Nitrogen (N) fertilizers are crucial for global food production, but their intensive use leads to significant environmental concerns, including air pollution from ammonia volatilization, groundwater contamination from nitrate-N leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More, and the emission of nitrous oxide , a potent greenhouse gas. A recent review published in Environments by Yudai Kohira and colleagues assesses the potential of 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, a carbon-rich material made from organic matter, to mitigate these N losses when applied to soils alongside N fertilizers. The study synthesizes quantitative data from research conducted over the past decade, offering valuable insights into how biochar’s unique properties can support more sustainable agricultural practices.

The review found that biochar can indeed reduce volatilization, though the effects vary. The biochar’s 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 is a dominant factor, with mitigation effects ranging from a 45.0% reduction to a 9.0% increase depending on its pH level. Specifically, a high ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content ( ge16 ) in biochar significantly increased volatilization by 25.1%, contrasting sharply with a 52.2% mitigation observed with le15 ash content. Moderate biochar application rates, up to 50 tons per hectare (t ha−1), were most effective, leading to a 13.3% to 27.8% reduction in NH3 volatilization. The greatest reductions, up to 52.2%, were seen in clayey soils, likely due to their high bulk density, which can be favorably altered by biochar.

For NO_3−−N leaching, biochar generally showed a mitigating effect. Biochar produced at higher 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 temperatures, which often results in a higher specific surface area (SSA), enhanced mitigation by up to 26.6%. This is primarily because biochar improves the soil’s water-holding capacity, reducing the movement of nitrate through the soil profile. Increasing the biochar application rate, especially above 51 t ha−1, also significantly reduced NO3−−N leaching. A notable finding was the pronounced mitigation (up to 39.0%) when biochar was co-applied with organic fertilizers compared to chemical fertilizers. This is attributed to the slower N release from organic fertilizers, allowing more time for biochar to adsorb and retain the nitrate. Pot experiments consistently showed stronger mitigation effects, with a 39.4% reduction, likely due to better soil homogeneity and enhanced root system interactions within a controlled environment.

Biochar also proved effective in mitigating N2O emissions. A higher SSA in biochar, often a result of higher pyrolysis temperatures, was linked to greater N2O trapping within its pores, leading to up to a 25.9% emission mitigation. Furthermore, increasing the biochar’s total carbon (TC) content (e.g., ge501 g kg−1), which resulted in a 28.3% to 33.7% mitigation, and a higher carbon-to-nitrogen (C/N) ratio (e.g., ge101), leading to a 34.2% mitigation, significantly reduced N2O emissions. This is likely due to promoted microbial N immobilization and a more complete denitrification process within the soil. However, the effect varied with soil type: clayey soils sometimes showed increased N2O emissions (up to 18.0%), while loamy and sandy soils exhibited mitigation (18.7% and 7.9% respectively). This suggests complex interactions with soil ammonium availability and physical structure, highlighting the need for tailored applications.

In conclusion, the review emphasizes that biochar applications offer a promising solution for reducing N losses and improving N use efficiency in agriculture, thereby fostering more sustainable systems. However, its effectiveness is not universal and depends heavily on matching biochar properties, such as SSA, pH, C/N ratio, and ash content, and application rates to specific soil types, fertilizer types, and the particular N loss pathway being targeted. Future research should focus on long-term field studies, deeper investigations into microbial mechanisms, and the development of predictive models to optimize biochar’s practical application in diverse agroecosystems.

Keywords: Biochar, Nitrogen Loss, Ammonia Volatilization, Nitrate Leaching, Nitrous Oxide Emissions, Sustainable Agriculture, 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, Soil Fertility, Specific Surface Area, Carbon-to-Nitrogen Ratio.

Source: Kohira, Y., Fentie, D., Lewoyehu, M., Wutisirirattanachai, T., Gezahegn, A., Ahmed, M., Akizuki, S., Addisu, S., & Sato, S. (2025). The Sustainable Management of Nitrogen Fertilizers for Environmental Impact Mitigation by Biochar Applications to Soils: A Review from the Past Decade. Environments, 12(6), 182.