"Forever chemicals" create boom-and-bust cycle in soil, disrupting global carbon processes

New research from the Wuhan University of Technology reveals the complex and contradictory effects of perfluoroalkyl substances (PFAS), commonly known as "forever chemicals," on soil ecosystems. A team led by authors Yulong Li and Lie Yang demonstrated that contaminants PFOA and PFOS trigger a dramatic two-phase response in soil. Initially, the chemicals stimulate a rapid release of carbon, but this is followed by a prolonged period of suppression, posing significant questions about the long-term health of contaminated soils and their role in the global carbon cycle.

The widespread presence of PFOA and PFOS in the environment is a growing concern due to their persistence and bioaccumulation. While many investigations have focused on their distribution and toxic effects on plants and animals, their influence on the fundamental geochemical processes within soil has been less understood. This inquiry sought to determine how these specific contaminants alter the mineralization of soil organic carbon (SOC), a vital process where microorganisms break down organic matter and release carbon, which influences both soil fertility and atmospheric carbon dioxide levels.

A Six-Month Soil Simulation

To examine the dynamic impacts of these chemicals, the scientists conducted a detailed 180-day laboratory culture experiment. Soil samples were prepared and treated with varying concentrations of PFOA and PFOS. Over the course of the incubation, the team meticulously measured changes in the soil organic carbon mineralization rate, the availability of dissolved organic carbon (DOC), soil enzyme activities, and the composition of the soil's bacterial and fungal communities. This controlled approach allowed for a precise observation of how the soil ecosystem responded over both short and extended timescales.

The results showed a distinct temporal shift. In the first 30 days, the presence of PFOA and PFOS created a positive priming effect, significantly increasing the rate of SOC mineralization by as much as 127%. This initial burst was driven by an accelerated consumption of the most accessible carbon sources in the soil, particularly DOC and its component hexose (sugars). The contaminants appeared to invigorate microbial activity, increasing the demand for carbon and energy and thereby quickening the decomposition process and carbon release.

From Abundance to Scarcity

This period of heightened activity was not sustainable. After 90 days, the trend reversed. Having exhausted the readily available DOC, the soil's carbon mineralization rate fell dramatically, dropping by 58% to 65% in the contaminated samples compared to the control group. The initial surge in microbial consumption led to a long-term nutrient deficit, effectively starving the microbial populations and inhibiting their ability to process organic matter. This finding was confirmed when researchers supplemented the soil with glucose, which promptly restored respiration rates, indicating that the microbes were limited by the lack of available carbon, not by direct chemical toxicity in the long run.

"Our findings reveal a deceptive paradox: while PFOA and PFOS appear to stimulate soil activity initially, they do so by rapidly depleting essential, bioavailable carbon," states corresponding author Lie Yang. "This 'boom-and-bust' cycle ultimately inhibits the soil's long-term ability to process carbon, with potentially significant consequences for nutrient cycling and ecosystem stability. It changes our understanding of how these persistent pollutants affect soil health on a functional level."

The investigation also identified profound shifts in the soil's microbial landscape. Both PFOA and PFOS altered the structure of bacterial and fungal communities. For instance, the relative abundance of Actinobacteria, a phylum critical for decomposing organic matter, was notably reduced in contaminated soils. Concurrently, other phyla like Proteobacteria and Acidobacteria, which are associated with polysaccharide degradation, became more dominant. These changes reflect a functional reorganization of the soil microbiome as it adapts to the chemical pressures and altered nutrient availability.

The conclusions drawn from this laboratory-scale experiment offer a critical window into the potential ecological damage caused by PFOA and PFOS contamination. The authors note that while these controlled conditions provide clear mechanistic insights, the next essential step is to conduct large-scale field experiments. Such studies are needed to confirm these findings in a more complex, natural environment and to fully comprehend the implications for global soil carbon pools and climate dynamics.

Corresponding Author: Lie Yang

Original Source: https://doi.org/10.1007/s44246-023-00088-8

Contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by [Yulong Li], [Bowei Lv] and [Zhendong Chen]. The first draft of the manuscript was written by [Yulong Li] and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.