Hydrogen sulfide and earthworm casts can synergistically increase the cycling rate of soil arsenic

Abstract

Earthworm casts are a seasonal and freshly formed part of soil that contains highly reactive compounds, including nutrients, e.g., phosphorus and sulfur, and toxic elements, e.g., arsenic (As). Casts are dissociated and lost due to biodecomposition or field care practices. However, there exists a knowledge gap concerning the dissociation of casts and the fate of associated compounds, particularly As. Dissociation of the microstructure of earthworm casts is hypothesized to result in the release of As, and in this study, this phenomenon was investigated under an atmosphere consisting of hydrogen sulfide (H₂S), a common anoxic gas in paddy environments. Batch experiments were carried out over a time span of 10 days. Factors such as zeta potential (ζ), hydrodynamic size (d_h) and transmission (T), which reflect possible electrostatic interactions and the dissociation dynamics of cast suspensions, were evaluated by dynamic light scattering and test tube-based analysis methods. The amounts of As and other elements (e.g., P, Si, Fe, dissolved organic carbon) released from the cast suspensions were also tracked. The results show that the ζ, d_h, and T values of the cast suspensions decreased over time, while pH increased. Cast samples exhibited a less negatively charged surface, smaller size, and higher dispersibility in the presence of H₂S compared to samples without H₂S. Intensified releases of Fe, Si and DOC were also observed in the presence of H₂S. This suggests that H₂S facilitated the dissociation of the casts by dissolving cast constituents such as Fe oxides, silicates and organic matter. It can be inferred that the formation/diffusion of H₂S in soil, particularly sulfate-containing soils, can accelerate cast cycling and mobilize cast-containing pollutants (e.g., As). Therefore, field management practices that reduce the reactivity of H₂S need to be considered.