Effect of biochar origin and soil type on the greenhouse gas emission and the bacterial community structure in N fertilised acidic sandy and alkaline clay soil.

Soil amendment with biochar has received increased attention because of its potential to i) sequester carbon and ii) reduce N2O emission when applied to N fertilised soils. To study the effect of biochar origin on greenhouse gas emission in two contrasting soil types, we used a robotized continuous flow incubation system and δ13C stable isotope approach to compare four biochar types (feed stock: olive mill, corn cob, pistachio shell, cotton stalk) in an alkaline clay soil and two selected biochar types (feed stock: olive mill and corn cob) in an acidic sandy soil. Furthermore, high-throughput sequencing of 16S rRNA genes was performed at the end of the incubation to investigate the effect of different biochars on bacterial community structure in the two different soils. In the alkaline clay soil, all biochar types in conjunction with N fertiliser decreased CO2 emissions up to 12% compared to the N added control treatment causing negative priming, whereas no significant effect of biochar addition on N2O emissions was observed. In contrast, application of olive mill biochar to the acidic sandy soil significantly increased soil pH, CO2, and N2O fluxes, whereas no significant effect of corn cob biochar addition was observed. There was a significant linear relationship between the biochar induced increase in soil pH and the biochar induced increase in soil born N2O emission. Additionally, we detected a clear variation in bacterial community structure in the acidic sandy soil (phyla Acidobacteria, Nitrospirare, and Arthrobacter) with the olive mill biochar addition. Overall, the amendment of different biochars failed to mitigate N2O emissions in both soil types when mineral fertiliser was added. Furthermore, amendment of olive mill biochar stimulated both N2O and CO2 emissions in the low pH sandy soil and altered the bacterial community structure, which was possibly related to its liming effect.