<scp>SOC</scp> Increase in <scp>UK</scp> Topsoils Is Most Likely due to <scp>SOC</scp> Vertical Redistribution: Comment on Bentley et al. (2025)

Percent cropland under conservation agriculture (CA) as function of time. Conservation agriculture—defined by permanent soil cover, minimal disturbance, and species diversification—expanded from virtually zero in 1990–220 M ha in 2020, or roughly 14% of global croplands (Kassam et al. 2022); (Figure 1). In the UK, reduced or no-till practices, once limited to trials and few farms, surged in adoption from the late 1990s onward. By 2010, 44% of arable land used reduced or no-till (DEFRA Farm Practices Survey), increasing to 50%–55% (including 7%–10% no-till) by the late 2010s. The reported SOC accrual since 2007 thus coincides with the widespread uptake of reduced-tillage practices. Bentley et al. (2025) argue that increased SOC in topsoils is strongly linked to sustainable land management uptake, particularly reduced tillage. Their position relies partly on Powlson et al. (2012), who reported SOC increases of 0.31 t ha−1 year−1 in the 0–30 cm layer following reduced tillage in the UK, combined with global SLM estimates (Freibauer et al. 2004; Smith 2004). However, studies assessing full soil profiles (to 1 m depth) consistently show that reduced or no-till practices primarily stratify SOC: gains near the surface are offset by losses at depth, resulting in negligible net SOC change. This pattern was first demonstrated by Baker et al. (2007) using global datasets and confirmed by subsequent synthesis studies (e.g., Luo et al. 2010; Liang et al. 2020) and long-term (> 40 years) investigations such as by Dimassi et al. (2013). For example, Luo et al. (2010), analysing 69 paired global experiments, found SOC increased by 3.15 ± 2.42 t ha−1 in the top 10 cm under no-till but decreased by 3.30 ± 1.61 t ha−1 in the 20–40 cm layer. These findings strongly suggest that recent UK topsoil SOC gains are more likely due to vertical redistribution than to true carbon sequestration. Because SOC can shift within the soil profile without changing total SOC, temporal monitoring must report changes across the entire profile to avoid misinterpretation and misguided policy. Although increased topsoil SOC can offer agronomic benefits, it is also more vulnerable to loss (e.g., Dimassi et al. 2013). Our understanding of SOC controls in croplands remains limited. Not only has reduced tillage been questioned for its ability to increase SOC, but cover cropping has recently been challenged as well (Chaplot and Smith 2024). If neither reduced tillage nor cover cropping reliably increases SOC stocks, identifying the true drivers becomes critical. Chaplot and Smith (2023) highlight the underappreciated role of fertilization: SOC losses tend to be less severe—and sometimes reversed—when N, P, and K are applied at rates aligned with crop and soil nutrient needs. Vincent Chaplot: conceptualization, investigation, writing – original draft, writing – review and editing. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.