Soil Microbes and Pesticides
Repercussions of Use
Soil is a vibrant ecosystem teeming with billions of microorganisms, such as bacteria, fungi, and archaea. These microbes play crucial roles in nutrient cycling, decomposing organic matter, supporting plant health, contributing to plant aromas and pigments, and facilitating photosynthesis. Contemporary agricultural methods heavily depend on pesticides—including herbicides, insecticides, and fungicides—which can substantially affect soil microbial communities. The relationship between soil microbes and pesticides is intricate; while some chemicals may hinder microbial diversity, these microbes can decompose and utilize others.
How Pesticides Affect Soil Microbial Communities
Pesticides can influence soil microbes in several ways, including:
Inhibition of microbial activity: Some pesticides have antimicrobial properties, reducing the population of beneficial bacteria and fungi.
Selective pressure: Certain microbes may develop resistance to pesticides, leading to shifts in microbial diversity.
Microbial degradation of pesticides: Some soil microbes can break down pesticides, transforming them into less harmful compounds.
Disruption of symbiotic relationships: Pesticides may interfere with plant-microbe interactions, such as nitrogen-fixing bacteria in legume roots or mycorrhizal fungi.
Examples of Pesticide-Microbe Interactions
1. Glyphosate (Herbicide) and Soil Microbes
Glyphosate, the active ingredient in Roundup, is the most widely used herbicide that targets the shikimate pathway—a metabolic route in plants and some bacteria. Research has shown that glyphosate can:
Reduce populations of beneficial soil bacteria, such as Pseudomonas and Bacillus, which help suppress plant pathogens (Van Bruggen et al., 2018).
Favor opportunistic and potentially harmful microbes, including some Fusarium fungi, are linked to plant diseases (Zobiole et al., 2010).
It persists in soil and is broken down by particular Pseudomonas and Acinetobacter species, which can use glyphosate as a phosphorus source (Mertens et al., 2018).
2. Atrazine (Herbicide) and Microbial Degradation
Atrazine is another common herbicide used to control broadleaf weeds. It is known for its persistence in soil and groundwater contamination risks. However, some microbes, such as Pseudomonas sp. strain ADP, can degrade atrazine as a nitrogen source. Mili et al., 2022, noted 25 additional species that can remediate atrazine. These microbes are crucial in bioremediation efforts to remove atrazine from contaminated soils (Udikovic-Kolic et al., 2012).
3. Neonicotinoids (Insecticides) and Nitrogen-Fixing Bacteria
Neonicotinoids, such as imidacloprid and clothianidin, are widely used to protect crops from insect pests. However, studies indicate they can negatively affect beneficial soil microbes, particularly nitrogen-fixing bacteria like Rhizobium and Bradyrhizobium. This can reduce plant nitrogen availability, leading to lower crop yields in legume-based farming systems (Kreutzweiser et al., 2008). Akter et al., 2023, published a review of over 200 published papers, noting that most of this research was conducted in a lab, not in the field. However, they did note that neonicotinoids reduce fungal populations in addition to bacterial populations. (Akter et al., 2023)
4. Chlorothalonil (Fungicide) and Fungal Diversity
Chlorothalonil is a broad-spectrum fungicide used to control fungal diseases in crops. While it effectively suppresses plant pathogens, it also reduces beneficial soil fungi, including mycorrhizal fungi, which help plants absorb nutrients. This can result in poorer soil health and reduced plant resilience to stress. (Drenovsky et al., 2004; Jiang et al., 2022). Repeated applications of chlorothalonil “significantly correlated with the change of the soil respiration, urease activity, sucrase activity, soil culturable bacteria, and culturable fungi biomass.” (Jian et al., 2022)
Strategies for Reducing the Negative Impact of Pesticides on Soil Microbes
To mitigate the harmful effects of pesticides on soil microbial communities, farmers and agronomists can adopt several practices:
Using microbial inoculants: Beneficial bacteria and fungi can be introduced to help maintain soil microbial diversity.
Implementing crop rotation and cover cropping: These practices promote microbial diversity and improve soil structure.
