Insights into CO2 and N2O emissions driven by applying biochar and nitrogen fertilizers in upland soil

Biochar and soil carbon sequestration hold promise in mitigating global warming by storing carbon in the soil. However, the interaction between biochar properties, soil carbon-nitrogen cycling, and nitrogen fertilizer application's impact on soil carbon-nitrogen balance remains unclear. Herein, we conducted batch experiments to study the effects and mechanisms of rice straw biochar application (produced at 300, 500, and 700 °C) on net greenhouse gas emissions (CO2, N2O, CH4) in upland soils under different forms of nitrogen fertilizers. The findings revealed that (NH4)2SO4 and urea significantly elevated soil carbon dioxide equivalent emissions, ranging from 28 to 61.7 kg CO2e/ha and 8.2 to 37.7 kg CO2e/ha, respectively. Conversely, KNO3 reduced soil CO2e emissions, ranging from 2.2 to 13.6 kg CO2e/ha. However, none of these three nitrogen forms exhibited a significant effect on CH4 emissions. The pyrolysis temperature of biochar was found negatively correlated with soil CO2 and N2O emissions. The alkaline substances present in biochar pyrolyzed at 500-700 °C raise soil pH, increase the ratio of Gram-negative to Gram-positive bacteria, and enhance the relative abundance of Sphingomonadaceae. Moreover, the co-application of KNO3 based nitrogen fertilizer and biochar increases the total carbon/inorganic nitrogen ratio and reduces the relative abundance of Nitrospirae. This series of reactions lead to a significant increase in soil DOC content, meanwhile reduces soil CO2 emissions, and inhibits the nitrification process and decreases the emission of soil N2O. This study provides a scientific basis for the rational application of biochar in soil.