Effect and mechanism of biochar on CO₂ and N₂O emissions under different nitrogen fertilization gradient from an acidic soil

"Nature based solutions" has been proposed at COP25 as an important venture for combating anthropogenic climate change, and soil biochar amendment have been proposed to have vast carbon sequestration potential. On the other hand, biochar carbon storage in soils is confronted with both biochar and soil carbon and nitrogen loss. The superposition of these two influences leads to complex variation in net greenhouse gas emissions from biochar-amended-soils. Nitrogen fertilization is a common agriculture practice in China and worldwide. To study the effects and mechanisms of biochar on soil net greenhouse gas emissions (CO₂, N₂O) under different nitrogen fertilization gradient in a ferrallitic soil, a soil column experiment was conducted. Maize straw derived biochar (pyrolysed at 500 °C) and nitrogen fertilizer (ammonium sulfate) were investigated at varying application rates. It was found that biochar amendment increased CO₂ emissions by 51.1%-57.1% and N₂O emissions by 50.0%-73.7%, respectively, when soil was incubated with 50 mg N/kg nitrogen fertilization. The N₂O emission in soil was dominated by nitrification, and the labile fraction of biochar played the dominant role in increasing soil CO₂ and N₂O emissions. Therefore, water or acid washing of biochar before its application would significantly reduce the net GHG emissions. When the nitrogen fertilization was applied at the unusually high level of 450 mg N/kg, the N₂O emissions mainly came from denitrification. Biochar amendment introduced less soil CO₂ emission increment, and suppressed N₂O emissions by inhibition of denitrification via adsorption protection mechanism (towards nitrogen) and aeration effect. A chain mechanism has been illustrated and results from this study suggest that biochar is best applied to an environment or the circumstance that maximizes adsorption protection mechanism and aeration effect to achieve total greenhouse gas emission reduction. This study therefore provides basis for the scientific sound application and regulation of biochar amendment in soils.