Soil carbon stock accrual under perennial cropping overestimated by many methods

Perennial crops are increasingly recognized for their potential to enhance soil carbon (C) stocks, owing to their continuous C inputs from extensive root systems and reduced C degradation due to minimal tillage. Integrating perennials into traditional arable agriculture is emerging as a promising strategy for C farming. However, inconsistencies in calculation methods across studies complicate direct comparison and hinder a comprehensive assessment of the changes in soil C stocks. Soil C stock is calculated by either fixed depth (FD) or equivalent soil mass (ESM) method, and the changes are either absolutely compared to the baseline soil or relatively compared to reference plots (i.e., adjacent fields representing previous land use or annual cropping systems). Here, we conducted a meta-analysis using 1545 paired observations from 110 publications to evaluate the changes in soil C stock under perennial cropping systems as estimated by different methods.

The results revealed significant biases introduced by calculation and comparison methods. In the topsoil (0–30 cm), compared to the baseline, the ESM method estimated a 6.1% (3.3–8.8%) increase in soil C stock under perennial cropping, whereas the FD method produced an 80% higher estimate (11.0%). Meanwhile, the relative changes (10.6%) based on the ESM method was 74% higher than absolute changes. In contrast, subsoil showed no significant absolute change, with the ESM method estimating a change of 4.9% (-2.9–12.7%). The effect of perennial cropping on soil C stock varied by system type. Grass monoculture, grass mixture, and short rotation coppice increased soil C stocks (7.9–15.4%), while incorporating perennials into crop rotations led to a decrease (-5.5%). Environmental factors also influenced the changes in C stocks. Soil C stock change was positively correlated to mean annual precipitation and temperature (p < 0.05), but negatively related to initial soil C content (p < 0.05). In the medium- and low-C soils (SOC < 20 g kg^-1), changes in C stocks were positively correlated to clay content and experimental duration.

Overall, our findings confirm that appropriately managed perennial cropping systems could enhance soil C stocks, with the changes primarily occurring in the topsoil. Furthermore, this study underscores the importance of selecting suitable calculation methods to ensure accurate estimates of C stock changes.