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Microplastics Influence Early Carbon Mineralization and Residue Decomposition Without Altering Net Nitrogen Mineralization
Summary
Microplastics from farm plastic mulch are building up in soil, and this study found they can temporarily slow down how quickly soil microbes break down plant matter and release nutrients like carbon and nitrogen. This matters because if nutrients don't become available when crops actually need them, it could affect soil health and how well crops grow over time — meaning the plastics we use to boost farming today might create hidden headaches for food production down the road.
Plastic film mulching is widely used in agriculture to improve crop productivity, but it leads to the accumulation of microplastics (MP) in soils, with poorly understood consequences for carbon and nutrient cycling. This project examined how MP influence carbon (C) and nitrogen (N) mineralization of organic substrates of differing quality. In a controlled 30-day soil incubation at 22°C, soils were treated with polyester (PE) or polypropylene (PP) microplastic fibers (1000 – 5000 µm) and incubated with green tea (GT; high-quality residue) or rooibos tea (RT; low-quality residue). Plastic type significantly affected respiration 24 hours into incubation (p=0.001), with PP reducing rates in GT (p=0.003) and RT (p=0.037) compared to soils without MP or with PE. An interactive effect between plastic and substrate was found 4 days into incubation (p< 0.001) with PE and PP increasing respiration in only in GT (p< 0.001). Plastic type had no effect on respiration during late incubation (p=0.232). A significant interaction occurred in residue mass loss (p=0.012), where GT exhibited lower mass loss when treated with PE (p=0.004) and PP (p=0.006) compared to no MP. Post-incubation total soil C was significantly higher in plastic-amended soils (p=0.002), while net N mineralization was unaffected (p=0.053). These findings suggest that microplastics induce transient disruptions in microbial activity and slow the physical decomposition of high-quality residues. This may lead to a mismatch between nutrient release and crop demand, indicating that long-term residue management strategies may require adjustment in soils with a legacy of plastic mulching.