Highway Maintenance “Drives” Carbon Release in Forests

Featured Image: Forest and highway between Trójmiasto and Gdynia, Northern Poland. Image courtesy Robin Hammam.

Paper: The proximity of a highway increases CO2 respiration in forest soil and decreases the stability of soil organic matter

Authors: Dawid Kupka, Mateusz Kania, Piotr Gruba

There has been a lot of talk about transportation as of late with America’s “Build Back Better Act”.  While these political decisions are partially informed by scientific research around climate change, particularly in the United States (where 30% of greenhouse gas emissions result from transportation by road, rail, and air each year), the negative impacts of transportation infrastructure on the climate and local ecosystems are often lost in political discussions.  In a new study in Scientific Reports, Kupka and colleagues discuss the broader impacts of highway maintenance on nearby forest soil ecosystems, finding that roadways themselves can increase carbon dioxide emissions by disrupting local carbon cycles.

Soil around the globe holds more carbon than the atmosphere.  Forest soils are especially unique as they support a complex carbon cycle between established evergreens, herbaceous plants, animals, and massive amounts of detritus.  Kupka, Kania, and Gruba defined how roadways disrupt this balance through inputs of carbon pollution and runoff of deicer in colder environments, leading to the release of carbon from the forest soil in the form of the greenhouse gas, carbon dioxide (CO2).

Forest soils hold onto carbon compounds in the form of complex and large molecules folded up like a tight knot of string; these carbon compounds are known as soil organic matter.  These molecules appear as an indigestible lunch for micro-organisms living in the soil and the root systems of plants; such micro-organisms are unable to untangle the “string”, so that the soil organic matter remains stored in the soil.  However, if something were to untangle the knot for the micro-organisms, they would be awarded a nice meal of carbon, and, like us humans, would exhale CO2 as they digest.  The balance between knotting and untangling these complex carbon molecules are mediated by several factors including geographical location of the forest, weather, the types of plants and animals, and more.  So how exactly can the addition of a highway alter this balance and lead to the production of CO2 from the organic matter in the forest soil?

Kupka and colleagues addressed this question by sampling forest soil around and away from the S7 highway in central Poland.  In the lab, they trapped any CO2 present in the soil using acids and bases and measured all the carbon present as it related to salts, metals, and complex compounds.  Using the ratios of total carbon, CO2, and all other compounds in the soil, they were able to identify the potential chemical interactions from the soil organic matter all the way to the eventual production of CO2 after micro-organism digestion.  It became clear that close proximity to the highway greatly increased CO2 release.  The cause is likely the deicer solution added to the highway which leech salts from the road surface into the surrounding soils.  In the soils, the salts increase the pH and alter a major factor in the balance of soil organic matter from stable knots to freely untangled carbon available for metabolism by micro-organisms.

This research bolsters the body of knowledge surrounding the environmental effects of current infrastructure.  A simple addition of salts to a roadway has unintended yet clear effects on the surrounding regional and global carbon cycle, as it causes more greenhouse gases to be released from originally stable carbon.  Research into alternative and updated infrastructure practices directly and immediately mitigate these consequences, allowing our societies to better combat climate change.

Highway Maintenance “Drives” Carbon Release in Forests by Jessica Buser-Young is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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