Common Fungicide Quietly Rewires Soil Bacterial Metabolism, Study Finds

A widely used agricultural fungicide appears to do more than kill fungal pathogens: it can substantially reshape the way soil bacteria metabolise carbon, according to a study published in the journal BioRisk. The research, led by scientists working with soils from the Sofia region of Bulgaria, examined how Quadris, a commercial formulation of the strobilurin fungicide azoxystrobin, affects bacterial communities in two contrasting soils — a grassland loamy sand and a cropland clay loam — neither of which had a known history of fungicide use. The findings add to growing evidence that pesticides designed to target fungi can ripple outward through soil ecosystems, touching organisms that are officially classified as non-targets.

Strobilurin fungicides are among the most heavily deployed crop protection chemicals in the world. First patented in 1996, azoxystrobin ushered in a family of related compounds — including pyraclostrobin, kresoxim-methyl and trifloxystrobin — that now protect cereals, fruits, vegetables, turf and ornamentals against an unusually broad spectrum of diseases, from downy and powdery mildews to rusts and blights. Their mode of action is precise: the molecules bind to the quinol oxidation site of cytochrome b, a key component of the mitochondrial respiratory chain, effectively shutting down fungal energy production. But because respiration is such a fundamental process, scientists have long worried that the chemicals could disturb non-target organisms, including the bacteria and archaea that drive nutrient cycling beneath our feet.

To probe this question, the team constructed laboratory mesocosms — 2-kilogram portions of each soil — and treated them with Quadris at four concentrations of the active ingredient azoxystrobin: a zero-dose control, 2.90 mg per kg, 14.65 mg per kg and 35.0 mg per kg. The mesocosms were maintained at 60 percent of their maximum water-holding capacity and incubated in darkness at 22 degrees Celsius for 120 days, preventing the breakdown of the fungicide by sunlight. Samples were then pulled at intervals of one, 30, 60, 90 and 120 days, allowing the researchers to track both the chemical fate of the fungicide and the metabolic response of the bacterial communities over time.

The centrepiece of the study was a technique called community-level physiological profiling, performed with Biolog EcoPlates. These microplates contain 31 different carbon sources, which the researchers grouped into five chemical guilds: carbohydrates, polymers, carboxylic acids, amino acids and amines/amides. By measuring how vigorously soil bacteria oxidised each substrate, the team could compute the average well colour development, or AWCD — a proxy for overall catabolic activity — and map the detailed physiological fingerprint of each community. The approach reveals not just how active the bacteria are, but which biochemical menus they prefer.

The results were striking. Across both soil types, Quadris depressed the overall catabolic activity of bacterial communities, and the decline persisted for at least two months before a partial recovery and, in some treatments, a late-stage stimulation by day 120. Notably, the magnitude of the drop in AWCD was largely independent of both soil type and fungicide concentration, suggesting a robust, systemic effect on bacterial metabolism. At the same time, the fungicide consistently suppressed the bacterial utilisation of amines and boosted that of amino acids in both soils, while its effects on carbohydrates, carboxylic acids and polymers depended closely on the specific soil environment and dose applied.

The two soils told clearly different stories about the fungicide’s environmental fate. Azoxystrobin dissipated far more slowly in the clay loam than in the loamy sand: the half-life ranged from 36.5 to 86.6 days in the sandy soil but stretched from 130.8 to 212.1 days in the clay-rich soil, classifying the compound as low to moderately persistent in the former and highly persistent in the latter. The researchers attribute this to differences in texture, pH and organic matter — the loamy sand was moderately acidic at pH 5.63, while the clay loam was near-neutral at 6.99 — which likely govern how bioavailable the chemical is to microorganisms. A more bioavailable fungicide, they reason, degrades faster but also exerts greater toxicity.

The fungicide also disturbed the soils’ chemistry. Applications lowered pH significantly in the loamy sand — by up to 14 percent at the highest dose — a change the authors link partly to the formation of azoxystrobin acid, the major degradation product, and possibly to other ingredients in the commercial formulation. Nitrogen pools shifted as well: nitrate concentrations rose overall in both soils, by as much as 70 percent at the low dose in the clay loam, while ammonium fell by 15 percent in the loamy sand and 39 percent in the clay loam. Because nitrification and mineralisation underpin nutrient turnover, such shifts could, the authors suggest, disrupt soil quality and productivity over the long term.

Perhaps the most subtle insight came from analysing metabolic diversity. Rather than losing the ability to use particular carbon sources outright — a change in

Subject of Research: Non-target effects of the fungicide Quadris (azoxystrobin) on bacterial functional profiles in loamy sand and clay loam soils

Article Title: Differences in bacterial functional profiles from loamy sand and clay loam textured soils under fungicide Quadris R impact

Article References: Petkova, M., Kenarova, A., Boteva, S., Georgieva, S., Chanev, C., & Radeva, G. (2022). Differences in bacterial functional profiles from loamy sand and clay loam textured soils under fungicide QuadrisR impact. BioRisk, 17, 31-43. https://doi.org/10.3897/biorisk.17.76939

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.76939

Keywords: azoxystrobin, strobilurin fungicides, soil bacteria, soil health, CLPP, Biolog EcoPlates, soil microbiology, pesticide non-target effects, nutrient cycling, soil texture, catabolic activity, mesocosm experiment