Reducing pesticide application rates: Applying pesticides only when necessary and at the lowest effective dose minimizes microbial disruption.
Choosing biopesticides: Some microbial-based pesticides, such as Bacillus thuringiensis (Bt) and Trichoderma, provide pest control while supporting soil health.
Increasing active carbon in the soil: Active carbon, also known as labile carbon, is the form of carbon that living beings, like microbes and other organisms, can use as building blocks for life. Carbon and carbon pools are a vast topic not exposed in conventional (or even today’s organic) agricultural systems. Bokashi provides labile carbon that increases overall soil health. (Jackson et al., 2017, cover details of the complexity of this subject.)
Conclusion
Pesticides profoundly impact soil microbial communities, influencing nutrient cycling, plant health, and soil fertility. While some microbes can break down pesticides, others suffer population declines, leading to imbalances in the soil ecosystem. Understanding these interactions can help develop sustainable agricultural practices that protect crops and the microbial life supporting them.
References
Akter, S., Hulugalle, N. R., Jasonsmith, J., & Strong, C. L. (2023). Changes in soil microbial communities after exposure to neonicotinoids: A systematic review. Environmental Microbiology Reports, 15(6), 431. https://doi.org/10.1111/1758-2229.13193
Castrejón-Godínez, M. L., Tovar-Sánchez, E., Valencia-Cuevas, L., Rosas-Ramírez, M. E., Rodríguez, A., & Mussali-Galante, P. (2021). Glyphosate Pollution Treatment and Microbial Degradation Alternatives, a Review. Microorganisms, 9(11), 2322. https://doi.org/10.3390/microorganisms9112322
Drenovsky, R. E., Duncan, R. A., & Scow, K. M. (2004). Impact of chlorothalonil application on soil microbial communities. Applied Soil Ecology, 25(3), 225-237.
Jackson, R. B., Lajtha, K., Crow, S. E., Hugelius, G., Kramer, M. G., & Piñeiro, G. (2017). The ecology of soil carbon: pools, vulnerabilities, and biotic and abiotic controls. Annual review of ecology, evolution, and systematics, 48(1), 419-445.
Jiang, J., Yang, Y., Wang, L., Cao, S., Long, T., & Liu, R. (2022). Effects of Chlorothalonil Application on the Physio-Biochemical Properties and Microbial Community of a Yellow–Brown Loam Soil. Agriculture, 12(5), 608. https://doi.org/10.3390/agriculture12050608
Kreutzweiser, D. P., Good, K. P., Chartrand, D. T., Scarr, T. A., & Thompson, D. G. (2008). Effects of insecticide-treated wood on emergence and development of aquatic insects. Environmental Toxicology and Chemistry, 27(3), 645-654.
Mertens, M., Höss, S., Neumann, G., Afzal, J., & Reichenbecher, W. (2018). Glyphosate, a chelating agent—Relevant for its activity in plants and soil? Journal of Experimental Botany, 69(11), 2539-2563.
Mili, C., Kalita, S., & Roy, S. (2022). Microbes as a potential bioremediation tool for atrazine-contaminated soil: A review. Journal of Applied Biology & Biotechnology, 10.
Udikovic-Kolic, N., Scott, C., Martin-Laurent, F., & van der Meer, J. R. (2012). Evolution of atrazine-degrading capabilities in the environment. Applied and Environmental Microbiology, 78(14), 4862-4869.
Van Bruggen, A. H., He, M. M., Shin, K., Mai, V., Jeong, K. C., Finckh, M. R., & Morris, J. G. (2018). Environmental and health effects of the herbicide glyphosate. Science of The Total Environment, 616, 255-268.
Zobiole, L. H. S., Bonini, E. A., de Oliveira, R. S., Kremer, R. J., Ferrarese, L. F. M., & Ferrarese-Filho, O. (2010). Glyphosate affects chlorophyll, nodulation, and nutrient accumulation of glyphosate-resistant soybean. Journal of Agricultural and Food Chemistry, 58(7), 4517-4522.