Biochar, a charcoal-like material made from plant waste, has been widely promoted as a promising tool for improving soil health and storing carbon in farmland. But a new long-term study suggests that its effects are more complex than previously understood.
In a 12-year field experiment across two contrasting cropland soils in China, researchers found that biochar significantly increased microbial necromass carbon in the topsoil, especially carbon derived from fungi. However, the same treatment reduced microbial necromass carbon in the subsoil. The findings show that biochar can strengthen soil carbon storage near the surface, but its deeper-soil effects require closer attention.
Microbial necromass carbon refers to the remains of dead bacteria and fungi. Although the term may sound unappealing, this material is one of the most important building blocks of stable soil organic carbon. As microbes grow, die, and leave behind cell residues, part of this material becomes protected in soil and can remain there for long periods. Because of this, microbial necromass is increasingly viewed as a key indicator of long-term soil carbon sequestration.
"Biochar is often discussed as a carbon-rich amendment, but our study shows that its influence on soil carbon also depends strongly on how it reshapes microbial life and death in different soil layers," said the study's corresponding author. "The surface soil and deeper soil did not respond in the same way."
The researchers applied wheat-straw biochar once at the start of the experiment and sampled soils 12 years later. They studied two soil types: a carbon-rich Entisol and a carbon-poor Ultisol. In the topsoil, biochar increased microbial necromass carbon by 23.3% in the Entisol and 39.0% in the Ultisol. The increase was mainly driven by fungal necromass carbon, suggesting that fungi played a particularly important role in turning biochar-amended soils into stronger carbon reservoirs.
The study also found that biochar improved several topsoil conditions linked to microbial growth, including nutrient availability, microbial biomass, and microbial efficiency. In other words, microbes in the surface layer appeared better able to convert resources into biomass and, eventually, stable necromass carbon.
In contrast, the subsoil told a different story. At depths of 20 to 40 cm, biochar reduced microbial necromass carbon by 17.9% to 30.4% across the two soil types. The researchers linked this decline to lower nitrogen availability, higher microbial metabolic stress, and increased enzyme activity associated with microbial nutrient mining. These changes may have made microbes more likely to break down existing microbial residues rather than build up new stable carbon.
To test whether their field results matched broader patterns, the team also conducted a meta-analysis of 85 observations from 23 peer-reviewed studies. The analysis showed that biochar increased topsoil microbial necromass carbon in 83.5% of cases, with an average increase of 10.2%. The effect was stronger in soils with low initial organic carbon and higher sand content, and it became more pronounced over time, peaking after about 10 years.
The findings carry practical implications for climate-smart agriculture. Biochar can help croplands store more carbon, but the benefits may depend on soil type, application rate, time since application, and soil depth.
"Our results suggest that long-term biochar strategies should not only ask how much carbon is added, but where that carbon is stabilized in the soil profile," the authors said. "Considering soil depth will be essential for accurately evaluating biochar's role in climate mitigation and sustainable farming."
Overall, the study provides new evidence that biochar promotes microbially mediated carbon accumulation in topsoil, while also revealing potential limitations in deeper soil. The authors emphasize that future soil carbon assessments should include long-term experiments and deeper soil sampling to avoid overestimating or underestimating biochar's true climate benefits.
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Journal Reference: Song, K., Liu, Z., Ma, R. et al. Depth-dependent microbial necromass carbon accumulation responses to long-term biochar amendment in croplands. Biochar 8, 78 (2026).
https://doi.org/10.1007/s42773-026-00577-0
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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.
