Evaluating soil organic carbon stock changes induced by no-tillage based on fixed depth and equivalent soil mass approaches

Abstract

It is controversial whether no tillage (NT) should be regarded an effective agro-ecosystem management to increase soil organic carbon (SOC). Some studies suggest NT leads to a notable accumulation of SOC stock across the whole sampling profile over the long-term, whereas other studies argue that the effect of duration on SOC stock increase is limited. Similar controversy is common with respect to the role of climatic conditions in SOC changes. In addition, despite the gradual recognition of the importance of equivalent soil mass (ESM) compared to fixed depth (FD) approach, in terms of SOC stock evaluation, the ESM approach mainly focuses on the direct impact of bulk density but ignores the potential indirect effect on SOC concentration. To further understand the SOC sequestration mechanisms under NT, responses of SOC stock after the adoption of NT were investigated based on FD and ESM approaches. The results showed that NT significantly changed the SOC stock distribution in different soil depths according to both FD and ESM, but no significant changes were observed when soil sampling deeper than 50 cm. Notably, the results illustrated that the use of FD overestimated the SOC stock in the soil surface due to the increased bulk density under NT, but underestimated the SOC stock in 30–70 cm soil layers compared to ESM. Consequently, the overestimate of SOC based on FD may not be as high as previously assumed across the sampling depth. Mean annual temperature and mean annual rainfall did not lead to any significant changes in SOC stock of the overall soil profile or different layers, indicating the limited impacts of climatic conditions on carbon sequestration after the adoption of NT. Long-term NT resulted in a significant accumulation in SOC stock in the top 5 cm of the surface soil, but SOC stock changes with time in soil layers deeper than 5 cm was not significant in both FD and ESM approaches. The overall changes in SOC stock as time increased were not significant across the whole profile based on FD, but was weakly significant based on ESM. The results illustrate that using the overall SOC stock change of the whole sampling soil profile deeper than 30 cm masks the beneficial change in SOC stock in the 0–5 cm soil over long-term adoption of NT. The biased interpretation of depth factor is the main reason that fuels the debate over whether long-term NT is beneficial for the accumulation of SOC stock, and hence must be properly considered. Overall, the data illustrate that the depth factor is not only important in terms of assessing total SOC stock changes under NT, but also critical when evaluating the effectiveness of FD and ESM, as well as the impact of experimental duration on SOC increase. The depth factor should thus be comprehensively considered in SOC stock assessment and more field experiments sampling to deeper depth are urgently needed to clarify whether NT is beneficial for SOC sequestration and climate mitigation.