Editorial NexoScience
If you have ever walked past a vegetable field wrapped in plastic mulch, dug into compost that once held a synthetic teabag, or washed a fleece jacket that shed fibers into the wastewater, you have already contributed a few grains of microplastic to soil somewhere on the planet. These tiny fragments, generally smaller than five millimeters, have become one of the most persistent forms of pollution on Earth, and scientists have spent the last several years trying to understand what they do once they end up underground.
Soil already holds three times more carbon than the entire atmosphere. That is why even small shifts in how soil handles carbon can ripple out into the planet’s broader climate math. A team of researchers pulled together data from 110 peer reviewed studies, covering 580 paired comparisons between soil exposed to microplastics and soil that was not, to figure out whether this quiet contamination is nudging that balance in one direction or another.
The headline number looks encouraging at first glance. Across a concentration range from barely detectable traces up to 280 grams of plastic per kilogram of dry soil, exposure to microplastics raised soil organic carbon by an average of 25.62% and dissolved organic carbon by 11.92%. Microbial biomass carbon rose by 11.40%, microbial biomass nitrogen by 19.09%, and root biomass climbed by 5.15%. On paper, that reads like plastic contamination is somehow helping soil bank away more carbon.
There is a catch buried in the chemistry. Plastic itself is almost entirely made of carbon, in some types as much as 99.99% by weight. When researchers measure soil organic carbon in a sample laced with plastic fragments, part of what they are detecting is simply the plastic itself, not new carbon that soil biology has captured and stabilized. The authors are candid about this limitation, noting that future work needs better methods to separate carbon that soil naturally holds from carbon that arrived stuck inside a piece of debris.
The plants growing in that soil are not benefiting from any of it. Net photosynthetic rate dropped by 14.44% in microplastic exposed soil, and the effect grew stronger as plastic concentration increased. Bulk density fell by 2.01% and the stability of soil aggregates, the clumps that keep soil structure intact and resist erosion, dropped by 16.15%. Soil invertase, an enzyme microbes rely on to break down carbohydrates, fell by 12.94%. None of these are signs of a healthy soil system, even if the raw carbon numbers look positive.
The type and size of the plastic changes the story further. Polyethylene, polypropylene and polystyrene fragments drove the strongest increases in soil organic carbon, while polyethylene terephthalate and polyvinyl chloride barely moved it. Particles between 50 and 150 micrometers in size boosted carbon measurements the most, while pieces larger than 150 micrometers still raised organic carbon readings but suppressed soil respiration, the process by which soil releases carbon dioxide back into the air. Biodegradable plastics, somewhat counterintuitively, produced a bigger jump in dissolved organic carbon than conventional ones, since microbes can actually break them down and use the carbon they release.
None of this means dumping plastic into farmland is a viable carbon strategy. The researchers are explicit that their findings should not be read as an endorsement of careless plastic disposal. What they describe instead is a measurement problem tangled up with a real ecological one: soil that looks like it is storing more carbon may simply be storing more plastic, while quietly losing the structural and biological health that lets it support plant life in the first place.
The study, “Global Responses of Soil Carbon Dynamics to Microplastic Exposure: A Data Synthesis of Laboratory Studies,” was led by Yangzhou Xiang, Matthias C. Rillig, Josep Peñuelas and colleagues, and published in Environmental Science & Technology in 2024. It is available under a CC BY-NC-ND open access license, DOI 10.1021/acs.est.3c06177.
