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Do glyphosate based herbicides harm soil organism such as worms?

Category: Environment and Wildlife

Several published scientific reviews and risk analyses conclude that the use of glyphosate-based herbicides poses a minimal risk to soil microorganisms, earthworms and invertebrates. Studies conducted in Canadian forests also show limited or no effect on soil organisms when tests use realistic exposure and environmental conditions.

Under typical use of glyphosate-based herbicides in forest vegetation management, soil organisms would have minimal exposure to glyphosate or its residues simply because most of the spray cloud is intercepted by the targeted competing vegetation. The majority of studies conducted with realistic exposure levels and environmental conditions demonstrate no significant impacts on soil microorganisms or their ecological functions such as nitrogen transformations. Similarly, field studies pertinent to use in Canadian forest ecosystems typically show no significant effects of glyphosate based herbicides on soil micro-organisms or macro-organisms such as earthworms, ground beetles or snails and slugs.

In summarizing 15 different studies pertaining to potential effects of glyphosate based herbicides on terrestrial invertebrates Sullivan and Sullivan 2003 noted that responses are variable and principally the result of changes in vegetation structure. Ratcliff et al. (2006) examined the effects of a glyphosate-based herbicide formulation applied at the recommended field rate to clay loam and sandy loam forest soils. The authors reported no major changes in microbial community structure assessed by several different methods. Treatment at much higher (100 x) rates simulating a spill of undiluted material resulted in a short-term stimulation of bacteria and minimal change to the fungal community. Preston and Trofymow (1989) reported no significant effects of glyphosate on either soil fauna or microflora populations, or several parameters of nitrogen transformations mediated by soil microbes in lab and field studies in British Columbia. Fletcher and Freedman (1986) conducted laboratory studies with two leaf litter and one forest floor substrate and found that the threshold for glyphosate effects on litter decomposition was more than 50 times higher than residue concentrations that occur in the field after silvicultural herbicide treatments. Duchesne et al. (1999) reported that total catch of carabid beetles in pitfall traps was not affected herbicide treatments including glyphosate (Vision) one year post treatment and that carabid species richness and diversity were slightly higher or equivalent to that observed in untreated controls or sites receiving mechanical vegetation control treatments. As part of this broader study, Houston et al. (1998) noted that conifer release treatments with the glyphosate-based herbicide Vision had no significant effect on basal respiration, microbial biomass carbon, metabolic quotients or nitrogen in either organic or mineral soils. They found that fungal species richness and community structure of check (77 species) and Vision treated plots (81 species) were similar. However, only 40 fungal species common to both Vision and check plots, and that forest harvesting increased fungal community richness and diversity in both near-surface organic and deeper mineral soils. Hawkins et al. (1997) examined effects on terrestrial gastropods in this study and reported that during the first growing season after release, neither surface-active densities nor species richness of gastropods were affected by the alternative vegetation management techniques tested, which included aerial application of a glyphosate-based herbicide. In follow-up studies, Prezio et al. (1999) noted that densities of surface active gastropods were 50-60% lower in all sites receiving vegetation management treatments including herbicide, brushsaw and mechanical clearing as compared to untreated controls 2 and 3 years post treatment, with some trends toward recovery apparent in year 3. Effects were attributed to decreased litter deposition and altered near ground microclimate on treated sites.

Busse (2001) reported that microbial respiration was unchanged at expected field concentrations and that long-term repeated application of glyphosate had minimal effect on seasonal microbial activity. A number of other studies also demonstrate that glyphosate herbicide treatments do not significantly reduce soil microbial populations or impair key microbial functions (e.g. Haney et al. 2000; Hart and Brookes 1996; Wardle and Parkinson 1990). Estok et al. (1989) examined the effects of glyphosate on ectomycorrhizal fungi and found 2 of 3 species showed significant growth reduction effects only at concentrations greater than 100 ppm, with the third having significant effects at 1 ppm. The authors noted that tests on fungi growing on agar medium predispose fungi to herbicide toxicity. Dalby et al. (1995) reported no effect of glyphosate on survival and condition of 4 species of earthworms exposed at “recommended” rates, although these rates were not specified. Similarly, Edwards and Bholen (1992) noted that glyphosate at exposures ranging from 1 to 100 ppm in soils had no toxic effects on earthworms. Risk assessments (PMRA 2015; Durkin 2003; Giesy et al. 2000) conclude that typical use patterns of glyphosate do not pose an acute or chronic risk to earthworms or other soil organisms.

