Comparison of equivalent soil mass approaches to estimate soil organic carbon stocks under long-term tillage

In agricultural ecosystems, soil organic carbon (SOC) which is affected by management practices, is important for soil health, food security, and climate change mitigation. However, accurately assessing the influence of agricultural management practices on SOC storage is a challenge, due in part to the uncertainty of calculation approaches used to estimate SOC stocks. Although equivalent soil mass (ESM) is widely recommended over the fixed depth (FD) approach, few field studies directly compare FD with different ESM approaches. Hence, the magnitude of potential difference in estimated SOC stocks among different approaches is not well known. Here, we collected soil cores (0–60 cm depth) from a 24-yr experiment (Ridgetown, Ontario, Canada) to investigate (1) the effect of two tillage systems (conventional tillage: CT, moldboard plowing to ∼20 cm deep; no-tillage with zone tillage: NT/ZT) on different SOC estimates (n=448), including concentration and stocks estimated by FD and ESM (cubic spline interpolation model: ESMcubic_spline, linear interpolation model: ESMlinear, and non-modeling fit: ESMnon_model); and (2) the relative difference of SOC stocks estimated by FD and ESM under NT/ZT system. The tillage effect on SOC stock was more pronounced than SOC concentration (P<0.05; except for 0–5 cm depth), indicating that using SOC concentration alone appears to be unreliable for evaluating tillage system effects on carbon sequestration. Furthermore, FD overestimated SOC stocks under NT/ZT (P<0.05), mainly due to greater soil bulk density than CT (P<0.05). Specifically, in the 0–60 cm depth, FD overestimated about 15% (or 30.6 Mg ha−1) of cumulative SOC stocks than ESMcubic_spline. However, the differences in SOC stocks estimated among three ESM approaches (ESMcubic_spline, ESMlinear, and ESMnon_model) were negligible (P>0.05; Cohen’s d<0.2), suggesting that these approaches may work equally well when soil depth increment is small (< 10 cm). Overall, we recommend using the ESM approach to calculate SOC stock, especially when comparing treatments where soil bulk density varies. Our findings may help to guide policy decision-making towards more accurately quantifying SOC stock when considering climate change mitigation practices.