Sorption and retention of herbicides in vertically oriented earthworm and artificial burrows

In many temperate region no-tillage agro-ecosystems, the vertically oriented burrows of the nightcrawler earthworm (Lumbricus terrestris L.) function as preferential flow paths and may promote the rapid downward transport of surface-applied chemicals. The burrows of nightcrawlers, however, are lined with a material that is enriched in organic C relative to the surrounding soil matrix that may affect transport of organic chemicals. We investigated the effect of this lining on the sorption and retention of four herbicides: atrazine [6-chloro-N-ethyl-N-(1-methylethyl)-1,3,5-triazine- 2,4-diamine], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N(2-methoxy- 1-methylethyl)acetamide], dicamba (3,6-dichloro-2-methoxybenzoic acid), and primisulfuron [3-[4,6-bis(difluoromethoxy)-pyrimidin-2yl]-1-(2-methoxy carbonylphenyl sulfonyl) urea]. Batch sorption isotherms were determined using burrow lining and bulk soil collected from seven depth intervals from 0 to 50 cm. Sorption of the more strongly sorbed herbicides (atrazine and metolachlor) was enhanced by a factor of up to three on burrow lining relative to bulk soil, while there was little or no enhancement of the weakly sorbed herbicides (dicamba and primisulfuron). Transport and retention of these herbicides was measured in intact burrows in undisturbed soil blocks. Concentrations of the two strongly sorbed herbicides decreased during flow through nightcrawler and artificial (unlined) burrows, with the largest decreases occurring in earthworm burrows. Concentrations of the two weakly sorbed herbicides decreased very little during flow through either type of burrow. Retained herbicide concentrations were higher in nightcrawler than in artificial burrow walls, and decreases in concentration from the surface to the second 1-mm wall layer were greater in nightcrawler than in artificial burrow walls. These results indicate the material lining nightcrawler burrows may significantly retard herbicide transport during lateral flow into and out of burrows.