The international research, published in Carbon Research, was co-led by Dr. Jie Zhou from Nanjing Agricultural University and Dr. Davey L. Jones from Bangor University. Their team compared the effects of polypropylene (PP) and polylactic acid (PLA) microplastics, added to topsoil at realistic field concentrations (0.2% w/w). While total soil carbon levels remained steady, the underlying chemistry and microbial dynamics shifted dramatically.
PLA, the biodegradable plastic, caused a 32% decline in plant-derived lignin - the stable carbon compounds typically preserved in soil - by attracting slow-growing microbes that decomposed complex organic matter. However, it also boosted microbial necromass, the dead remains of microbes, by 35%, enhancing one of soil's most important long-term carbon storage pathways. Fungal residues became dominant, contributing 24% to total soil carbon compared with 11% in PP-treated soils, helping stabilize soil structure.
Yet the benefits came with trade-offs. PLA's high carbon but low nitrogen content triggered microbial nitrogen starvation, forcing microbes to recycle their own biomass - reducing bacterial necromass by 19%. "In trying to adapt to PLA, microbes start cannibalizing their own biomass," said Dr. Jones. "It's a survival strategy, but it could undermine long-term soil fertility and carbon stability."
By contrast, conventional PP microplastics suppressed microbial growth altogether, starving soil ecosystems of energy and reducing carbon cycling efficiency. "PP doesn't feed the soil - it starves it," noted Dr. Jones.
The findings underscore that biodegradable does not mean harmless. "These materials interact with living systems in complex ways we're only beginning to understand," said Dr. Zhou. "We must design biodegradable plastics that support soil health, not compromise it."
With agricultural plastics increasingly replacing traditional materials, the study highlights the urgent need for new standards and smarter designs that consider soil as a living ecosystem, not merely a substrate for crops.
Research Report:Biodegradable microplastics decreased plant-derived and increased microbial-derived carbon formation in soil: a two-year field trial
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