BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More is widely considered a valuable tool for sustainable agriculture, but its effects can be complex. The success of biochar application depends not only on its intrinsic properties but also on its interaction with the specific soil it is applied to. In a study published in the Journal of Hazardous Materials Advances, researchers Guillermina Cantou, Jim J. Wang, Baoyue Zhou, Jeong-Min Lee, and Jong-Hwan Park investigated the ecotoxicological effects of two highly contrasting biochars when applied to two distinct soil types. This research provides a crucial framework for understanding how to match the right biochar to the right soil, ensuring both agricultural benefits and environmental safety.

The study used two types of biochar: cattle manure biochar (CMB) and rice husk biochar (RHB). These biochars had very different properties due to their source material. CMB was highly alkaline with a 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 of 11, high electrical conductivity (12 dS m−1), and a high potassium content (89 g kg−1). In contrast, RHB had a much larger specific surface area, measuring 174 m2g−1 compared to CMB’s 1.7 m2g−1. The researchers then tested the ecotoxicity of these biochars in isolation using a battery of four bioassays, including tests on bacteria, algae, and plants. The results were starkly different: CMB caused strong toxicity in all tests, while RHB showed no negative effects whatsoever and was even found to stimulate growth. This initial finding classified CMB as a highly toxic material (hazard class IV) when considered alone.

However, the study’s most significant finding emerged when the biochars were applied to two different soils: an acidic clay soil (Alligator) and a calcareous silt loam (Norwood). The highly alkaline and seemingly toxic CMB demonstrated a profoundly beneficial effect on the acidic Alligator soil. The biochar significantly reduced the toxicity of the soil to the algae Pseudokirchneriella subcapitata, decreasing the growth inhibition from 54% to a mere 16%. This reduction in toxicity was so substantial that it reclassified the soil’s hazard level from class III (acute hazard) to class I (no acute hazard). This beneficial effect is attributed to the high alkalinity of the CMB, which acted as a liming agent to immobilize toxic aluminum ions (Al3+) in the acidic soil. This demonstrates that a biochar considered “toxic” in isolation can be a highly effective tool for mitigating existing soil toxicity under the right conditions.

In a key contrast, when the same CMB was applied to the calcareous Norwood soil, which already had a high pH, the biochar’s effect was different. In this case, CMB slightly increased the soil’s toxicity, particularly affecting the luminescence of Vibrio fischeri and the root growth of Sorghum saccharatum seedlings. This outcome highlights the context-dependent nature of biochar’s effects, as the high pH and salinity that were beneficial in one soil became a liability in another.

The rice husk biochar (RHB), on the other hand, consistently improved the ecotoxicological profiles of both soil types. In the Norwood soil, it showed a significant beneficial effect on V. fischeri, and in the Alligator soil, it also reduced toxicity, though to a lesser extent than CMB. The study’s results emphasize that a biochar’s ecotoxicological impact is not a fixed property but rather a function of its complex interaction with the soil. Therefore, relying solely on a biochar’s chemical composition or testing it in isolation is insufficient for predicting its real-world environmental safety and effectiveness. The authors conclude that site-specific evaluations using bioassays are essential for developing safe and effective biochar management strategies that are tailored to specific soil conditions and management goals.

SOURCE: Cantou, G., Wang, J. J., Zhou, B., Lee, J.-M., & Park, J.-H. (2025). Ecotoxicity of two contrasting soils with biochar derived from cattle manure and rice husk. Journal of Hazardous Materials Advances.