Antibiotic resistance is a growing global health concern, and soils are now recognized as major environmental reservoirs of resistance genes. In modern cropping systems, widespread use of plastic mulch films fragments into microplastics that change soil microbial communities and can promote persistence and horizontal transfer of antibiotic resistance genes.
Large amounts of crop straw are generated each year in major grain-producing regions, and open burning, though officially restricted, still occurs in many areas. This practice leaves black carbon rich ash in surface soils, often in the same fields where plastic mulch fragments accumulate, but the combined influence of these residues on antibiotic resistance dynamics has until now remained unclear.
A study published on 18 November 2025 in the journal New Contaminants by Fei Wang and colleagues at Beijing Normal University examined how black carbon from straw burning interacts with residues from conventional and biodegradable plastic mulch films in soil crop systems. The team focused on intensive farming conditions where plastic mulching and straw residues commonly coexist.
The researchers set up a combined soil incubation and soil soybean pot experiment using two mulch types, conventional polyethylene and a biodegradable plastic, along with two black carbon scenarios representing direct addition and in situ straw burning. Over a three month period covering key soybean growth stages, they measured plastic aging, soil physicochemical properties, enzyme activities, microbial community structure, and the abundance, mobility, and microbial hosts of antibiotic resistance genes and mobile genetic elements in bulk soil, rhizosphere soil, rhizoplane, phyllosphere, and seeds.
Soil burial roughened both polyethylene and biodegradable film surfaces through abrasion, while straw burning caused rapid thermal deformation and more intense perforation of the plastic. The biodegradable film showed stronger surface roughening under black carbon addition due to preferential biodegradation, whereas thinner polyethylene was more susceptible to heat damage.
Spectral analyses indicated oxidation and aging of polyethylene and surface degradation of the biodegradable plastic, with straw burning exerting stronger effects than black carbon addition alone. These physical changes were accompanied by shifts in soil chemistry and enzyme activities linked to nutrient cycling.
In soils with polyethylene residues, pH tended to decrease while nitrate and available phosphorus increased, whereas biodegradable films raised pH but lowered these nutrient levels. Black carbon altered both patterns and contributed phosphorus and potassium to the soil. Enzymes such as alkaline phosphatase, urease, peroxidase, and catalase responded in different ways depending on mulch type and black carbon treatment, reflecting changes in nutrient transformation processes.
Gene profiling showed that plastic mulch films alone increased soil antibiotic resistance gene abundance, with stronger effects under polyethylene. In contrast, black carbon consistently lowered resistance gene levels and in some cases cut abundances by nearly half, while also markedly suppressing transfer of these genes from soil into plant tissues.
During the reproductive stage of soybean growth, straw burning reduced the abundance of antibiotic resistance genes in leaves by more than 75 percent and in seeds by up to 80 percent. Network and multivariate analyses found that resistance genes and mobile genetic elements were closely associated with dominant bacterial groups including Proteobacteria, Firmicutes, Bacteroidota, and Actinobacteriota, and that habitat type was the main driver of resistance and mobile element patterns.
Although straw burning temporarily disturbed microbial diversity, soil communities recovered within three months in the experimental system. The results indicate that black carbon constrained the spread of antibiotic resistance genes without causing lasting harm to soil health or nutrient cycling.
The study concludes that black carbon generated from straw burning can act as a mitigating factor for antibiotic resistance risks in plastic mulched agroecosystems. By reducing resistance gene abundance in soils and limiting their movement into edible plant parts, black carbon may help lower the chance that these genes enter the food chain.
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