Biochar and crop residues store carbon in soil through distinct pathways, long-term field study finds
image: Soil organic carbon accumulation mechanisms in soil amended with straw and biochar: entombing effect or biochemical protection?
Credit: Yuhan Yuan, Yao Liang, Hongguang Cai, Jingchao Yuan, Cuilan Li, Hang Liu, Chang Zhang, Lichun Wang & Jinjing Zhang
A new long-term field study reveals that biochar and crop residues help soils store carbon in fundamentally different ways, offering new insights for climate-smart agriculture and carbon sequestration strategies.
Soils are the largest terrestrial carbon reservoir on Earth, and even small increases in soil organic carbon can play a significant role in reducing greenhouse gases and improving soil health. However, the mechanisms by which different agricultural practices build and stabilize soil carbon have remained unclear.
In research published in Biochar, scientists conducted a nine-year field experiment in Northeast China to compare how maize straw and straw-derived biochar influence soil carbon accumulation. The results show that while both approaches increase soil organic carbon, they do so through distinct biological and chemical processes.
“Understanding how different soil amendments store carbon is essential for designing effective strategies to mitigate climate change,” said one of the study’s authors. “Our findings show that not all carbon inputs behave the same once they enter the soil.”
The study evaluated four treatments: mineral fertilizers alone, fertilizers combined with maize straw, fertilizers combined with biochar, and an untreated control. Both straw and biochar significantly increased soil organic carbon compared to fertilizer alone, but biochar had the stronger effect, boosting carbon levels by over 40 percent, compared to about 24 percent for straw.
The key difference lies in how carbon is stabilized in the soil.
Straw promotes carbon storage mainly through what researchers call an “entombing effect.” As microbes break down plant material, they leave behind residues known as microbial necromass. These microscopic remains become physically protected within soil aggregates, effectively locking carbon away.
In contrast, biochar enhances carbon storage through “biochemical protection.” Produced by heating biomass in low-oxygen conditions, biochar contains highly stable, aromatic carbon structures that resist decomposition. These chemically resistant forms of carbon persist in soil for long periods, contributing to long-term carbon sequestration.
The study also found that fungi play a central role in soil carbon accumulation under both treatments. Fungal-derived residues contributed more to stable carbon pools than bacterial residues, highlighting the importance of fungal activity in soil ecosystems.
Interestingly, while straw increased microbial activity and biomass more strongly, biochar led to greater chemical stability of soil carbon. This suggests a trade-off between biologically driven carbon inputs and chemically resistant carbon forms.
“Our results highlight two complementary pathways for building soil carbon,” the researchers noted. “Straw supports microbial processes that convert plant material into stable residues, while biochar directly adds persistent carbon that is less prone to degradation.”
These findings have important implications for agriculture and climate policy. Returning crop residues to the soil can enhance microbial-driven carbon storage, while converting residues into biochar may provide a more durable carbon sink. Combining both approaches could potentially maximize soil carbon sequestration while improving soil fertility.
With growing global interest in carbon farming and negative emissions technologies, understanding these mechanisms is critical for scaling sustainable land management practices.
The researchers emphasize that long-term field studies like this are essential for capturing the full impacts of soil amendments over time. Future work will explore how these processes evolve under different climates, soils, and cropping systems.
Overall, the study provides new evidence that biochar is a powerful tool for enhancing soil carbon storage and supporting climate mitigation efforts, while also underscoring the continued value of traditional practices like straw return.
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Journal Reference: Yuan, Y., Liang, Y., Cai, H. et al. Soil organic carbon accumulation mechanisms in soil amended with straw and biochar: entombing effect or biochemical protection?. Biochar 7, 33 (2025).
https://doi.org/10.1007/s42773-025-00431-9
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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.