Soil organic carbon (SOC) is a strong determinant of soil fertility through its positive effects on soil structure and soil chemical and biological properties which in turn stimulate primary production. The objective of this work was to simulate field sites that represent different land uses and management practices in three continents, in order to identify the most important factors controlling soil structure dynamics and C sequestration across different climatic and lithological conditions as well as to quantify the rates of the aforementioned processes. The carbon, aggregation, and structure turnover (CAST) model was used to simulate SOC sequestration, aggregate formation, and structure dynamics in three field sites including nontilled soils of natural ecosystems and tilled soils of agricultural fields in Europe (Critical Zone Observatories (CZO) of the SoilTrEC network) and one site in North America. Derived data from the simulations’ results of SOC stocks and water-stable aggregate (WSA) particle size distribution, together with the respective results of three additional sites (Damma Glacier CZO, Milia (Greece), and Heilongjiang Mollisols (China)), were statistically analyzed in order to determine the factors affecting SOC sequestration and soil structure development. The natural ecosystems include nontilled soils covered with natural local vegetation, while the agricultural sites include cultivated and tilled soils covered with crops. The natural ecosystems were represented by Damma Glacier CZO (Switzerland), Heilongjiang Mollisols (China), Koiliaris CZO (Greece), Clear Creek (United States), and the Slavkov Forrest CZO (Czech Republic), whereas the agricultural field sites were located at Heilongjiang Mollisols (China), Koiliaris CZO (Greece), Clear Creek (United States), Marchfeld CZO (Austria), and Milia (Greece). Principal component analysis (PCA) identified clay content, bulk density, climatic conditions (precipitation and evapotranspiration), organic matter (OM), and its decomposition rates as the most important factors that controlled soil structure development. The relative importance of each of these factors differs under differing climatic and lithological conditions and differing stages of soil development. Overall, the modeling results for both natural ecosystems and agricultural fields were consistent with the field data. The model reliably simulated C and soil structure dynamics in various land uses, climatic conditions, and soil properties providing support for the underlying conceptual and mathematical modeling and evidence that the CAST model is a reliable tool to interpret soil structure formation processes and to aid the design of sustainable soil management practices.
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