Chinese Study Finds Clay Minerals and Soil Depth, Not Sunlight Alone, Control Mulch Film Breakdown
14 November 2026, Baoding, CN: A laboratory study by researchers in China and Australia has found that the speed at which biodegradable plastic mulch film breaks down in farm soil depends heavily on the type of clay mineral present and how deep the film is buried, not primarily on sunlight exposure as had generally been assumed, raising new questions about how predictably these films behave once plowed into different soils.
Biodegradable mulch film is thin plastic sheeting laid over soil to suppress weeds and hold in moisture. It has been promoted as a lower-residue alternative to conventional polyethylene film because it is designed to break down in the ground rather than requiring removal after harvest. Two materials dominate the market: PBAT, or polybutylene adipate terephthalate, and PLA, or polylactic acid. Both are marketed as biodegradable, but how quickly and completely they actually break down under real farming conditions has been poorly understood.
The study, published online October 12 in the open-access journal New Contaminants, a title produced in partnership with Shenyang Agricultural University, was led by corresponding author Wei Liu of Hebei University’s College of Chemistry and Materials Science in Baoding. Co-authors are affiliated with the Agricultural University of Hebei, the Chinese Academy of Agricultural Sciences and Deakin University in Australia.
How the plastics were tested
The researchers cut small squares of PBAT and PLA film and exposed them to ultraviolet light while suspended in water mixed with one of two common soil clay minerals, kaolinite or montmorillonite, sampling the films at 0, 15, 30, 45 and 60 days. In a separate experiment, they placed film either on the surface or buried 10 centimeters deep in sterilized soil for up to 60 days, a step designed to isolate the effects of soil minerals and moisture from biological activity such as microbial action. They tracked chemical changes using electron spin resonance, a technique that detects short-lived reactive molecules called free radicals, and liquid chromatography-mass spectrometry, which identifies the specific breakdown products formed.
Under ultraviolet exposure, PBAT degraded roughly twice as fast as PLA, reaching a comprehensive aging index, a composite score the researchers used to track physical and chemical breakdown, of 24.02 compared with 12.98 for PLA. Clay minerals made a significant difference: with kaolinite present, PBAT lost about 10.7 percent of its weight under UV light over the test period, compared with roughly 4.8 percent under UV light alone, because kaolinite absorbs light more efficiently and generates more reactive oxygen species, mainly hydroxyl radicals, that attack the polymer’s chemical bonds. Montmorillonite produced a smaller effect than kaolinite in the same tests. Soil minerals in general, dominated by silica and aluminum oxide, also sped up aging of the films independent of the specific clay type.
The most counterintuitive result came from the burial experiment. Even though ultraviolet light barely penetrates 10 centimeters of soil, buried film degraded more than film left on the surface, pointing to soil moisture and mineral chemistry, rather than sunlight, as the dominant force driving breakdown underground. Chemical analysis showed the breakdown occurred mainly through two pathways: cleavage of ester bonds, the chemical links holding the polymer chain together, and oxidation, both of which produced smaller molecules including oligomers, carboxylic acids and alcohols.
The breakdown process also released additives built into the plastic during manufacturing, including antioxidants and stabilizers. The researchers ran computational toxicity screening on the degradation products and flagged some as having potential ecological toxicity, though they did not quantify the scale of that risk. Liu, the corresponding author, said the environmental behavior of these plastics “is far more complex than simply disappearing into the soil,” and that faster degradation “does not necessarily mean complete environmental safety.” The team called for longer field studies tracking chemical migration and ecological risk under real farming conditions.
The findings carry commercial weight because biodegradable mulch film use is expanding as China, the European Union and other jurisdictions tighten restrictions on conventional polyethylene film, which leaves persistent plastic fragments in farmland. Manufacturers have marketed PBAT and PLA blends as a cleaner replacement, but this study suggests degradation rates cannot be assumed to be uniform across regions, since clay mineral content and soil depth vary widely between farms. The authors noted that earlier research found PBAT mineralized by only 0.9 percent to 16 percent after 120 days in soil, underscoring how much remains unknown about the long-term fate of these materials once they are plowed under.
Also Read: Crystal Crop Protection to Launch Rs 455 Crore IPO on November 18
Global Agriculture is an independent international media platform covering agri-business, policy, technology, and sustainability. For editorial collaborations, thought leadership, and strategic communications, write to pr@global-agriculture.com
