Effect of rice-straw biochar on nitrous oxide emissions from paddy soils under elevated CO₂ and temperature

In paddy ecosystems, the feedback of N₂O emissions on climate change as well as the effects of biochar on N₂O emissions will have a profound impact on global warming and rice production. In order to investigate the effect of biochar amendment on N₂O emissions from paddy soils under elevated atmospheric CO₂ concentration (700ppm) and air temperature (ambient +3°C), a microcosm study was performed in plant growth chambers with rice plant and rice-straw-derived biochar. N₂O emissions from urea-fertilized paddy soils had a significant increase with the biochar amendment during midseason drainage under ambient CO₂ concentration and air temperature. However, N₂O emissions was suppressed under simultaneous elevated CO₂ concentration and air temperature, regardless of whether biochar was amended; under this condition the stimulating effect of biochar was attenuated. Reduced mineral N concentrations and increased DOC concentrations could inhibit N₂O emission at simultaneously elevated CO₂ concentration and air temperature. Increased soil pH and variation in the abundance of archaeal and bacterial amoA genes indicated that biochar amendment could stimulate ammonia oxidation-induced N₂O emission during nitrification. Moreover, the liming effect of biochar was lessened under elevated CO₂ concentration and temperature, which may contribute to the attenuated stimulating effect of biochar on N₂O emissions. More attention should to be paid to the effect of soil pH-induced changes in ammonia oxidation, when mitigating N₂O emissions in urea-fertilized paddy soils with biochar amendment at elevated CO₂ concentrations and air temperatures.