Linking Topographical Ring Features to Geochemical and Geophysical Anomalies

Abstract

Circular features in forests seen from air have been studied for several decades at different locations around the world. Forest rings, as they are called in Canada’s boreal forests, express several geochemical (pH, carbonate content) and geophysical (surface potential) anomalies on their 20–30 m wide ring edges. Although it has been proposed that microbial processes may cause these anomalies, the exact mechanisms of ring formation are still unknown. We focused on the Thorn North forest ring in Ontario, Canada to correlate the surface potential anomaly to soil gas concentrations. Field measurements showed that the surface potential drop at the ring edge center is framed by peaks in CO₂ production, which is linked to O₂ depletion and methane generation. Carbon isotope signatures were found to drop to lighter values (down to −20‰), suggesting increased respiration. Higher concentrations of uronic acids bound to extracellular polymeric substances were found, indicating that the surface potential anomaly is linked to respiration. 16S rRNA gene sequencing of shallow soil did not indicate a dominant microbial group on the edges; instead, principal component analysis showed that the microbial composition was controlled by the substrate (clayey vs. sandy soil), therefore future studies should focus on deeper ground layers.

Keywords:

• Boreal forests
• forest rings
• soil microbiology
• soil respiration
• surface potential

Acknowledgments

We appreciate scientific advice given by Cheng Zhong, Barbara Sherwood Lollar, and Hamnah Majeed. The authors received the support of Kayla Dell, Kevin Ho and Andrew Hicks in the field.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The Ontario Geological Survey provided financial support and facilitated site access. Financial support was also provided by the Natural Sciences and Engineering Research Council of Canada (Discovery grant to DSA: RGPIN-2020-05289 and Discovery grant to MD: RGPIN-2020-06184), the Faculty of Graduate Studies and Research of the University of Alberta (travel grant to KvG).