Do stable Mg isotope ratios in ecosystem compartments reflect the increasing incorporation of Mg into biotic cycles as vegetation succession advances?

Magnesium (Mg) is an essential nutrient for plant growth, particularly because of its important role in photosynthesis. Thus, Mg availability plays a crucial role in ecosystem development on freshly deposited material such as glacial debris. Gaining insight into the processes that drive vegetation succession is essential for addressing the challenges posed by the rapidly expanding glacier retreat areas worldwide. The subtropical Hailuogou glacial retreat area (approximately 3000 m a.s.l., 1950 mm precipitation, mean annual temperature 4.2 °C), southwest China, with its rapid development from bare soil to a full conifer-dominated mixed forest in <80 years, provides an ideal environment to study the biogeochemical cycling of Mg in the early stage of soil and vegetation development. The quantification of Mg fluxes together with the interpretation of changing Mg isotope ratios (δ²⁶Mg values) in soil and plants provide a promising approach to investigate the development of the Mg cycle during vegetation succession.

Along the Hailuogou chronosequence, 33% of the initial Mg stock in the uppermost 10 cm of the debris was lost within 37 years of soil development, which was attributable to leaching of exchangeable Mg and dissolution of chlorite. In spite of this loss, the δ²⁶Mg values of the organic layer, which developed simultaneously with the vegetation succession, were not correlated with site age and remained mostly unchanged at an average of -0.34±0.10 ‰ (SD, n=15). The δ²⁶Mg values in the organic layer resembled those of the tree leaves (-0.33±0.20 ‰, n=9) and were higher than those in 3 yr-old (-1.16±0.26 ‰, n=15) and 1 yr-old (-0.57±0.21, n=15) needles. The fact that the needles were isotopically lighter than the soil organic layer suggests that there is a Mg-isotopically heavy component in the litter of the conifer-dominated forest, such as woody components and/or seeds and fruits. Mg remobilization from older to younger plant compartments primarily occurs as organo-complexes, where Mg²⁺ forms strong covalent bonds. At chemical equilibrium, organo-Mg complexes tend to favor the heavy ²⁶Mg over ²⁴Mg, which accumulates in the free cytosolic Mg²⁺. The higher δ²⁶Mg value of leaves than of the 1 yr-old needles might indicate that most of the comparatively high Mg demand of the leaves of deciduous trees is covered by Mg that was re-translocated prior to leaf abscission and reused by the following generation of leaves, while less Mg was taken up from the exchangeable soil pool (−0.86 ± 0.13 ‰, n = 5). Tree roots were Mg-isotopically heavier than the exchangeable Mg pool, supporting previous findings that plants prefer to incorporate ²⁶Mg relative to ²⁴Mg, mainly because of the equilibrium fractionation by Mg binding to the root surfaces. With increasing ecosystem age, the roots increasingly accumulated isotopically heavy Mg.

Our results reveal that Mg is quickly incorporated into biotic cycles as vegetation succession progresses at the scale of years to decades resulting in a decoupling from soil processes such as weathering and leaching. As a consequence, the Mg isotope ratios in plant compartments are more dominated by plant-internal processes than by soil-plant transfer.