The research published in the journal Biochar by authors Yucan Dong, Merve Tunali, and Bernd Nowack highlights the complex dual nature of biochar as both a potential fertilizer and a possible pollutant. By analyzing a massive dataset spanning over a decade of research, the study concludes that biochar does not have a single, universal effect on the environment. Instead, its impact depends heavily on the specific properties of the biochar, the type of soil it is added to, and the specific organisms living in that soil. While much of the previous scientific literature has focused on the clear benefits biochar provides for agricultural crop yields, this comprehensive analysis brings much-needed attention to the risks posed to soil fauna and microbes.

The findings reveal a significant divide in how different groups of soil organisms respond to biochar applications. Plants generally experience a positive growth response, averaging a 10 percent increase in productivity across the studied conditions. However, the outlook for soil animals and bacteria is considerably more concerning. The data shows that biochar application is statistically linked to a nearly 10 percent negative effect on these organisms overall. More specifically, when looking at survival rates rather than just growth, the hazardous impact on soil life jumps to a 21 percent decrease. This suggests that while a farmer might see taller crops, the hidden ecosystem beneath the surface, including essential creatures like earthworms, may be struggling to survive.

The physical and chemical properties of the biochar itself play a decisive role in determining whether the outcome is beneficial or hazardous. One of the most critical factors identified is the temperature at which the biochar is produced. Biochar created at temperatures below 550 degrees Celsius generally exerts a positive influence on the soil. In contrast, biochar produced at higher temperatures is often associated with significantly negative effects. This is likely because higher heat can increase the concentration of heavy metals and other pollutants like polycyclic aromatic hydrocarbons. Furthermore, the acidity or alkalinity of the material is vital. Biochar with a high 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 was found to be particularly toxic to soil animals, sometimes reaching a 100 percent mortality rate for earthworms when the pH exceeds 10.

Soil conditions also dictate the success of biochar as a 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. The study found that biochar is most effective when applied to acidic soils, where its natural alkalinity helps buffer the soil and improve nutrient availability. In these environments, the positive effects are most pronounced. However, in soils that are already neutral or alkaline, the addition of high-pH biochar can lead to increased salinization and a reduction in phosphorus availabilityPhosphorus is another essential nutrient for plant growth, but it can sometimes be locked up in the soil and unavailable to plants. Biochar can help release phosphorus from the soil and make it more accessible to plants, reducing the need for chemical fertilizers.   More, which ultimately harms the ecosystem. This indicates that biochar should not be used as a one-size-fits-all solution but must be carefully matched to the existing chemistry of the land.

The amount of biochar applied to the soil is another major predictor of risk. The analysis suggests a clear negative correlation between the application rate and the health of soil organisms. Lower application rates, typically below 50 grams per kilogram of soil, are associated with a nearly 10 percent positive effect. As the dose increases to a high level, defined as more than 100 grams per kilogram, the effect swings sharply negative, resulting in a 32 percent hazardous impact. This suggests that even a beneficial material can become a pollutant if used in excessive quantities, likely because too much biochar can trap essential nutrients and make them unavailable to the organisms that need them.

To help navigate these complexities, the researchers successfully developed a machine learning tool that can predict the impact of biochar with high accuracy. By feeding the computer information about biochar and soil properties, the model achieved a 79 percent accuracy rate in classifying whether a specific application would be beneficial or hazardous. This technological advancement provides a promising way for land managers to perform quality control and determine the safest application rates before large-scale use. Ultimately, the study advocates for a more cautious and standardized approach to biochar production and application to ensure that efforts to store carbon in the soil do not inadvertently damage the delicate biological networks that keep our ground healthy and productive.

Source: Dong, Y., Tunali, M., & Nowack, B. (2026). Fertilizer or pollutant: Analyzing the effects of biochar on soil organisms using machine learning. Biochar, 8(28).