Biochar benefits may fade faster than expected, 8-year field study finds

Biochar is often promoted as a long-lasting solution for improving soil fertility, raising soil pH, storing carbon, and helping crops access nutrients. But a new 8-year field study published in Biochar suggests that its benefits may be more temporary than many farmers and land managers expect.

Researchers tracked how different biochar application rates affected soil nutrient availability in two contrasting agricultural soils under continuous tobacco cultivation from 2018 to 2025. The study tested five application rates, 0, 5, 15, 20, and 40 tons per hectare, across a sandy loam Dystrudept soil and a clay loam Hapludult soil in Guizhou Province, China.

The findings were clear. Biochar produced strong early improvements in soil pH, soil organic carbon, and available nutrients, but most of these benefits gradually weakened and disappeared within 6 to 8 years. In many cases, the strongest effects lasted only about 3 to 5 years.

“Biochar is still a valuable soil amendment, but our results show that it should not be viewed as a permanent, one-time solution under intensive cropping,” said corresponding author Caibin Li. “The key message is that biochar management needs to be adjusted by soil type, application rate, and time.”

At the beginning of the trial, higher biochar rates sharply increased soil pH and nutrient levels. In the clay loam Hapludult soil, soil organic carbon reached 83.59 grams per kilogram at the highest application rate of 40 tons per hectare. Available potassium, phosphorus, calcium, magnesium, and other nutrient indicators also increased strongly after biochar addition.

However, those gains did not last indefinitely. Over time, crop nutrient uptake, leaching, fertilization effects, and biochar aging appeared to reduce the differences among treatments. By the later years of the experiment, soils receiving different biochar rates often looked statistically similar to untreated soils.

One surprising result was that the sandy loam soil retained biochar effects longer than the clay loam soil, even though clay-rich soils are often expected to hold nutrients more effectively. The researchers suggest that under intensive cultivation, crop demand and nutrient movement through the soil can override simple predictions based on texture or cation exchange capacity.

The study also identified an important threshold for application rate. A moderate rate of 20 tons per hectare offered the best balance between effectiveness and persistence. The highest rate, 40 tons per hectare, produced the largest short-term response but did not extend the duration of benefits. This means that adding more biochar may raise costs without providing longer-lasting improvements.

Structural equation modeling showed that soil pH played a central role in regulating nutrient availability, especially for base cations such as calcium and magnesium. As the liming effect of biochar weakened over time, nutrient advantages also declined.

“Our findings suggest that farmers may benefit more from optimized or periodic biochar applications than from a single high-dose application,” Li said. “Future biochar strategies should focus on when, where, and how much to apply, rather than assuming that one application will work forever.”

The authors note that the study used one type of biochar made from tobacco stems and focused on continuous tobacco cultivation, so results may differ for other biochars, crops, and management systems. Still, the 8-year dataset provides rare long-term field evidence that can help guide more realistic and cost-effective biochar use in agriculture.

The study highlights a practical shift in biochar management: biochar can improve soils, but its agronomic benefits under intensive farming may need renewal over time.

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Journal Reference: Zhang, J., Li, C., Xu, M. et al. Biochar application rates regulate soil nutrient availability: evidence from an 8-year field study across two soils. Biochar 8, 115 (2026).   

https://doi.org/10.1007/s42773-026-00623-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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