Gas entrapment and microbial N₂O reduction reduce N₂O emissions from a biochar-amended sandy clay loam soil

Nitrous oxide (N₂O) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of N₂O emissions with nitrogen fertilizer application being the main driver of rising atmospheric N₂O concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate N₂O emissions from soils. However, the underlying processes that cause N₂O emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used ¹⁵N tracing techniques and quantitative polymerase chain reaction (qPCR) to investigate the impact of biochar on mineral and gaseous nitrogen dynamics and denitrification-specific functional marker gene abundance and expression. In accordance with previous studies our results showed that biochar addition can lead to a significant decrease in N₂O emissions. Furthermore, we determined significantly higher quantities of soil-entrapped N₂O and N₂ in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced N₂O emission mitigation is based on the entrapment of N₂O in water-saturated pores of the soil matrix and concurrent stimulation of microbial N₂O reduction resulting in an overall decrease of the N₂O/(N₂O + N₂) ratio.