Invisible but pervasive, microplastics are washed into oceans and blown through the air. Now, scientists have uncovered that microplastics change agricultural crop growth.
Here's what we know: Most of these particles are fibers (up to 92 percent). The remainder are generally fragments (around 4 percent). Both types are secondary microplastics, meaning they come from larger plastic waste breaking down in the environment. Primary microplastics are small beads or pellets generally produced for industrial applications. These could be accidentally released, but at this time are not the primary source of microplastics in the environment.
A new study released last week explores the impact of microplastics on terrestrial systems, specifically agriculture. The researchers were interested in changes in the soil, microbes and plants exposed to microplastics.
The researchers added different types of microplastics into the soil: polyamide beads (a primary microplastic), polyester fibers (the most common type of secondary microplastic) and four different plastics in the form of fragmenta (another type of secondary microplastic). They added microplastics at a similar concentration to that of soils exposed to high levels of human activity. Then, they grew spring onions in soil with and without the different microplastic types. Here's what the scientists found:
Plant growth, as well as microbial activity, can be enhanced by the presence of microplastics, particularly polyester fibers.
The plastic fibers are by far the most common type of microplastic in soils. Both the polyamide beads and polyester fibers increased general microbial metabolic activity. Polyester fibers enhanced the colonization of the spring onions by beneficial fungi, called arbuscular mycorrhizal fungi. These fungi exchange nutrients with their host plant in return for sugars and other carbon compounds the fungi need to grow. When plastic fibers were in the soil, there were more of these structures used to exchange compounds between the fungi and plant. Moreover, water availability in the soil was higher when it was treated with microplastics, though this effect was reduced by the presence of plants.
Microplastics similar in size, shape and composition to soil have minimal to no effect on crops.
One of the six microplastic types tested, the fragments—all four types—did not change microbial activity or plant growth. The fragments were similar in size and shape to the natural soil particles where the study was conducted. Additionally, the chemical composition of the four plastics used to make the fragments were carbon and hydrogen. These elements aren't limited in the environment and therefore don't increase plant growth when more is added. In contrast, nitrogen is a limiting nutrient for plants and is present in polyamide beads, a primary plastic. Further supporting this idea, the researchers observed higher concentrations of nitrogen in plant tissue growing in soil with polyamide beads, which is a plastic polymer that does contain nitrogen.
The scientists are hesitant to say that microplastics increase the overall growth of plants. Further research on additional types of microplastic tested with different plant species and ecosystems is needed. They caution that "pervasive microplastic contamination in soil may have consequences for plant performance, and thus for agroecosystems and terrestrial biodiversity".