Earthworms can mobilize soil arsenic through their casts

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Arsenic (As) mobilization in paddy soils is a concern due to potential transfer to rice.
• Earthworm casts are investigated for their role in arsenic mobility and cycling.

Purpose of the Study:

• To determine if earthworms mobilize arsenic (As) through their casts in paddy soils.
• To identify factors influencing As mobility within earthworm casts.

Main Methods:

• Fractionation and batch experiments were used to analyze As forms and mobility in cast samples.
• Colloidal properties of cast suspensions were examined to understand As release mechanisms.
• Samples were collected from 23 paddy fields in the Red River delta.

Main Results:

• Earthworm casts had lower median As concentrations (5.11 mg kg⁻¹) than surrounding soil (6.7 mg kg⁻¹).
• Arsenic release from casts is linked to reductive dissolution of iron oxides, organic matter decomposition, and competition from P, Si, and DOC.
• Cast dissociation can release As-containing colloids.

Conclusions:

• Earthworm casts may accelerate the arsenic cycle in paddy soils.
• Increased As mobility in casts could lead to greater human exposure.
• The cotransport of As with cast-induced colloids warrants further investigation.