Soils are among the planet's largest carbon reservoirs, yet salinity and land degradation threaten their ability to store carbon and support agriculture. Improving carbon storage in salt-affected farmland could therefore offer a dual benefit: restoring soil health while supporting climate change mitigation.
In a study published in Biochar, researchers from the Chinese Academy of Agricultural Sciences and collaborating institutions discovered that combining manure with straw or biochar significantly increased soil organic carbon, but through two distinct pathways. Straw primarily stimulated microbial activity and carbon transformation, while biochar improved soil structure and the physical protection of carbon.
"Both straw and biochar can improve carbon storage in saline soils, but they work in different ways," said Hongyuan Zhang, a corresponding author of the study. "Understanding these differences is essential for developing soil management strategies that improve fertility and support carbon sequestration."
The researchers conducted a two-year field experiment in China's Yellow River Delta, where high soil salinity and low organic matter levels pose major challenges to agricultural productivity. Four fertilizer treatments were compared, including conventional chemical fertilizer, manure supplementation, and manure combined with either straw or biochar.
The team examined soil carbon, microbial communities, enzyme activities, and soil pore structures to understand how each treatment influenced carbon accumulation.
Both straw and biochar combinations increased soil organic carbon by approximately 19% compared with manure alone in 2023. However, the underlying processes differed substantially.
When straw was combined with manure, it provided readily available organic materials that stimulated soil microorganisms. This increased carbon-cycling enzyme activities and promoted the accumulation of microbial necromass, the remains of dead microorganisms that contribute to soil organic matter.
Compared with manure alone, the straw treatment increased microbial necromass carbon by 10.8% and fungal necromass carbon by 11.2%. These findings suggest that microbial processing plays an important role in transforming straw-derived carbon into soil carbon.
Biochar followed a different route. Rather than primarily stimulating microbial carbon transformation, it enhanced soil aggregation and pore structure, helping protect organic carbon within larger soil particles.
The biochar treatment increased large soil aggregates by 40.9% and improved aggregate stability by 31.9%, compared with manure alone. It also increased soil porosity, creating a more favorable physical environment for carbon retention.
Microbial community analyses revealed another important distinction. Straw reduced the abundance of certain Actinobacteria-dominated microbial groups, while biochar promoted fungal groups dominated by Ascomycota that were positively associated with soil carbon storage.
Together, these findings highlight two complementary carbon sequestration strategies: biological transformation driven by straw and physical protection enhanced by biochar.
The researchers emphasize that longer-term field observations are needed to determine how these mechanisms influence carbon persistence over time.
"Our findings show that selecting organic amendments should involve more than measuring how much carbon they add to soil," Zhang explained. "Understanding how carbon is processed and protected can help guide more effective restoration of degraded agricultural land."
The study provides new insights for improving saline soil management, supporting sustainable agriculture, and strengthening soil-based carbon sequestration strategies.
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Journal Reference: Bai, X., Zhou, J., Dong, R. et al. Divergent pathways of straw versus biochar integration with manure to enhance SOC sequestration in coastal saline soils. Biochar 8, 139 (2026).
https://doi.org/10.1007/s42773-026-00656-2
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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.
