Effect of biochar origin and soil pH on greenhouse gas emissions from sandy and clay soils

Emissions of greenhouse gases (GHGs), such as carbon dioxide (CO₂) and nitrous oxide (N₂O) have great impact on global warming and atmospheric chemistry. Biochar addition is a potential option for reducing GHGs emissions through carbon (C) sequestration and N₂O mitigation. However, the influences of biochar on C and nitrogen (N) transformations in soil are still unclear, resulting in a poor understanding of the mechanisms of N₂O mitigation effects of biochar. Here we carried out two soil incubation experiments to investigate the influence of two common biochars addition (corn cob and olive pulp) with ammonium sulfate on CO₂ and N₂O emissions from two contrasting soil types (acidic sandy and alkaline clay soil). Furthermore, four extracellular enzymes activities that related to C and N cycling, i.e. cellobiohydrolase, chitinase, xylanase and β-glucosidase, were analyzed to gain insights into the underlying mechanisms of biochar’s effects on CO₂ and N₂O evolutions. Contrasting effects of two biochars on CO₂ and N₂O emissions were observed in the two different soils. The corn biochar addition had no significant effect on CO₂ and N₂O emissions in the alkaline clay soil, but significantly decreased CO₂ emissions by 11.8% and N₂O emissions by 26.9% in the acidic sandy soil compared to N-fertilizer only treatment. In contrast, olive biochar addition showed no significant effect on CO₂ emissions but decreased N₂O emissions by 34.3% in the alkaline clay soil, while in the acidic sandy soil addition of olive biochar triggered about a twofold higher maximum CO₂ emission rate and decreased N₂O emissions by 68.4%. Up to 50–130% higher specific CO₂ emissions (per unit of C-related enzyme activity: cellobiohydrolase, chitinases and β-glucosidase) were observed after addition of olive biochar compared to corn biochar addition in the acidic sandy soil. We concluded that biochar’s effects on N₂O and CO₂ emissions are more pronounced in acidic soils. Alkaline biochar’s NO mitigation potential in acidic soils seems to be dependent on soil NO₃⁻ content as drastically higher N₂O emissions were measured in early phase of the experiment (where soil NO₃ ⁻ was high) and significantly lower N₂O fluxes were obtained in later phases (with lower soil NO₃⁻ content).