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) and nitrous oxide (NO) emissions. However, the effects of biochar amendment on NO and CH reduction in soil under rotation with different soil moisture contents is not well understood. To understand CH and NO 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 NO and CH emissions following soil amendment with tobacco straw biochar at rates of 0, 10, 40 and 80 t·ha (B0, B10, B40 and B80, respectively). NO and CH 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 in the UT season. During both the UT and SR seasons, with biochar amendment at 40 and 80 t·ha, the soil bulk density (BD) decreased, while the soil organic matter (SOM) and available potassium (Av. K) contents increased. NO 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 emission decreased with the rate of biochar application and the methanotroph pmoA gene copy number in soils and increased with the methanogenic archaea 16Sr 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 reduction.