Effects of earthworm burrowing on arsenic biotransformation and mobility: implications for roxarsone-bearing poultry litter application

Abstract

This study investigates the effects of earthworm burrowing on arsenic mobility and speciation in soil. The use of roxarsone in the poultry industry as a feed supplement is an increasing anthropogenic source of arsenic. Most of the arsenic is excreted in the litter, later used as agricultural fertilizer. With larger pushes for soil conservation, the decrease in tillage allows earthworm populations to increase and burrows to become more permanent. The influence of earthworms on soil mixing is controlled by two main components: physical displacement and earthworm feeding habits.

Mesocosm experiments were constructed to test the hypothesis that earthworms and burrows provide means for biotransformation and redistribution of arsenic. By controlling the thickness and location of a contaminated layer within the soil columns, arsenic concentrations can be used to indicate bioturbation rates by fitting depth profiles to a simple 1-D advection/diffusion model with a spatial decay term that varies with depth. Synchrotron X-ray methods reveal 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. Overall implications are that 1) burrows serve as macropores allowing rapid transport 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 bioavailable as arsenic is transported through the subsurface.