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.

中文翻译：

---

基于固定深度和等效土壤质量方法评估免耕引起的土壤有机碳库变化

摘要 免耕（NT）是否应被视为一种有效的农业生态系统管理以增加土壤有机碳（SOC）存在争议。一些研究表明，NT 会导致整个采样曲线中长期 SOC 储量的显着积累，而其他研究则认为持续时间对 SOC 储量增加的影响是有限的。关于气候条件在 SOC 变化中的作用，类似的争议很常见。此外，尽管等效土壤质量 (ESM) 与固定深度 (FD) 方法相比的重要性逐渐得到认可，但在 SOC 储量评估方面，ESM 方法主要关注容重的直接影响，而忽略了潜在的间接影响。 SOC 浓度的影响。为了进一步了解 NT 下的 SOC 封存机制，基于 FD 和 ESM 方法研究了采用 NT 后 SOC 储量的反应。结果表明，根据 FD 和 ESM，NT 显着改变了不同土壤深度的 SOC 储量分布，但当土壤取样深度超过 50 cm 时，未观察到显着变化。值得注意的是，结果表明，由于 NT 下堆积密度的增加，FD 的使用高估了土壤表面的 SOC 储量，但与 ESM 相比，低估了 30-70 cm 土层中的 SOC 储量。因此，基于 FD 对 SOC 的高估可能不像之前假设的那样高，跨越采样深度。年平均温度和年平均降雨量不会导致整个土壤剖面或不同层的 SOC 储量发生任何显着变化，表明在采用 NT 后气候条件对碳固存的影响有限。长期 NT 导致表层土壤顶部 5 cm 的 SOC 储量显着积累，但在 FD 和 ESM 方法中，深度超过 5 cm 的土壤层中 SOC 储量随时间的变化不显着。随着时间的增加，SOC 储量的整体变化在基于 FD 的整个配置文件中并不显着，但基于 ESM 是微弱的。结果表明，使用深度超过 30 厘米的整个采样土壤剖面的整体 SOC 储量变化掩盖了长期采用 NT 后 0-5 厘米土壤中 SOC 储量的有益变化。对深度因子的偏见解释是引发关于长期 NT 是否有利于 SOC 储量积累的争论的主要原因，因此必须适当考虑。总体而言，数据表明，深度因子不仅在评估 NT 下总 SOC 储量变化方面很重要，而且在评估 FD 和 ESM 的有效性以及实验持续时间对 SOC 增加的影响时也很重要。因此，在 SOC 储量评估中应综合考虑深度因素，迫切需要更多的实地实验采样到更深的深度，以明确 NT 是否有利于 SOC 封存和减缓气候变化。