Biochar decreases nitrogen oxide and enhances methane emissions via altering microbial community composition of anaerobic paddy soil

Biochar application to agricultural soil is an appealing approach to mitigate nitrous oxide (N₂O) and methane (CH₄) emissions. However, the underlying microbial mechanisms are unclear. In this study, a paddy soil slurry was incubated anaerobically for 14d with biochar amendments produced from rice straw at 300, 500, or 700°C (B300, B500, and B700) to study their influences on greenhouse gas emissions. Illumina sequencing was used to characterize shift of soil bacterial and archaeal community composition. After peaking at day 1, N₂O emission then sharply decreased to low levels while CH₄ started to emit at day 3 then continually increased with incubation. Compared to control soil (57.9mgkg^-1 soil), B300, B500, and B700 amendments decreased N₂O peak emission to 17.9, 1.28, and 0.59mgkg^-1, mainly due to increased soil pH. In contrast, the amendments enhanced CH₄ production from 58.2 to 93.4, 62.6, and 63.4mgkg^-1 at day 14 due to increased soil dissolved organic carbon. Abundance of denitrifying bacteria (e.g., Bacilli, 7.07-13.6 vs. 16.9%) was reduced with biochar amendments, especially with B500 and B700, contributing to the decreased N₂O emissions. However, larger pore size of B500 and B700 (surface area of 68.1 and 161m²g^-1) than B300 (4.40m²g^-1) favored electron transfer between bacteria and iron minerals, leading to increased abundance of iron-reducing bacteria, (e.g., Clostridia, 48.2-50.6 vs. 33.3%), which competed with methanogens to produce CH₄, thereby leading to lower increase in CH₄ emission. Biochar amendments with high pH and surface area might be effective to mitigate emission of both N₂O and CH₄ from paddy soil.