A new long-term field study reveals that biochar, a carbon-rich material derived from biomass, can significantly enhance soil carbon storage by stimulating microbial processes. However, the benefits are not uniform throughout the soil profile, highlighting the importance of looking deeper beneath the surface.
"Soil microbes are central to long-term carbon storage, and our study shows that biochar can reshape how microbial carbon is formed and stabilized," said the study's lead author. "But these effects depend strongly on soil depth and nutrient conditions."
Soils represent the largest terrestrial carbon reservoir, storing more carbon than the atmosphere and vegetation combined. A major portion of this carbon exists not as plant debris, but as microbial necromass, the remains of dead microorganisms. This form of carbon is particularly stable and can persist in soils for long periods, making it critical for climate change mitigation.
To better understand how biochar influences this process, researchers conducted a 12-year field experiment in two contrasting cropland soils. They measured microbial necromass carbon in both topsoil and subsoil layers and complemented their findings with a global meta-analysis of 23 studies.
The results showed a clear pattern. In the topsoil, biochar significantly increased microbial necromass carbon by up to 39 percent, with particularly strong gains in fungal-derived carbon. This increase was linked to improved nutrient availability, greater microbial biomass, and enhanced microbial efficiency. In simple terms, biochar created a more favorable environment for microbes to grow and convert carbon into stable forms.
However, the story changed below the surface. In the subsoil, biochar reduced microbial necromass carbon by as much as 30 percent. The researchers attribute this decline to nutrient limitations. Biochar tends to retain nutrients in the upper soil layers, leaving deeper layers relatively deprived. As a result, microbes in the subsoil increase their metabolic activity to acquire nutrients, accelerating the breakdown of existing carbon rather than building new stores.
"This depth-dependent response is critical," the authors explained. "If we only look at surface soils, we may overestimate the long-term carbon sequestration potential of biochar."
The study also found that the effectiveness of biochar depends on environmental conditions. Greater increases in microbial carbon were observed in soils with low initial organic carbon, sandy textures, and in warmer and wetter climates. Long-term application was also essential, with the strongest effects appearing after ten years.
The accompanying meta-analysis confirmed that biochar generally increases microbial necromass carbon across diverse ecosystems, with positive responses observed in more than 80 percent of cases. On average, biochar increased microbial-derived carbon by about 10 percent globally.
Despite these gains, the researchers noted that microbial carbon represents only part of the total soil carbon pool. Because biochar also directly adds stable carbon to soil, the relative contribution of microbial necromass to total soil carbon may decrease even as its absolute amount increases.
Overall, the findings provide new insights into how biochar interacts with soil microbial systems to influence carbon cycling. They also emphasize the need for long-term and depth-specific assessments when evaluating soil carbon sequestration strategies.
"Biochar holds great promise for sustainable agriculture and climate mitigation," the authors said. "But optimizing its benefits requires a deeper understanding of how soil systems function across space and time."
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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.
