How does biochar influence soil nitrification and nitrification-induced N₂O emissions?

Nitrification is a major pathway of N₂O production, especially in aerobic soils. The amendment of soils with biochar has been suggested as a promising solution to regulate soil N cycle and reduce N₂O emissions. However, there is a lack of comprehensive and quantitative understanding of biochar impacts on soil nitrification and nitrification-induced N₂O emissions. In this study, a meta-analysis was conducted using data compiled across 95 peer-reviewed studies. Results showed that biochar in general significantly increased soil nitrification rate by 56 %, with overall no significant effect on nitrification-induced N₂O emissions, suggesting that biochar likely restricted the fraction of nitrified N emitted as N₂O emissions. The abundance of ammonia-oxidizing bacteria (AOB) was significantly increased by 37 % following biochar addition, but that of ammonia-oxidizing archaea (AOA) did not change significantly, indicating that the impact of biochar on AOB rather than AOA may play an important role in soil nitrification. The impacts of biochar on soil nitrification processes were heterogeneous depending on soil properties. Biochar increased soil nitrification rate and AOB abundance to a larger extent in poorly pH-buffered soils such as those with acidic pH (<5), low organic carbon (<10 g kg^-1), or poor texture (rich in either sand or clay), which may be attributed to the liming and structural effects of biochar that regulate soil pH and water-air status. The overall no significant effect of biochar on nitrification-induced N₂O emissions was due to a positive effect in acidic soils, a negative effect in alkaline soils, and little effect in neutral soils. This study provides a comprehensive insight into how different factors mediate the response of soil nitrification processes to biochar amendment, which contributes to a new understanding of biochar function in regulating soil N₂O emissions, and can assist in designing biochar projects that would benefit soil N cycle while minimizing undesirable side effects.