Gas entrapment and microbial NO reduction reduce NO emissions from a biochar-amended sandy clay loam soil.

Nitrous oxide (NO) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of NO emissions with nitrogen fertilizer application being the main driver of rising atmospheric NO concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate NO emissions from soils. However, the underlying processes that cause NO emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used 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 NO emissions. Furthermore, we determined significantly higher quantities of soil-entrapped NO and N in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced NO emission mitigation is based on the entrapment of NO in water-saturated pores of the soil matrix and concurrent stimulation of microbial NO reduction resulting in an overall decrease of the NO/(NO + N) ratio.