Nitrogen applied to agricultural soils does not always remain available to crops. Microorganisms can transform nitrate through several competing pathways, some of which conserve nitrogen in the soil while others ultimately remove it from the system. A new study published in Biochar shows that long-term biochar amendment can reshape this competition throughout the soil profile, promoting nitrogen retention in deep alkaline paddy soils.
"Our results show that the effect of biochar on nitrogen cycling cannot be understood by looking only at the surface soil," said corresponding author Lili Wang. "As soil depth increased, biochar shifted nitrate reduction toward a pathway that retains nitrogen as ammonium, while suppressing pathways associated with nitrogen loss."
The researchers studied an alkaline paddy field in Tianjin, China, where rice-straw biochar had been applied annually for more than five years. Soil was collected from four depths extending from the surface to 80 centimeters below ground. Using a nitrogen-15 tracer technique, the team simultaneously quantified three major nitrate reduction processes: denitrification, anaerobic ammonium oxidation, or anammox, and dissimilatory nitrate reduction to ammonium, known as DNRA.
These pathways have very different consequences for agricultural nitrogen. DNRA converts nitrate into ammonium, helping retain nitrogen in soil, while denitrification and anammox can convert reactive nitrogen into gaseous forms that leave the soil system.
The study revealed a striking depth-dependent pattern. Across the soil profile, the contribution of DNRA increased from 25.7% to as high as 91.1%, while the contribution of denitrification fell from 67.9% to 7.2%. Under biochar treatment, DNRA became increasingly favored below the surface. In the 60 to 80 centimeter layer, biochar increased the DNRA contribution by 37.0%, while reducing the contribution of denitrification by 27.8%.
The researchers also examined why this shift occurred. Statistical modeling identified soil pH, the ratio of soil organic carbon to nitrate, dissolved organic carbon, and ferrous iron as important controls. Biochar altered these environmental conditions differently at different depths, which in turn affected nitrogen-transforming microbial communities and functional genes.
In surface soil, biochar stimulated several nitrate reduction processes. Deeper in the profile, however, soil became more alkaline and carbon availability declined. These conditions increasingly suppressed denitrification and anammox while allowing DNRA to account for a larger share of nitrate transformation.
The findings suggest that long-term biochar application may help conserve nitrogen that would otherwise be lost from deeper alkaline paddy soils. This is particularly important because nitrogen transformations below the plow layer can influence whether nitrate remains in the soil, moves toward groundwater, or is converted into gaseous products.
The authors caution that the effects of biochar depend strongly on soil depth, initial soil pH, biochar aging, and changes in available carbon. Future studies should compare fresh and naturally aged biochar and directly measure nitrogen leaching and gaseous nitrogen losses.
The study provides new mechanistic evidence that biochar-based nitrogen management should consider the entire soil profile rather than focusing only on topsoil, especially in alkaline rice-growing regions.
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Journal Reference: Yang, Q., Zhang, G., Li, J. et al. Long-term biochar amendment increased dissimilatory nitrate reduction to ammonium concomitantly suppressing denitrification and anaerobic ammonium oxidation in deep alkaline paddy soil. Biochar 8, 136 (2026).
https://doi.org/10.1007/s42773-026-00645-5
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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.
