Weathering changes the long-term role of biochar in soil carbon storage and pollution control

Biochar is often described as a stable form of carbon that can improve soils, store carbon, and help immobilize pollutants. But once biochar is applied to farmland, contaminated sites, or other soils, it does not remain unchanged. A new review published in Biochar examines how biochar weathers over time and what those changes mean for soil health, carbon sequestration, and soil remediation.

Biochar is produced by heating biomass under limited oxygen. Because it contains a large fraction of relatively stable carbon, it has attracted growing attention as a tool for climate mitigation and sustainable soil management. It can also bind potentially toxic elements such as cadmium and arsenic, as well as some persistent organic pollutants, reducing their movement and biological availability in soil.

However, the review by Nanthi Bolan and colleagues shows that the story is more complex. In the field, biochar is exposed to rain, drying and wetting cycles, freeze and thaw events, oxygen, sunlight, soil minerals, plant roots, and microorganisms. These forces gradually alter the structure and surface chemistry of biochar.

“Biochar should not be viewed as a fixed material once it enters soil,” said lead author Nanthi Bolan. “It is a dynamic soil amendment. Understanding how it weathers is essential if we want to use it reliably for carbon storage, soil improvement, and contamination control.”

The review identifies three major weathering pathways. Physical weathering can break larger biochar particles into micro and nano-sized fragments. Chemical weathering can oxidize the biochar surface, producing more oxygen-containing functional groups such as carboxyl and phenolic groups. Biological weathering involves microbial colonization, biofilm formation, enzymatic reactions, and gradual changes driven by soil organisms and root exudates.

These changes can have both benefits and risks. Weathered biochar may develop new reactive surfaces that improve nutrient retention, water behavior, and metal binding. It may also provide habitats for microorganisms and contribute to soil aggregation. At the same time, fragmentation can increase particle mobility, and some weathering processes may reduce the long-term carbon storage potential of biochar or alter its ability to retain contaminants.

The authors emphasize that biochar weathering is controlled by many interacting factors. Feedstock type, pyrolysis temperature, particle size, soil pH, soil texture, mineral composition, microbial activity, climate, irrigation, fertilization, tillage, and crop rotation can all influence how quickly biochar changes and whether those changes improve or weaken its performance.

One important message from the review is that biochar-based carbon removal should be evaluated under realistic field conditions. Although some carbon may be lost through leaching, mineralization, or particle movement, long-term studies also show that the aromatic core of biochar can remain highly persistent in soil. The best outcomes are expected when biochar remains in the root zone, supports plant and microbial activity, and does not accelerate the loss of native soil organic matter.

For contaminated soils, the review highlights the need to monitor how aging affects pollutant immobilization. Weathering can strengthen or weaken contaminant binding depending on soil conditions and the type of pollutant. This means that short-term laboratory results may not fully predict long-term remediation performance.

The authors call for more long-term field studies, standardized weathering tests, and models that account for changing biochar properties over time. Such work could help farmers, land managers, and environmental engineers select the right biochar for the right soil and climate.

“Biochar remains a promising tool,” Bolan said. “But its long-term success depends on matching biochar properties with soil conditions and management practices. Weathering is not a side issue. It is central to how biochar works in the real world.”

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Journal Reference: Bolan, N., Mukherjee, S., Bolan, S. et al. Weathering of biochar: implications to soil health, carbon sequestration and soil remediation. Biochar 8, 102 (2026).  

https://doi.org/10.1007/s42773-026-00615-x  

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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. 

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