Abstract: The objectives of this study were to investigate the profile distribution and accumulation characteristics of soil organic carbon (SOC) and total nitrogen (TN) under different tillage treatments, and further to compare the effects of sampling depths and calculation methods on the evaluation of SOC and TN storages. Two field experiments were established at the Shandong Huantai (5 years) and Hebei Luancheng (17 years) sites. The experiments included three tillage treatments (with residue): conventional tillage (CT), rotary tillage (RT) and no-tillage (NT). Soil samples were collected down to 60 and 50cm depths at Huantai and Luancheng site respectively. Soil bulk density (b) and the distribution of SOC and TN concentrations were determined. The SOC and TN storages were calculated by the fixed depth (FD) and equivalent soil mass (ESM) methods. The results showed that soil depth significantly affected the soil b, the concentrations and storages of SOC and TN (P<0.001). Compared with CT, NT enhanced SOC and TN storages in the top layer, and increased the stratification ratio (SR) of SOC and TN concentrations, though the SR value of SOC at Luancheng site was not significant. RT (cf. CT) increased the storages of SOC and TN in the top layer and the SR value of TN concentration at the Luancheng site. Specifically, at the Huantai site, the SOC and TN storages under NT were 29% and 30% higher than that of CT in the 0−5cm soil layer (P<0.05), but were 8% and 10% lower in the 0−60cm soil profile. At the Luancheng site, the SOC storage in the NT and RT was higher by 10% and 14% than CT; but there was no significant differences in SOC and TN storages between tillage treatments in the deeper profiles (i.e., ≥20cm). Due to the varied soil b between the treatments in the surface layer, the FD method overestimated the SOC and TN storages in the NT soil at the Huantai site, but underestimated them at the Luancheng site. Therefore, to accurately assess the SOC sequestration induced by tillage conversion, the ESM instead of FD method was recommended to calculate SOC storages together with the ＂deeper sampling＂ strategy (≥30cm). Our study implicates that although conservation tillage has positive effect on soil quality, the potential for mitigating climate change through SOC sequestration should not be overestimated.

Key words: Conservation tillage, Soil organic carbon, Total nitrogen, Fixed depth, Equivalent soil mass