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

Abstract

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 remained 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 CO_2e/ha and 8.2 to 37.7 kg CO_2e/ha, respectively. Conversely, KNO₃ reduced soil CO_2e emissions, ranging from 2.2 to 13.6 kg CO_2e/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 presented in biochar pyrolyzed at 500-700 °C raised soil pH, increased the ratio of Gram-negative to Gram-positive bacteria, and enhanced the relative abundance of Sphingomonadaceae. Moreover, the co-application of KNO₃ based nitrogen fertilizer and biochar increased the total carbon/inorganic nitrogen ratio and reduces the relative abundance of Nitrospirae. This series of reactions led to a significant increase in soil DOC content, meanwhile reduced soil CO₂ emissions, and inhibited the nitrification process and decreased the emission of soil N₂O. This study provided a scientific basis for the rational application of biochar in soil.

Keywords: Biochar amendment; CO(2) and N(2)O emissions; Nitrogen fertilizer; Upland soil.