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 beta-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 beta-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 N2O 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).