In a recent paper published in Biochar, lead authors Ping Wu and Yingdong Fu, alongside co-authors Hailong Wang and Shuping Qin, investigated how reactive oxygen species generated by biochar influence carbon storage in different soil environments. Biochar is widely recognized as a effective tool for agricultural soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More and long-term atmospheric carbon sequestration. However, during pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, persistent free radicals become trapped within the biochar matrix. When exposed to oxygen and moisture, these free radicals continuously generate reactive oxygen species, including hydroxyl radicals. While hydroxyl radicals can directly oxidize soil organic matter, they are also known to cause structural damage to extracellular enzymes secreted by soil microbes. Because extracellular enzymes drive the decomposition of complex organic molecules into carbon dioxide, their suppression represents an unchartered pathway through which biochar might slow down carbon loss. The researchers sought to resolve whether biochar-derived hydroxyl radicals ultimately protect or deplete soil carbon pools across contrasting soil types.

The experimental findings reveal that the impact of biochar-derived hydroxyl radicals on soil carbon mineralization is highly dependent on soil properties. When raw biochar containing high levels of persistent free radicals was added to acidic Black soil and acidic Red soil, cumulative carbon dioxide emissions dropped significantly compared to unamended control soils. Specifically, raw biochar suppressed carbon dioxide emissions by 6.8 percent in Black soil and by 12.9 percent in Red soil. In contrast, when the free radicals in the biochar were chemically quenched prior to application, this protective effect disappeared completely. Adding quenched biochar to these acidic soils actually stimulated carbon dioxide emissions above control levels, proving that free-radical-derived hydroxyl radicals play a primary role in slowing soil respiration in acidic contexts.

The biological mechanism behind this suppression was confirmed by measuring the activity of key carbon-degrading enzymes, including beta-glucosidase, chitinase, and cellulase. In acidic soils rich in iron and aluminum oxides, extracellular enzymes readily adsorb onto mineral surfaces. Once bound to minerals, these enzymes become particularly susceptible to localized oxidative damage caused by hydroxyl radicals. Consequently, raw biochar reduced total enzyme activity and slowed the breakdown of solid soil organic carbon. When free radicals were scavenged, enzyme activity surged, accelerating the depolymerization of complex organic matter into dissolved organic carbon and elevating microbial carbon dioxide output.

Conversely, testing in neutral Fluvo-aquic soil revealed a completely different response driven by alternative biochemical dynamics. Adding raw biochar to neutral soil increased cumulative carbon dioxide emissions by 19.4 percent over the control. Applying quenched biochar stimulated carbon dioxide release even further, reaching an increase of 45.6 percent above control levels. In neutral soils with lower metal oxide content, extracellular enzymes do not adsorb strongly to mineral surfaces, rendering them less vulnerable to hydroxyl radical inhibition. In this environment, the positive priming effect triggered by soluble carbon components in biochar dominates, overriding enzyme suppression and accelerating carbon mineralization.

Direct chemical scavenging experiments across all three soil types further validated these dual protective pathways. When hydroxyl radicals were entirely scavenged from the soil system, enzyme activities increased sharply and soil organic carbon levels experienced a pronounced drop, accompanied by a rapid accumulation of dissolved organic carbon. These results indicate that hydroxyl radicals exert two complementary protective functions: they suppress enzyme-mediated breakdown of solid soil organic carbon and simultaneously destroy labile dissolved organic carbon before microbes can consume it.

Overall, the study demonstrates that biochar cannot be treated as a uniform carbon sink. The chemical radicals created during biochar production interact dynamically with soil mineralogy and pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More to determine net carbon balance. Minimizing persistent free radicals during pyrolysis may prevent short-term abiotic carbon loss in some soils, but maintaining radical activity is beneficial for preserving carbon in acidic, oxide-rich agricultural soils.

Source: Wu, P., Fu, Y., Wang, H., & Qin, S. (2026). Soil-specific protection of organic carbon by biochar-derived hydroxyl radicals associated with enzyme suppression. Biochar, 8(1), 126.