|Le Bayon, Renee-claire|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2005
Publication Date: 1/6/2006
Citation: Binet, F., Kersante, A., Munier-Lamy, C., Le Bayon, R., Belgy, M., Shipitalo, M.J. 2006. Lumbricid macrofauna alter atrazine mineralization and sorption in a silt loam soil. Soil Biology and Biochemistry. 38:1255-1263. Interpretive Summary: The herbicide atrazine is widely used in corn production, but can become an environmental concern if it moves offsite to surface or ground water. There is little information, however, on how earthworms affect the distribution and persistence of atrazine in soils. In this laboratory study, radio-labeled atrazine was used to investigate how two species of earthworms affect the fate of atrazine in soil. The results indicated that earthworms stimulated microbial activity and increased the rate of organic matter decomposition, thus should also increase atrazine breakdown. Nevertheless, atrazine breakdown was reduced by earthworms because of intimate mixing with soil during passage through the earthworms. This promoted binding of atrazine to the soil, which reduced its rate of decomposition by soil microorganisms. Longer persistence of atrizine may increase its potential to leach or be transferred offsite by surface runoff. This phenomenon may have to be taken into account when farmers are selecting crop and weed management practices.
Technical Abstract: Atrazine is a widely used herbicide and is often a contaminant in terrestrial and freshwater ecosystems. It is uncertain, however, how the activity of soil macrofauna affects atrazine fate and transport. Therefore, we investigated whether earthworms enhance atrazine biodegradation by stimulating herbicide degrading soil microflora, or if they increase atrazine persistence by facilitating herbicide sorption. Short (43 d) and medium term (86 d) effects of the earthworms Lumbricus terrestris and Aporrectodea caliginosa on mineralization, distribution, and sorption of U-ring-14C atrazine and on soil C mineralization was quantified in packed-soil microcosms using silt loam soil. A priming effect (stimulation of soil C mineralization) caused by atrazine supply was shown that likely lowered the earthworm net effect on soil C mineralization in atrazine-treated soil microcosms. Although earthworms significantly increased soil microbial activity, they reduced atrazine mineralization to 14CO2-C from 15.2% to 11.7% at 86 d. Earthworms facilitated formation of non-extractable atrazine residues within C-rich soil microsites that they created by burrowing and ingesting soil and organic matter. Atrazine sorption was highest in their gut contents and higher in casts than in burrow linings. Also, gut contents exhibited the highest formation of bound atrazine residues (non-extractable atrazine). Earthworms also promoted a deeper and patchier distribution of atrazine in the soil. This contributed to greater leaching losses of atrazine in microcosms amended with earthworms (3%) than in earthworm-free microcosms (0.003%), although these differences were not significant due to high variability in transport from earthworm-amended microcosms. Our results indicated that earthworms, mainly by casting activity, facilitated atrazine sorption, which increased atrazine persistence. As a consequence, this effect overrode any increase in atrazine biodegradation due to stimulation of microbial activity by earthworms. It is concluded that the affect of earthworms on atrazine mineralization is time-dependant, mineralization being slightly enhanced in the short term and subsequently reduced in the medium term.