Earthworms as agents for arsenic transport and transformation in roxarsone-impacted soil mesocosms: A μXANES and modeling study

• Published in: Geoderma 2010-05, Vol.156 (3-4), p.99-111
• Main Authors: COVEY, A. K, FURBISH, D. J, SAVAGE, K. S
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Arsenic; Biological and medical sciences; Cropping systems. Cultivation. Soil tillage; Displacement; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Microorganisms; Pollution, environment geology; Poultry; Soil (material); Soil tillage; Soils; Speciation; Surficial geology; Tillage. Tending. Growth control; Transformations; X-rays
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2010.02.004

Earthworms influence soil mixing during their feeding and burrowing activities by causing physical displacement and chemical transformations, potentially impacting the distribution and speciation of soil contaminants. This study investigates the effects of burrowing earthworms on arsenic speciation and mobility in soil mesocosms treated with roxarsone, an increasing anthropogenic source of arsenic used in the poultry industry as a feed supplement. Experiments were designed to test the hypothesis that earthworms and burrows provide means for biotransformation and redistribution of arsenic. Arsenic distribution was determined following earthworm bioturbation in artificial soil columns that were initially constructed with a contaminated layer of designated thickness and depth. The resulting depth profiles were used to indicate bioturbation rates by fitting them to a simple one-dimensional advection/diffusion model with a biodiffusivity that decreases with depth. Synchrotron X-ray methods reveal additional small-scale displacement of arsenic near the burrow and in the bulk soil. Microbeam X-ray Absorption Near Edge Structure spectra were collected to determine arsenic speciation. Arsenic initially introduced as roxarsone tended to be in a methylated form after 30 days. Within the earthworm tissue, arsenic-glutathione complexes were formed. Results are viewed in the context of poultry litter used as agricultural fertilizer. A decrease in tillage arising from soil conservation efforts allows earthworm populations to increase and burrows to become more permanent. This study suggests that (1) burrows serve as macropores allowing rapid transport of arsenic to greater depths during infiltration events; (2) bioturbation rates appear to be depth-dependent; and (3) earthworm soil mixing produces potentially mobile forms of arsenic that may become bioaccessible as arsenic is transported through the subsurface.