Abstract: The dynamics of calcium (Ca) and magnesium (Mg) in the forest floor and topsoil caused by anthropogenic and natural processes continue to be a concern in temperate forests. However, the impacts of abiotic and biotic variables as well as their interactions remain unclear, especially in areas undergoing long-term forest restoration. In this study, Ca and Mg concentrations in the forest floor and topsoil from 239 forest plots across the Loess Plateau were measured, and the effects of forest types, climate, soil properties, stand characteristics and nitrogen deposition were explored. The results showed significantly higher Ca concentrations in the forest floor (20.68 ± 8.04 mg/g) than in the topsoil (13.28 ± 12.83 mg/g), whereas Mg exhibited the inverse pattern (3.64 ± 1.09 and 10.11 ± 2.51 mg/g, respectively). The effect of forest types was only significant on forest floor Ca, and Ca concentrations were higher in broadleaf and mixed forests than in coniferous forests. Overall, Ca and Mg concentrations in forest floor and topsoil increased with latitudes while decreased with elevations, and the significance of the trends varied among forest types. Forest floor Ca and Mg were mainly influenced by environmental variables aboveground, i.e., basal area (BA) and mean annual precipitation (MAP), respectively; topsoil Ca and Mg were more affected by soil properties (soil C/N and pH, respectively). Those suggested a depletion of Ca belowground was associated with forest growth and enriched soil nitrogen, and the leaching of mobile Mg was correlated with rainfall and soil acidification. Besides, the impact of environmental variables on Ca-Mg balance (Ca/Mg ratio) belowground was primarily through the regulation of Ca. Elucidating the influence of environmental variables will improve our ability to predict future changes in base cations and thus forest soil health in the greening vegetated Loess Plateau.

Key words: Soil C/N, Basal area, Random-forest model, Temperate forests, Base cations, Elevation