Ancient charcoal kilns reveal how biochar changes forest soils over decades
Study shows that long-buried charcoal in Mediterranean forests becomes chemically transformed, interacts with local geology, and may support beneficial soil microbes
image: Biochar aging, soil microbiota and chemistry of charcoal kilns in Mediterranean forests
Credit: Giuseppina Iacomino, Mohamed Idbella, Luigi di Costanzo, Giandomenico Amoroso, Emilia Allevato, Ahmed M. Abd-ElGawad & Giuliano Bonanomi
Old charcoal kilns hidden in Mediterranean forests may offer a natural window into one of today’s most discussed soil technologies: biochar. A study published in Biochar shows that charcoal left in forest soils for decades undergoes clear chemical aging, develops oxygen-rich surface groups, and helps create distinctive soil microsites shaped by both biology and geology.
Biochar, a carbon-rich material produced when plant biomass is heated under limited oxygen, is widely studied for its potential to improve soils, retain nutrients, support plant growth, and store carbon. But many biochar studies rely on short-term experiments or artificially aged materials. Historical charcoal kilns provide a rare real-world model for understanding what happens when biochar-like material remains in soil over long periods.
Researchers studied two abandoned charcoal kiln platforms in Mediterranean beech forests in Southern Italy, located at Mount Gelbison and Mount Vesole. The sites share similar climate and vegetation but differ in parent rock, with Gelbison shaped by sedimentary flysch and Vesole influenced by limestone. The team compared soils inside the kiln platforms with nearby forest soils and analyzed soil chemistry, charcoal composition, microbial communities, and plant growth responses.
Their findings challenge some common expectations. The kiln soils did not consistently show higher pH, cation exchange capacity, or richer nutrient content than surrounding forest soils. Instead, the long-term effects of charcoal accumulation appeared more complex and strongly influenced by local soil and rock conditions.
A key result came from the charcoal itself. Using scanning electron microscopy, energy dispersive X-ray spectroscopy, and infrared spectroscopy, the researchers found that old charcoal particles had undergone substantial surface oxidation. Compared with recently produced beech charcoal, kiln charcoal showed a much higher oxygen-to-carbon ratio and clear evidence of oxygen-containing functional groups. These chemical changes are important because aged biochar surfaces can interact differently with minerals, nutrients, organic matter, and microorganisms.
The study also showed that local geology leaves a chemical fingerprint on aged charcoal. Charcoal from the limestone-influenced Vesole site was enriched in calcium, while charcoal from the sedimentary Gelbison site showed stronger enrichment in aluminum and silicon. This suggests that biochar aging in real soils is not a uniform process. Instead, it depends on the surrounding mineral environment.
“Historical charcoal kilns act like long-term field experiments that nature and human history have already created for us,” said corresponding author Mohamed Idbella. “By studying these sites, we can better understand how biochar behaves after years or decades in soil, not just after a few weeks in the laboratory.”
The researchers also used next-generation sequencing to examine bacterial and fungal communities. While kiln and surrounding soils did not differ drastically in overall microbial diversity, the kiln soils hosted distinctive microbial patterns. In plant bioassays with soybean, maize, and tomato, the results suggested that microbiota associated with kiln soils could have a more positive influence on plant growth than microbiota from nearby forest soils.
The authors note that the study does not mean all charcoal-rich soils behave the same way. Instead, it highlights the importance of site-specific conditions, especially parent rock, soil chemistry, and microbial composition.
The findings point to charcoal kiln platforms as valuable natural laboratories for biochar research. They may help scientists better predict how biochar ages, how it interacts with minerals and microbes, and how its long-term presence could affect soil functions and carbon storage in forest ecosystems.
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Journal Reference: Iacomino, G., Idbella, M., di Costanzo, L. et al. Biochar aging, soil microbiota and chemistry of charcoal kilns in Mediterranean forests. Biochar 6, 82 (2024).
https://doi.org/10.1007/s42773-024-00378-3
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.