Carbon budget by priming in a biochar-amended soil

• The presence of aged and fresh biochars reduced soil CO2 emissions.
• Aged and fresh biochars have contrasting effects on glucose mineralization.
• Repeated and continuous glucose additions have accumulated carbon in soil.
• The presence of aged or fresh biochar had low net carbon accumulation.

Understanding the balance between the priming effect and the retention of added organic carbon (C) in soil is important in evaluating the role of specific practices for terrestrial C sequestration. However, knowledge about the effects of biochar addition on soil net C accumulation following various C input patterns remains limited. We incubated soil with aged and fresh biochar, and added 14C-labeled glucose at three frequencies over 120 days: single, repeated, and continuous additions. Regardless of glucose addition, the presence of aged or fresh biochar reduced the accumulative CO2 emissions, by 23% on average, relative to the soil without biochar addition. Compared with the soil without biochar addition, glucose mineralization significantly increased by 1.4–2.0% in the soils with aged biochar, but decreased by 0.1–1.9% in the soils with fresh biochar. Relative to repeated and continuous glucose additions, the single addition may slightly overestimate priming effects, especially during the early incubation period. At the end of incubation, repeated and continuous additions showed a higher net C accumulation than the single addition. Regardless of the glucose addition frequency, net C accumulation was lower in soils with aged or fresh biochar than in the control. In summary, biochar amendment has an interactive effect with glucose addition frequency and thus affects the direction and magnitude of priming. Despite the direct contribution of biochar to C sequestration in soil, the low net C accumulation from glucose in the biochar-amended soil highlights the necessity of C balance accounting.