Efficiency of additional organic inputs for carbon sequestration in agricultural soils modulated by the priming effect and physical accessibility.

• Divergent responses of SOC dynamics to organic amendment (OA) inputs. • Additional OA inputs alter the decay of SOC pools and their physical accessibility. • Efficiency of OA inputs for SOC sequestration depends on baseline SOC content. • OA-induced changes in SOC turnover must be considered for carbon sequestration. Increasing carbon input (CI) to soil has been widely recommended (e.g., residue retention and manure application) to sequester more carbon in agricultural soils thereby mitigating climate change and improving soil quality. However, carbon sequestration may not respond linearly to additional CI due to their active interactions with the turnover of native soil organic carbon (SOC) via such as the priming effect and mediating SOC accessibility to decomposition. In this study, we collected SOC measurements from 22 long-term paired trials (12–142 years) with two levels of CI [i.e., a treatment with carbon input (+CI) and another treatment with less carbon input (-CI) compared with + CI] across the globe. We used these observed SOC to constrain a carbon model and analyse SOC turnover processes as impacted by CI to the soil. The results show that there are divergent responses of SOC turnover processes to CI, indicated by large variability of changes in decay rates of carbon pools, transfer coefficient of carbon flow from fast to slow pools, and physical accessibility to decomposition. Initial SOC at the start of the trial, the average amount of CI during the experiment, and soil bulk density are the three most important variables underlying such divergent responses. Simulation experiments suggest that these discrepancies in SOC turnover processes induced by CI have significant consequences on the efficiency of increasing carbon inputs for SOC sequestration. Ignoring the effect of additional CI leads to overestimation or underestimation of CI required to achieve typical SOC sequestration targets depending on baseline SOC content and temperature. Our results demonstrate the vital role of CI in regulating SOC turnover processes, which have site-specific consequences on the management of carbon inputs to sequester SOC. [ABSTRACT FROM AUTHOR]