Long-term application of rice straw biochar significantly alters the mechanisms governing dissimilatory nitrate reduction in alkaline paddy soils, shifting nitrogen transformation pathways in a depth-dependent manner. In a recent study published in Biochar, Qiannan Yang, Guilong Zhang, Jie Li, Hu Li, Lukas Van Zwieten, and Lili Wang evaluated the impacts of surface-applied biochar at 0.05 metric tons per hectare per year across soil depths down to 80 centimeters. The findings provide new mechanistic insights into how biochar application influences nitrogen retention and loss by altering local soil chemistry and key microbial functional gene abundances. The research highlights how sustainable soil management practices directly affect microbial ecosystems below the surface.

In upper soil layers spanning 0 to 20 centimeters, biochar application increased overall microbial activity and raised the local pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More from 6.81 to 7.43. This shift stimulated overall dissimilatory nitrate reduction processes, increasing the potential rates of denitrification, anaerobic ammonium oxidation, and dissimilatory nitrate reduction to ammonium. In topsoil, denitrification remained the dominant pathway, accounting for over 70 percent of total nitrate reduction due to high dissolved organic carbon availability and elevated ferrous iron concentrations acting as electron donors. These upper layer dynamics demonstrate how surface conditions foster rapid biological processing of chemical compounds.

In deeper soil strata between 20 and 80 centimeters, biochar amendment exhibited a distinctly different regulatory effect. Biochar migration and alkaline accumulation increased deep soil pH levels above 8.0, exceeding the optimal functional threshold for key denitrifying enzymes. Simultaneously, biochar amendment lowered the ratio of soil organic carbon to nitrate in deeper layers. These altered parameters led to a marked suppression of specific functional genes associated with denitrification, causing denitrification rates to drop sharply to 1.23 nanomoles per gram per hour in the deepest 60 to 80 centimeter layer.

The anaerobic ammonium oxidation pathway was similarly constrained in deep soil layers under biochar treatment. Oxidation rates fell to 0.32 nanomoles per gram per hour at the 60 to 80 centimeter depth, accounting for less than 2 percent of total nitrate reduction. This suppression resulted from the elevated soil pH combined with substrate limitation, as reduced denitrification rates decreased the availability of essential nitrite intermediates required for anaerobic oxidation activity.

Conversely, dissimilatory nitrate reduction to ammonium became the primary nitrate reduction pathway in deep soil layers under biochar management. The relative contribution of this nitrogen-conserving process increased from 25.7 percent in topsoil to 91.1 percent in the 60 to 80 centimeter layer. Higher available phosphorus levels maintained under biochar treatment provided a protective effect that sustained microbial activity despite low organic carbon availability. By converting soil nitrate into plant-available ammonium rather than gaseous dinitrogen or nitrous oxide, deep-layer biochar processes mitigate subsurface nitrogen leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More and atmospheric emissions, offering a sustainable strategy to improve fertilizer efficiency in alkaline agricultural systems.

Source: Yang, Q., Zhang, G., Li, J., Li, H., Van Zwieten, L., & Wang, L. (2026). 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.