11
Nov
Weed Killer Use Destroys Soil Life and Ecosystems, Paper Finds
(Beyond Pesticides, November 11, 2022) A paper published in Trends in Ecology & Evolution in late October sounds an unnerving alarm about the globally ubiquitous use of herbicides and the ecological destruction being caused. It asserts that widespread environmental contamination with these herbicide compounds is influencing soil, plant, and animal microbiomes in ways that are not only not well understood, but also, can have significant impacts on the functioning of organisms and their ecosystems — with evolutionary implications. Impacts of herbicides on microbiota in soils include, for example, those on nutrient cycling, and altered organism and plant performance, which can affect pollination and animal consumption of plants. This research reinforces what Beyond Pesticides wrote in covering a 2021 study: “The popular herbicide glyphosate negatively affects microbial communities, indirectly influencing plant, animal, and human health. Exposure to sublethal concentrations of glyphosate shifts microbial community composition, destroying beneficial microorganisms while preserving pathogenic organisms.”
Herbicides are a category of pesticide used to control weeds in agriculture and commercial forests, on managed landscapes, byways, gardens, and lawns, and directly on surface waters to control aquatic weeds. They are designed to kill “target” plant species considered undesirable in any of those circumstances. Herbicide use has exploded in the past two decades, in large part due to the advent of the agro-biotech industry’s deployment of genetically modified, herbicide tolerant crop seeds that pair with herbicide use.
This increased use has ramped up the development of weeds’ resistance to multiple herbicides. Glyphosate formulations (including the infamous Roundup) are the most commonly used, in agriculture, horticulture, silviculture, and urban environments. In the aggregate, glyphosate contributes mightily to global environmental contamination; other widely used herbicides include [triazines (e.g., atrazine), acetochlor and metolachlor, paraquat, and dicamba. Residues of herbicides are found in soil, water, non-target plants, animals, and humans, and are associated with pollinator and insect declines and biodiversity losses, compromise of other organisms (including keystone species), ecosystem dysfunction, and human health anomalies.
The study authors also note that adjuvant, “inert” ingredients in herbicide formulations can sometimes be even more toxic to non-target organisms than the active ingredients themselves, and that in the U.S., such co-formulants are not required to be tested for toxicity to non-target organisms. To make matters worse, information about such adjuvants is usually considered “proprietary” and therefore, is not shared with regulators or the public. Beyond Pesticides has covered this “inert” ingredient phenomenon.
If present patterns persist, the use of herbicides is predicted, by BusinessWire, to increase annually by 2–3% through 2025. Most of that increase is expected in the agricultural sector because of (1) increasing resistance to herbicides by weed species, (2) an increase in agricultural intensity in Central and South America and the Asian/Pacific region, and (3) the ongoing development of new herbicide formulations (in part because of #1) and “herbicide use education” in developing markets.
Though herbicides are designed for target species, they also expose nontarget plants, animals, humans, and ecosystem function to risks. This study focuses on the compounds’ effects on microbiota in flora, fauna, and soils. The authors assert that, “While many herbicides were initially considered safe for non-target taxa, as their mechanism of action was thought to be absent in these organisms, it has been understood only recently that herbicides may have profound effects on non-target taxa via alterations of microbial communities and microbial function in soil, plants, and animals. Given the imperative role of microbes in driving eco-evolutionary adaptations since the origin of life, and that microbes and their hosts comprise coevolving, multipartite entities [known as holobionts], a comprehensive understanding of the risks associated with altered microbiomes is needed.” (A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit through symbiosis.)
The assumption that herbicides would be safe for nontarget taxa was based on the idea that their modes or mechanisms of action — how the compounds actually work to kill or disable weeds — were lacking in nontarget organisms. More-recent research has shown, however, that these compounds’ mechanisms of action can have profound effects on the microbial communities harbored by non-target organisms. These communities, or microbiota, are present in all living things and are critical to healthy organism function — and to optimal immune response in particular, a primary task being the control of pathogens. When herbicides damage or kill a plant or animal’s resident microbes, they alter the organism’s ability to execute this protective function.
The study identifies classes or modes of action for a host of herbicide active ingredients, including whether they act directly or indirectly on microbiota, and their respective effects on soil, plant, or animal microbiomes. Among the modes (and sample compounds) that have direct impacts on microbes are:
• ACC (acetyl-CoA carboxylase) inhibitors (e.g., diclofop-methyl, haloxyfop)
- ALS (acetolactate synthase) inhibitors (sulfonylureas, triazolopyrimidines)
- EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) inhibitors in the shikimate pathway (glyphosate)
- glutamine synthetase inhibitors (glufosinate)
Effects on resident microbiomes include those that damage microbes’ role in nutrient cycling, compromise immune response, alter soil carbon and phosphorous dynamics, and degrade population levels.
The mechanisms that exhibit indirect impacts, including on cellular metabolism and hormone synthesis, are auxin-like herbicides (2,4-D, dicamba); photosystem (related to photosynthesis) inhibitors (triazines, paraquat, diphenyl ether); and gibberellin (plant hormone that stimulates stem elongation, germination, and flowering) inhibitors (acetochlor, metolachlor, pendimethalin). The indirect impacts on microbiota include those that degrade bacterial diversity, erode microbial community structure, and disable nitrogen-fixing bacteria.