Ratcliff AW, Busse MD, Shestak CJ. Changes in microbial community structure following herbicide (glyphosate) additions to forest soils. Applied Soil Ecology. 2006; 34:114-24

Preston, SM, Trofymow JA. Effects of glyphosate (Roundup) on biological activity of two forest soils. In Reynolds PE (ed) Proceedings of the Carnation Creek Workshop, Nanaimo, 7-10 December 1987. Forestry Canada/British Columbia Ministry of Forests, Victoria, British Columbia, 1989, pp 122.

Fletcher K, Freedman B. Effects of the herbicides glyphosate, 2,4,5-trichlorophenoxyacetic acid, and 2,4-dichlorophnoxyacetic acid on forest litter decomposition. Canadian Journal of Forest Research. 1986;16:6-9.

Duchesne LC, Lautenschlager RA, Bell FW. Effects of clear-cutting and plant competition control methods on carabid (Coldeoptera:Carabidae) assemblages in northwestern Ontario. Environmental Monitoring and Assessment. 1999;56:87-96.

Houston, APC, Visser, S, Lautenschlager RA.. Response of microbial process and fungal community structure to vegetation management in mixedwood forest soils. Can. J. Bot. 1998; 76(4):630-640.

Hawkins JW, Lankester MW, Lautenschlager RA, Bell FW. Effects of alternative conifer release treatments on terrestrial gastropods in northwestern Ontario. The Forestry Chronicle. 1997;73(1):91-8.

Prezio JR, Lankester MW, Lautenschlager RA, Bell FW. Effects of alternative conifer release treatments on terrestrial gastropods in regenerating spruce plantations. Canadian Journal of Forest Research. 1999; 29(7):1141-8.

Busse MD, Ratcliff AW, Shestak CJ, Powers RF. Glyphosate toxicity and the effects of long-term vegetation control on soil microbial communities. Soil Biology & Biochemistry. 2001;33(12/13):1777-89.

Haney RL; Senseman SA; Hons FM. Effect of glyphsoate on soil microbial activity and biomass. Weed Science Society 2000 48:89-93.

Hart MR; Brookes PC. Soil microbial biomass and mineralisation of soil organic matter after 19 years of cumulative field applications of pesticides. Soil Biology & Biochemistry; 1996; 28 (12): 1-1649.

Wardle, DA, Parkinson D. Influence of the herbicide glyphosate on soil microbial community structure. Plant and Soil 1990; 122:29-37.

Estok D, Freedman B, Boyle D. Effects of the Herbicides 2,4-D, Glyphosate, Hexazinone, and Triclopyr on the Growth of Three Species of Ectomycorrhizal Fungi. Bulletin of Envirnmental Contamination and Toxicology. 1989;42(6):835-9.

Dalby PR, Baker GH, Smith SE. Glyphosate, 2,4-DB and dimethoate: Effects on earthworm survival and growth. Soil Biology and Biochemistry. 1995;27:1661-2. (as cited by Tatum 2004).

Edwards, CA, Bolen PJ. The effects of toxic chemical on earthworms. Reviews of Environmental Contamination and Toxicology. 1992, 125:23-99.

PMRA (Pest Management Regulatory Agency). Proposed re-evaluation decision – Glyphosate. PRCD2015-01. 13 April 2015. Available electronically at: pmra.publications@hc-sc.gc.ca

Durkin PR. Glyphosate - Human health and ecological risk assessment report. Syracuse Environmental Research Associates Inc, Fayetteville NY 2003.

Giesy JP, Dobson S, Solomon KR. Ecotoxicological risk assessment for Roundup® herbicide. Rev. Environ. Contam.Toxicol. 2000; (167):35-120.

Sullivan TP, Sullivan DS. Vegetation management and ecosystem disturbance: impact of glyphosate herbicide on plant and animal diversity in terrestrial systems. Environ. Rev. 2003; 11:37-59.