Tillage intensity affects total SOC stocks in boreo-temperate regions only in the topsoil-A systematic review using an ESM approach

Shifting from high intensity (HT) to intermediate intensity (IT) or no tillage (NT) practices has been credited as being a promising agricultural management option towards climate change mitigation due to carbon (C) sequestration in the soil. The consequences of conversion from HT to mainly NT on soil organic carbon (SOC) have been subject to a number of meta-analyses revealing either a positive or non-significant effect. In this study, we used the equivalent soil mass (ESM) approach to evaluate SOC stock changes following the conversion from HT to IT and NT considering studies compiled within a systematic review. In order to maximize the use of available evidence, we used different substitution methods for imputing missing information on the variance of both SOC and bulk density (pb). Based on a total of 101 long-term field trials ( > 10 years), the positive effect of IT and NT compared to HT was found to be limited to the topsoil (0-30 cm depth). Estimated SOC stock increases for this particular depth ranged from 3.22 +/- 1.48 to 3.50 +/- 1.60 (HT vs. IT) and 4.19 +/- 1.82 to 4.23 +/- 1.92 Mg ha(-1) (HT vs. NT). Calculating stocks based on fixed depth layers and without consideration of the equivalent soil mass, respectively, resulted in an overestimation of the increase with 15 (HT vs. IT) and 47% (HT vs. NT and IT vs. NT). Due to shallow sampling depth, HT vs. IT and IT vs. NT comparisons were limited to 0-30 cm depth, but the effect of HT to NT conversion could also be determined for 0-60 cm. The results indicate that the NT sequestration potential is overvalued when neglecting deeper depths, since the SOC storage capacity was reduced to < 0.10 Mg ha(-1) yr(-1). No linear relation between relative SOC stock changes and climate, soil, and cropping systems was found.