Mass budgets and stable isotope ratios of Ca and Mg in ecosystems along a 120 yr-old glacial retreat chronosequence in the subtropical Gongga mountains

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The stabilization of bare soil originating from glacial retreat by vegetation succession is crucial for reducing landslide, erosion, and water risks in mountainous areas. A major driver of the speed of vegetation establishment is the release of nutrients like Ca and Mg from the substrate by weathering and their transfer to the plants. We propose to determine the rates with which release from substrate and incorporation into biological cycling of Ca and Mg occur at a subtropical glacial retreat chronosequence with fast vegetation succession to a forest in less than 100 yr in China. To quantify these rates, we will establish mass balances by determining the change of the total stocks of Ca and Mg in all major ecosystem compartments (mineral soil, organic layer, vegetation) and bulk deposition rates along a ca. 120-yr chronosequence reaching from bare soil to full forest at a short distance using a space-for-time-substitution approach. To elucidate the processes behind the observed stock changes, we will develop a new method to determine 44/42Ca isotope ratios using a double spike of 46Ca and 43Ca and establish a published method to determine 26/24Mg ratios in our laboratory and measure stable isotope ratios of Ca and Mg in rocks, soils, plants, and ecosystem solutions. Because processes like weathering, plant uptake, adsorption, or leaching are reflected by changes of the stable isotope ratios, we expect an improved process understanding by the simultaneous establishment of mass budgets and determination of stable isotope ratios of Ca and Mg.