Herbicides alter microbial communities through multiple pathways; factors that influence such alterations include differing vulnerability to the compounds across microbe type; some microbes’ utilization of herbicides as nutritional sources; and functional changes that can cascade to have “community-wide” impacts. An example of that last is that healthy microbiomes exhibit successful, long-term self-regulation; herbicide exposures can have damaging effects on that ability.
Soil- and root system–associated microbes are critical to functioning ecosystems, and herbicides’ impacts on them depend on several elements: the compounds’ chemical composition and mode of action, soil health, and climate, among others. The dynamics of herbicides in soil microbiota are complex, and thus, can be hard to predict. Examples the study cites are these: “Glyphosate negatively affects shikimate pathways present in the majority of microbes, but their genetic resistance to glyphosate varies. Therefore, some of the resistant and glyphosate-degrading microbes that can use glyphosate as a nutrient source may become prevalent in the microbial community. . . . Similarly, in some environments atrazine may not affect the overall microbial community, while in other environments it can decrease soil microbial biomass or increase atrazine-degrading bacteria due to strong selection favoring them, thus leading to atrazine degradation.” (The shikimate pathway is one of many physiological pathways that impact plant defense and signaling chemistry.)
The study concludes that the ecological and evolutionary consequences for microbial soil communities are poorly understood and require further research. But the authors posit that other research has demonstrated a negative correlation between pesticide use and (1) beneficial soil- and root-associated microbes, and (2) herbicide-modulated nutrient cycling.
The team also asserts that herbicide residues can cause disruptions in dynamic relations between mycorrhizal fungi and their associated plant communities, and reduction in abundance of nitrogen-fixing bacteria. They note: “As many plant traits, including growth, phenology, and resistance to abiotic stressors and pathogens, are modulated by rhizosphere microbiome, changes in rhizosphere composition and functioning are likely to be reflected in host fitness and growth. . . . [S]ublethal doses of glyphosate [for example] can potentially disrupt virtually all plant above-ground interactions with other coevolving organisms, such as pathogens, plant-mutualistic microbes, herbivores, and pollinators.”
The study also points to deleterious impacts on animal gut and skin microbiomes, which play important roles in digestion, pathogen management, and neurobehavioral coordination. Glyphosate, the paper notes, “has been shown to increase pathogenic and decrease symbiotic bacteria, which may affect the susceptibility of bees to viral and fungal pathogens, with survival effects cascading to the ecosystem level. . . . [H]erbicide-altered plant microbiomes and/or metabolomes in plant leaves, pollen, and nectar may alter the exposure and consumption of pollinators and herbivores, which can have cascading effects on their gut microbiomes and, therefore, the health of the pollinators and herbivores.” This is a demonstration of how herbicide-driven alterations in animal-host gut microbiomes can lead to ecosystem-level changes.
Last, the research addresses the “widely known evolutionary consequence of repeated herbicide exposure” — selection for increased herbicide resistance in soil bacteria. This resistance, the authors assert, can feed back to the ecosystem level when changes in the microbial community composition influence soil processes; they cite nitrogen and carbon flows as examples. They add, “Long-term exposure to herbicides may influence not only microbial evolution but also the evolution of the animal hosts driven via microbes,” and cite the example of a particular wasp variety’s chronic atrazine exposure causing adaptive gut changes that then exerted selective pressure on its host genome.
The health of microbial communities is hugely important. These tiny organizations of organisms maintain individual plant, animal, and human health, and that of ecosystems. Altering these communities — particularly in our soils, through prolonged assault with toxic herbicides (and other pesticides) — can have, the authors assert, “far-reaching, long-term, and unforeseen impacts on ecosystems.” We are witnessing these chemical impacts unfold in the current trend toward a collapse of biodiversity that threatens all life on Earth. (Other factors contribute, of course —the climate crisis, human-caused development that destroys habitat, pollution, overexploitation of natural resources, and problematic invasive species.)
But this research (and more here) identify a threat that has great potential to accelerate the distortion and potential destruction of organisms and ecosystems. The issue of herbicides’ and pesticides’ impacts on microbiomes, especially in our soils, needs more research and certainly, should be part of the U.S. Environmental Protection Agency’s (EPA’s) risk evaluations of herbicides and pesticides.
Meanwhile, as Beyond Pesticides advocates with growing volume and urgency, the “fix” for insect, weed, and animal “pests” (in agriculture, and in other land and building management) is not the agrochemical industry’s never-ending chase of evolving organisms’ resistance to chemical assaults with new, more, and more-intensive chemical applications. This approach will never “win” the contest with the living world’s mutation-plus-selection strategy for organismic survival.
What can work is a change in approach, on the order of “work smarter, not harder.” Organic approaches to agriculture, in particular, but to all land and pest management, are effective, holistic, protective, and benign ways to deal with pests, and can achieve production and land management goals — without the toxic, systemic, destructive, and sometimes unknown, impacts of chemical saturation of the environment and ecosystems, natural resources, and organisms across the living spectrum.
Learn more about the environmental, health, and socioeconomic benefits of organic, and please advocate for this critical transition (see our Tools for Change). You can reach out to us for assistance with this work in your community: email us at [email protected] or call 202.543.5450.
Source: Trends in Ecology & Evolution
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
