Q: I have questions on the breakdown of plastic in the soil, and its effect on soil microbe life and function. I’ve read that plastic is non-biodegradable, lasts forever unless broken down by sunlight, and that it inhibits microbial life. Many county landfills are giving away mulch made of chopped yard waste. This mulch also contains small pieces of chopped plastic, which comes mainly from the plastic bags that residents use to transport leaves, twigs, etc.
As soil is in need of organic material enrichment, I would like to use this mulch, but not at the expense of killing microbes: the life of the soil. Does plastic affect microbes in soil? Also do plants absorb plastic chemicals, which means I should not use this on vegetables, fruits?
A: To my knowledge, there has not been any specific research to measure the impact of the plastic contaminants in compost on soil microbes.
What I do know is that scientists have studied the impact of antimicrobial compounds in municipal biosolids on soil microbes. These antimicrobial compounds–namely the chemicals triclosan and triclocarban–were formulated to kill any and all bacteria. You can find these chemicals in many household products that advertise that they are “antimicrobial.” Examples include a range of soaps and certain brands of toothpaste. Because we use these products in our homes, they enter the municipal wastewater system. Biosolids are the nutrient-rich byproduct of wastewater treatment and are often used to fertilize crops like wheat and corn. They are present in biosolids as small chemicals that are often attached to the organic matter in the biosolids.
Scientists tested to see if these antimicrobial compounds would harm soil bacteria. They found that the benefits of biosolids–which are very high in food for microbes–strengthened the soil microbes much more than the antimicrobial compounds hurt them. Total populations of microbes, respiration, and nutrient cycling were all measured.
Compost will be very similar to biosolids in that it provides food for soil microorganisms, which as you say are the life of the soil. It’s almost certain that the benefits of adding compost will far outweigh any negative effects from the plastics. There is one other thing to consider: Plastic contaminants will be enormous compared to the microbes. That means the microbes won’t try to eat the plastic–it’s like a mouse trying to eat an elephant. This is another reason plastic pieces are unlikely to hurt soil bacteria.
To find research papers on this, search for O’Connor at University of Florida. He has done a lot of research on triclosan and triclocarban with other authors.
–Answered by Sally Brown, University of Washington
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