Technique tracks carbon as soil microbes munch plastic

Read the full story in Chemical & Engineering News.

To keep weeds at bay and retain soil moisture, farmers apply plastic “mulch” to their fields. But these thin polymer films are tough to collect again, and accumulating plastic can reduce soil fertility. Biodegradable plastic mulches avoid those issues and are gaining prominence. Researchers have now developed what some experts say is the most comprehensive method yet for tracking where carbon from one of these polymers goes in soil.

To distinguish polymer carbon from soil carbon, Michael Sander and colleagues at ETH Zurich used a mulch film, poly(butylene adipate-co-terephthalate), with 13C at certain spots within the polymer. They filled glass bottles with polymer samples and 60 grams of soil. Over six weeks, the team followed release of isotope-labeled CO2—a sign that soil microbes had metabolized the polymer—with cavity ring-down spectroscopy. They tested different polymer samples with isotope labels in each of three different subunits (shown). The microbes broke down all the polymer components, but the rate of isotope-labeled CO2 release differed depending on where the label was within the polymer. On the basis of NMR studies, the team concluded that the difference was largely due to different rates of enzyme hydrolysis for the various polymer subunits (Sci. Adv. 2018, DOI: 10.1126/sciadv.aas9024).

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