Methane and Nitrous Oxide Flux after Biochar Application in Subtropical Acidic Paddy Soils under Tobacco-Rice Rotation

Biochar amendment is a good means of mitigating methane (CH 4 ) and nitrous oxide (N 2 O) emissions. However, the effects of biochar amendment on N 2 O and CH 4 reduction in soil under rotation with different soil moisture contents is not well understood. To understand CH 4 and N 2 O flux from soil with biochar amendment under water-unsaturated and water-saturated conditions, a field experiment was conducted in a tobacco-rice rotation field in subtropical China to investigate N 2 O and CH 4 emissions following soil amendment with tobacco straw biochar at rates of 0, 10, 40 and 80 t·ha −1 (B0, B10, B40 and B80, respectively). N 2 O and CH 4 emissions were monitored by a closed-chamber method in the water-unsaturated tobacco (UT) and water-saturated rice (SR) seasons during the 2015 planting season. The soil pH increased from 5.4 in the control to 6.1 in the soil amended with biochar at 80 t·ha −1 in the UT season. During both the UT and SR seasons, with biochar amendment at 40 and 80 t·ha −1 , the soil bulk density (BD) decreased, while the soil organic matter (SOM) and available potassium (Av. K) contents increased. N 2 O flux was significantly greater in UT than in SR in the controls but decreased with the application of biochar during both the UT and SR seasons. The cumulative CH 4 emission decreased with the rate of biochar application and the methanotroph pmoA gene copy number in soils and increased with the methanogenic archaea 1 6Sr DNA gene copy number in soils during the rice-cropping season. These results indicated that biochar amendment could decrease methanogenic archaea and increase of methanotroph pmoA gene, which are the mechanistic origin for CH 4 reduction.