Characteristics of earthworm burrow lining affecting atrazine sorption

Vertically oriented earthworm burrows exist in many agricultural sites, especially in no-tillage soils where the burrows are left intact from year to year. Earthworm burrows can function as preferential flow conduits and may enhance downward transport of surface-applied pesticides. Earthworm activity induces changes in the soil characteristics of the burrow walls that may affect pesticide sorption and transport. Little information is available describing the properties of the lining material. Earthworm (Lumbricus terrestris L.) burrow lining and bulk soil at seven depth intervals from 0 to 50 cm were characterized with respect to pH, clay content, total organic C (TOC), water-soluble organic C (WSOC), and alkaline phosphatase (AP) activity. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] sorption-desorption isotherms were conducted on the same materials. Burrow linings were found to have a narrower range of pH and clay content than the bulk soil within the 0- to 50-cm depth. Levels of TOC and WSOC in burrow linings were two to three times higher than in the bulk soil at the profile surface. This difference increased to eight times at the 35-cm depth although concentrations were lower than at the profile surface. Activity of AP in burrow lining from 0 to 20 cm was three times greater than in the surface bulk soil, and at depths of 20 to 40 cm AP activity in burrow lining was similar to that of the surface bulk soil. Freundlich Kf values for atrazine sorption and desorption were higher for burrow lining than bulk soil at almost every depth. The largest differences (1.5- to 3-fold) occurred in the 15- to 30-cm depth range. Multivariate regression analysis of atrazine sorption-desorption indicated that of the parameters measured, TOC could account for most of the variability in atrazine sorption-desorption.