Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (Ca Exch ) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca 2? can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner-and outer-sphere bridging by Ca 2? can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO 3 ) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al-or Fe-to Ca, and their respective effects on SOC dynamics.

Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation from C3 to C4 22-36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a

Aim:To investigate the potential of a large range of soil variables to improve topoclimatic models of plant species distributions in a temperate mountain region encompassing complex relief. Location:The western Swiss Alps. Methods:Fitting topo-climatic models for >60 plant species across >250 sites with and without added soil predictor variables (>30). Testing included the following: (a) which soil variables improve plant species distribution models; (b) whether an optimal subset of soil variables can improve models for the majority of species and habitat types and (c) how much variation in plant species distributions soil variables alone explain.Results: Geochemical variables (i.e. CaO, pH and inorganic carbon) and a drainage indicator (i.e. bulk soil water content) improved the predictive abilities of the models across the large majority of alpine plant species. The improvement of the models after the addition of soil information varied strongly between plant species and habitat types, but a trade-off was found between the number of soil variables and the associated gain in model performance. Finally, across all species, one specific combination of soil variables -bulk soil water content + total phosphorus +δ 13 C -outperformed the commonly used topo-climatic variables.Main conclusions: Several soil variables significantly increased the predictive power of plant species distribution models in the temperate mountain region. Geochemical and drainage variables proved most important. K E Y W O R D Secological niche, habitat suitability, predictive power, random-stratified sampling, soil variables, species distribution models (SDMs), Swiss Alps, topo-climate

No abstract

Forest soils of coastal British Columbia, Canada, may store significant amounts of organic matter because of the cool climate and high forest productivity of the area. The objectives of this study were to determine the distribution of soil organic carbon (SOC) in the profile and to identify the most important predictors of SOC in Podzols of a forested watershed in southwestern British Columbia. We sampled 9 soil profiles in undisturbed forest plots by morphological horizon and measured SOC using a dry combustion method. We also determined soil pH, texture, moisture content, total nitrogen, loss on ignition, and pyrophosphate-and oxalate-extractable Fe and Al.The average soil profile stored 15.9 kg C/m 2 over a depth of 100 cm, which is higher than SOC stocks estimates for inland Canadian forests. The organic layer (LFH) only accounted for one fourth of the C stock. Sixty percent of the profile SOC (including the forest floor) was found in the subsoil of depth greater than 20 cm. Studies of SOC dynamics that only sample the topsoil are therefore inappropriate.Although the clay concentration was low (È5%), the clay fraction accounted for one third of SOC. This suggests that organo-mineral interactions were an important factor for SOC storage. The major predictors of SOC in the mineral horizons were organically complexed Al and Fe and short-range order inorganic material. Crystalline clays also seemed to play a role in organic matter accumulation, but were not as important as poorly crystalline compounds. In the organic layer, organically complexed Fe forms correlated negatively with SOC, indicating that the amount of Fe available for adsorption to organic matter is limited. Organically complexed Al did not show the same negative association, suggesting the existence of a mechanism for upward translocation of Al into the FH horizon.