Abstract
An improved identification of the environmental variables that can be used to predict the content of soil organic carbon (SOC) stored belowground is required to reduce uncertainties in estimating the response of the largest terrestrial carbon reservoir to environmental change. Recent studies indicate that some metal cations can have an active role in the stabilization of SOC, primarily by coordinating the interaction between soil minerals and organic matter through cation bridging and by creating complexes with organic molecules when their hydration shells are displaced. The effective cation exchange capacity (CEC eff.) is a measure that integrates information about available soil surfaces to which metal cations are retained. Therefore, we critically tested the relationship between CEC eff. and SOC content using regression analyses for more than 1000 forest sites across Switzerland, spanning a unique gradient of mean annual precipitation (640-2500 mm), elevation (277-2207 m a.s.l), pH (2.8-8.1) and covering different geologies and vegetation types. Within these sites, SOC content is significantly related to CEC eff., in both topsoils and subsoils. Our results demonstrate that in Swiss forest topsoils (5.5, between 59 and 83 % of subsoil CEC eff. originates from exchangeable calcium, whereas in acidic soils exchangeable aluminum contributes between 21 and 44 % of the CEC eff. Exchangeable iron contributes to less than 1 % of the variability in CEC eff. Overall this study indicates that in Swiss forests subsoils, CEC eff. strongly reflects the surface of soil minerals to which SOC can be bound by metal cations. The strength of the relationship between CEC eff. and SOC content depends on the pH of the soil, with the highest amount of variation of SOC content explained by CEC eff. in subsoils with pH >5.5.
Highlights
Our predictions on the feedback between the terrestrial carbon (C) cycle and climate change are still uncertain, largely due to the difficulty in accurately quantifying the stabilization of soil organic C (SOC) (Jobbágy and Jackson, 2000; Todd-Brown et al, 2013; Friedlingstein et al, 2014; Jackson et al, 2017)
We explored which type of soil surfaces and which cations reflect the variation of CEC eff. and thereby may influence soil organic carbon (SOC) content
The Swiss forest sites from which the soils were sampled are distributed between 277 and 2207 m a.s.l. They are characterized by a mean annual precipitation (MAP) ranging between 636 and 2484 mm, and mean annual temperatures (MAT) ranging between 0.1 and 12.0◦C
Summary
Our predictions on the feedback between the terrestrial carbon (C) cycle and climate change are still uncertain, largely due to the difficulty in accurately quantifying the stabilization of soil organic C (SOC) (Jobbágy and Jackson, 2000; Todd-Brown et al, 2013; Friedlingstein et al, 2014; Jackson et al, 2017). Measuring and predicting mechanisms of SOC preservation is challenging due to the long-time periods that are needed to detect changes, as well as the complexity of the soil physicochemical properties which effectively stabilize SOC (van der Voort et al, 2016; Harden et al, 2018). Conceptual advances highlighted that polyvalent metal cations such as Al3+ and Ca2+ can play a significant role in the stabilization of SOC by binding organic compounds to mineral surfaces through exchangeable bridges and ionic bonds (Oades, 1988; Rasmussen et al, 2018; Rowley et al, 2018). In soils where soil organic matter contributes negligibly to CEC eff., we assessed how the variance of SOC content was explained by CEC eff. in comparison to other environmental variables
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