for As immobilization.

Soil aeration and redox potential as function of pore connectivity unravelled by X‐ray microtomography imaging

“…These values are close to the ones previously reported with 0.048 ± 0.008 for ε Pt reaction and 0.085 ± 0.007 cm 3 cm À3 for ε Pt aeration from the same study site (Dorau et al, 2018). Besides the overall linkage between ε and E H , Nakamura et al (2018) demonstrated the benefit from these kind of pedotransfer functions to water management practices, for example, when ε is known but data about reduced and toxic species in soil solution is missing. They found that dissolved arsenic (As) was only appreciable when ε was smaller 0.04-0.06 cm 3 cm À3 at a plot that featured a similar particle size distribution compared with our study.…”

“…In another study, root elongation rate of radiata pine seedlings was close to zero at ε < 0.05 cm 3 cm À3 , a finding that was independent of soil texture (Zou et al, 2001). This indicates that a similar tipping range in ε indicated the transition from reducing towards oxidising soil conditions since As was rapidly immobilised (Nakamura et al, 2018) and root elongation was enhanced by invading O 2 above the ε threshold (Zou et al, 2001).…”