Isotope Tags Track What Happens to Plastic Left in Farm Soil

Polyethylene microplastic buried in agricultural soil for 22 months lost 0.12% of its carbon to microbial breakdown, according to a paper published Sept. 1 in the journal Microplastics and Nanoplastics by Hannah Forsyth and six co-authors.

The team mixed 37.5 milligrams of carbon-13-labeled polyethylene powder into 100 grams of soil from an agricultural field near Dijon, France, sealed it in bottles and pumped CO2-scrubbed air through them for 672 days. The heavy isotope let them distinguish CO2 released by the plastic from CO2 released by the soil itself. After 22 months, 0.12% of the added plastic carbon had been converted to CO2, they report.

Before the incubation, the polyethylene was exposed to UV light equivalent to about 130 days of sunlight in central Europe, the paper's stand-in for mulch film weathered on a field surface. Eight setups ran in parallel, four with plastic and four without, sampled every 28 days.

The rate fell as the experiment ran. Extrapolated to a full year, the rate implied by the first four weeks of incubation was 0.32% a year, falling to 0.03% a year by the final four weeks. The faster early phase was due to microbes consuming the UV-damaged surface of the particles first.

Fitting linear and exponential models to the 672 days of data, the authors roughly estimate the half-life of polyethylene microplastic in soil at between 600 and 2,250 years. That is a projection forward from a 22-month experiment rather than a measurement, and the paper presents the modeling as an illustration of how hard the rate is to extrapolate.

Micro- and nanoplastic particles were also found in pores inside soil aggregates 1 to 2 millimeters across, and 0.066% of the added plastic carbon was measured in soil microbial biomass at the end. The authors write that the results "demonstrate the high persistence of polyethylene in agricultural soils and its gradual physical integration into soil structures."

Carbon-13 labeling had been used on plastic in marine and lake water, and on biodegradable polymers in soil, but not on a conventional plastic such as polyethylene in soil.