Does soil compaction affect the mobility and transformation behavior of Arsenic -A case study of an Alfisol

Soil compaction induced by frequent field traffic often causes surface waterlogging in wheel rut areas during the rainy season. In arsenic (As)-contaminated farmlands, this process creates varying redox conditions that affect As mobility and transformation at different depths. This study examined the effects of compaction on As behavior in a clay loam Alfisol (from A horizon) under periodic waterlogging over 66 days. Four soil columns were packed with two bulk densities of 1.3 g cm⁻³ and 1.65 g cm⁻³ to simulate different compaction treatments across a 0–40 cm profile: no compaction(CK), low compaction(LC), medium compaction (MC), and heavy compaction (HC) with corresponding compacted thickness of 0 cm, 16 cm, 24 cm, and 32 cm. Results showed that compaction significantly decreased soil air permeability (K_a) and redox potential (Eh). During waterlogging, excess water stagnated in compacted layers, creating reducing conditions that promoted the release of the mobile As (III) species into pore water. In the first two waterlogging cycles, As(III) concentrations in pore water peaked at 5.6, 6.2, 7.3, and 8.0 μg L⁻¹ under CK, LC, MC, and HC, respectively, while the average Fe-(hydr)oxide-bound As decreased by 22.9 %, 22.9 %, 23.7 %, and 26.5 %, respectively. During the subsequent waterlogging cycles, with the increase of K_a, Eh, and microbial activity of aerobic species the As was stabilized on soil particles in the fractions of specifically bound As and Fe-(hydr)oxide-bound As with reduced mobility. These findings highlight the potential risks of soil compaction-induced As transformations, particularly the conversion of less mobile As(V) to the more toxic and mobile As(III), posing environmental and health concerns.