Gas entrapment and microbial N(2)O reduction reduce N(2)O 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.