Turning livestock manure into biochar has been hailed as a climate-friendly way to recycle waste, lock away carbon, and make farming more sustainable. But new research suggests that this "green" solution may not be as stable as once believed—especially in regions with harsh winters.
A study published in Biochar by scientists from Monash University and Xinjiang University found that repeated freeze–thaw cycles—the kind of seasonal temperature swings common in northern climates—can damage biochar and trigger the release of toxic heavy metals it was supposed to lock up.
"Biochar is often seen as a win–win technology: it helps manage livestock waste and reduces environmental risks," said lead researcher Xingdong Wang. "But our results show that when biochar is exposed to freeze–thaw aging, it can actually become more fragile and release pollutants back into the environment."
The team tested chicken manure biochars made at different production temperatures (350 °C, 550 °C, and 750 °C). Surprisingly, the biochars made at the highest temperature—once considered the most durable—were the most vulnerable to cracking and oxidation during freeze–thaw stress. This breakdown released greater amounts of heavy metals such as zinc, copper, and lead, which can harm crops, soil organisms, and potentially contaminate groundwater.
In some cases, the bioavailable levels of zinc and copper in aged biochar were hundreds of times higher than in fresh samples—well above safe limits set for plants. "This shows that the way biochar is made has long-term consequences," explained co-author Victor Wei-Chung Chang of Monash University. "We can't just assume that higher-temperature biochars are automatically safer or more stable."
The findings highlight an important lesson for climate-smart agriculture: it's not enough to evaluate biochar when it is freshly produced. Farmers, policymakers, and researchers must also consider how environmental conditions—like freezing and thawing—will affect biochar years after it has been applied to soil.
The authors stress that biochar remains a promising tool for sustainable farming and waste recycling, but more research is needed to design materials that can resist weathering and keep heavy metals locked away safely. Potential solutions include protective surface treatments or additives that reduce metal mobility.
"This is a reminder that there's no quick fix in environmental science," said Chang. "If we want biochar to truly deliver climate and agricultural benefits, we have to think about its whole lifecycle in real-world conditions."
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Journal reference: Wang, X., Zhu, G., Yi, Y. et al. Reassessing the role of pyrolysis temperature: freeze–thaw aging challenges heavy metals stability in biochar. Biochar 7, 86 (2025). https://doi.org/10.1007/s42773-025-00479-7
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