Biochar decreases nitrogen oxide and enhances methane emissions via altering microbial community composition of...

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.