Biochar can reduce N2O production potential from rhizosphere of fertilized agricultural soils by suppressing bacterial denitrification

Biochar-fertilizer combination is a promising strategy to improve environmental quality while maintaining agronomic performance. Yet, its effect on soil nitrification and denitrification and their causal pathways remains less explored. Here in a pot experiment growing pakchoi (Brassica Chinensis) under two nitrogen (N) fertilizer rates (100 and 200 mg N kg−1) in combination with four biochar levels (0%, 0.5%, 1% and 2% w/w), we investigated the potential of N2O production-related bacterial and fungal nitrification and denitrification from both non-rhizosphere and rhizosphere, as well as the abundance of bacterial and fungal genes associated with N2O production. Results revealed strong inorganic fertilizer and biochar-driven changes in the potential of N2O production as well as the gene abundance associated with these processes. However, bacteria and fungi showed distinct responses to inorganic fertilizer and biochar amendment. Greater N fertilization rates increased the abundance of bacterial genes and bacterial-related nitrification and denitrification potential by 89 ± 2.3% and 70 ± 4.1%, respectively; but did not affect fungal gene abundance or activities. By contrast, the increased application rate of biochar, although increased fungal activities overall by 21 ± 1.8%, resulted in a decline of the abundance of N-cycle bacterial genes and bacterial-related nitrification and denitrification, especially in the rhizosphere, a hotspot of soil microbial activities and greenhouse gas emissions. As a result, the N2O production potential from denitrification increased in the non-rhizosphere but decreased in the rhizosphere with biochar application. Also, structural equation modeling showed a greater bacterial contribution to total denitrification than that of fungi in the non-rhizosphere but showed an opposite trend in the rhizosphere soil. These findings, together with greater soil C content but lower NO3−-N with the increased rate of biochar application, suggest that partially substituting inorganic fertilizers with biochar can reduce potential N2O production from bacterial nitrification and denitrification.