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

Nitrous oxide (N 2 O) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of N 2 O emissions with nitrogen fertilizer application being the main driver of rising atmospheric N 2 O concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate N 2 O emissions from soils. However, the underlying processes that cause N 2 O emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used 15 N 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 N 2 O emissions. Furthermore, we determined significantly higher quantities of soil-entrapped N 2 O and N 2 in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced N 2 O emission mitigation is based on the entrapment of N 2 O in water-saturated pores of the soil matrix and concurrent stimulation of microbial N 2 O reduction resulting in an overall decrease of the N 2 O/(N 2 O + N 2 ) ratio.