Plastic in the soil, but not as we know it: Biodegradable microplastics rewire carbon storage in farm fields

We often think of plastic pollution as a problem of oceans and seabirds. But beneath our feet, in the quiet dark of agricultural soils, a new kind of contamination is unfolding—one with profound implications for climate, crops, and carbon.

A pioneering two-year field study has revealed that biodegradable microplastics, often hailed as eco-friendly alternatives to conventional plastics, are quietly reshaping the chemistry of farmland soils in unexpected and complex ways. Published on August 22, 2025, in Carbon Research as an open-access original article, this research was co-led by Dr. Jie Zhou from the College of Agriculture at Nanjing Agricultural University, China, and Dr. Davey L. Jones from the School of Environmental and Natural Sciences at Bangor University, UK—a powerful Sino-British collaboration bridging soil science, microbiology, and climate resilience. The team investigated how polypropylene (PP)—a common conventional plastic—and polylactic acid (PLA)—a widely used biodegradable plastic—affect soil organic carbon (SOC) in real-world agricultural conditions. Both were added at realistic concentrations (0.2% w/w) to topsoil (0–20 cm), with an unamended plot serving as control. While neither plastic changed the total amount of carbon stored, the story beneath the surface was dramatically different.

The Biodegradable Paradox: Green Plastic, Complex Consequences

The surprise? PLA—the “eco” plastic—had the strongest impact on carbon composition.

It reduced plant-derived lignin in the soil by 32%, meaning fewer stable carbon compounds from roots and crop residues were being preserved. Why? Because PLA attracted a special group of microbes known as K-strategists—slow-growing, efficient decomposers that specialize in breaking down tough, carbon-rich materials like lignin. “These microbes see PLA as a feast,” explains Dr. Zhou. “But in doing so, they also ramp up enzymes that degrade other stubborn carbon compounds, including those that help lock carbon away long-term.” Yet PLA also boosted microbial necromass—the dead remains of bacteria and fungi—by 35%, a key but often overlooked pathway for carbon storage. This boost came from increased microbial diversity (+5.3%) and more complex microbial networks (+11%), creating a richer, more resilient soil ecosystem. Even more striking: fungal necromass became the dominant player, contributing 24% to SOC under PLA, compared to just 11% with conventional PP. Fungi, it turns out, thrive on PLA and help glue soil particles into stable macroaggregates, physically protecting carbon from decomposition.

The Nitrogen Trap: When Biodegradable Plastics Starve Microbes

But there’s a catch. PLA is rich in carbon but poor in nitrogen—an imbalance that triggers microbial nitrogen limitation. To survive, soil microbes began breaking down their own kind—specifically bacterial necromass, which dropped by 19%. The evidence? A strong negative correlation between bacterial remains and enzymes that scavenge nitrogen from the soil. “In trying to adapt to PLA, microbes start cannibalizing their own biomass,” says Dr. Jones. “It’s a survival strategy, but it could undermine long-term soil fertility and carbon stability.”

Conventional Plastic: A Different Kind of Damage

Meanwhile, polypropylene (PP) told a different story. Rather than altering microbial behavior, it suppressed microbial growth through carbon deprivation and the leaching of toxic additives. This led to reduced synthesis of necromass overall, weakening one of soil’s main carbon storage engines. “PP doesn’t feed the soil—it starves it,” says Dr. Jones. “It’s like putting a blanket over a garden: nothing grows underneath.”

Why This Matters for Climate and Farming

Soil is the second-largest carbon reservoir on Earth—bigger than all the world’s forests combined. How carbon is stored—whether from plants or microbes—determines how long it stays out of the atmosphere. This study shows that even biodegradable plastics can disrupt this delicate balance, shifting carbon storage from plant-based to microbial-based forms, with uncertain long-term consequences. “We can’t assume ‘biodegradable’ means ‘benign’,” warns Dr. Zhou. “In soil, these materials interact with living systems in complex ways we’re only beginning to understand.”

A Triumph of International Soil Science

The collaboration between Nanjing Agricultural University and Bangor University exemplifies the power of global science to tackle pressing environmental challenges.

Dr. Zhou’s expertise in soil biogeochemistry and Dr. Jones’ leadership in microbial ecology have produced one of the most detailed field assessments to date of microplastic impacts on carbon dynamics. The College of Agriculture at Nanjing Agricultural University continues to lead in sustainable agriculture research in China, while Bangor University’s School of Environmental and Natural Sciences remains at the forefront of ecosystem studies in the UK.

The Bottom Line: Rethinking the Future of Farm Plastics

Mulching films, seed coatings, irrigation tapes—plastics are deeply embedded in modern agriculture. As farmers shift toward biodegradable options to reduce pollution, this study serves as a crucial reality check. “Biodegradable plastics aren’t a silver bullet,” says Dr. Zhou. “We need to design them not just to break down, but to break down in ways that support, not disrupt, soil health.” The findings call for smarter regulations, better material design, and a deeper understanding of soil as a living system—not just a growing medium. So the next time you hear “biodegradable,” remember: in the soil, the truth is buried deeper than the plastic itself. And thanks to the work of Dr. Jie Zhou, Dr. Davey L. Jones, and their team, we’re one step closer to uncovering it.

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• Title: Biodegradable microplastics decreased plant-derived and increased microbial-derived carbon formation in soil: a two-year field trial
• Keywords: Microplastic; Soil organic carbon; Plant lignin; Microbial necromass; Microbial life strategy
• Citation: Guo, X., Zhang, W., Lu, Y. et al. Biodegradable microplastics decreased plant-derived and increased microbial-derived carbon formation in soil: a two-year field trial. Carbon Res. 4, 61 (2025). https://doi.org/10.1007/s44246-025-00231-7 

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About Carbon Research

The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.

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