Is the content and potential preservation of soil organic carbon reflected by cation exchange capacity? A case study in Swiss forest soils

Abstract. The content of organic carbon (C) in soils is not stable, but depends on a number of environmental variables and biogeochemical processes that actively regulate its balance. An improved identification of the environmental variables that can be used as predictors of soil organic C (SOC) content is needed to reduce uncertainties of how the soil C reservoir will respond to environmental change. Although several simulations rely on the amount of clay to reproduce changes in the balance of SOC, recent efforts have suggested that other soil physicochemical properties may serve as better predictors. Here we tested whether the effective cation exchange capacity (CEC eff.), may be a more suitable predictor of the content and potential preservation of SOC as compared to the mere quantification of clay-size particles. We further assessed how various climatic, vegetation and edaphic variables explain the variance of SOC content across different soil depths and soil pH classes. A set of more than 1000 forest sites across Switzerland, spanning a unique gradient of mean annual precipitation (636–2484 mm), altitude (277–2207 m a.s.l), pH (2.8–8.1) and representing different geologies and soil orders was used as a case study for this linear model analysis. Our results showed that CEC eff. has the largest explanatory potential of SOC content (35 % of response variance in the complete mineral soil profile) as compared to the amount of clay (which only explained 7 % of the response variance in the complete mineral soil profile) and other environmental variables. CEC eff. is strongly linked to SOC especially in the top mineral soil (0–30 cm depth) with the larger presence of organic matter. At deeper soil depths most of the variance in SOC is instead explained by climate, which in Switzerland is related to a greater weathering activity and translocation of organic C through leaching with increasing mean annual precipitation. We further observed soil pH to have a complex influence on SOC content, with CEC eff. being a dominant variable controlling SOC content at pH > 4.5 in the upper mineral soil and pH > 6 in the subsoil. Since CEC eff. is an edaphic property which is intimately associated to both the conditions that shaped the soil and the current edaphic physicochemical conditions, these findings indicate that considering CEC eff. as an integrative proxy for the potential preservation of SOC and its alteration could improve future predictions of how the soil C reservoir will feed back to environmental change.