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
Final Report Abstract
The retreat of glaciers is exposing new terrains to primary plant succession around the globe. To improve the understanding of vegetation development along a glacial retreat chronosequence, we (i) evaluated a possible link between base metal (Ca, Mg, K, Na) supply and vegetation establishment, (ii) determined the rates of the establishment of soil and plant base metal stocks, (iii) estimated the size of the main base metal fluxes, (iv) characterized the composition of the glacial debris to elucidate the sources of base cations, (v) determined the base cation release kinetics from topsoils along the chronosequence with a weathering experiment, and (vi) established a method to determine stable Mg isotope ratios in soils and plants in our laboratory with which we determined stable Mg isotope ratios to further elucidate the Mg cycling processes along the chronosequence. We determined base metal stocks and δ26Mg values in the soil organic layer, the mineral soil, and in leaves/needles, trunk, bark, branches and roots of the dominating shrub and tree species and estimated fluxes of atmospheric deposition, plant uptake and leaching losses along the 127-yr Hailuogou chronosequence. We used photomicrography of thin sections to identify the carbonate minerals in the parent rocks and conducted a weathering experiment at pH 3 (pHstat). Total ecosystem Ca and Mg stocks (to 0.1 m mineral soil depth) decreased along the chronosequence, while those of K and Na were unrelated with ecosystem age. Fortyfour and 30% of the initial stocks of Ca and Mg, respectively, were leached during the first 47 years, at rates of 130±10.6 g m^-2 year^-1 Ca and 35±3.1 g m^-2 year-1 Mg. The organic layer accumulated at a mean rate of 288 g m^-2 year^-1 providing a bioavailable base metal stock, which was especially important for K cycling. The glacial debris was composed of a mix of metasedimentary and meta-volcanic calc-silicate rocks, amphibolite, mica schist, and quartzite, which all contained at least some carbonates. Although the total Ca concentration of the glacial debris was only about double that of Mg, K, and Na, during the first day of the pHstat experiment, the released mass of Ca was >10 times higher than that of Mg and K, and even ca. 100 times higher than that of Na. The size of the carbonatic fast-reacting Ca pool decreased quickly in the first ca. 40 yr, after which a silicatic slow-reacting pool matched the fast-reacting pool with a size of 1.9±0.6 mg g^-1 Ca. In contrast, for Mg, K and Na the slowreacting pool dominated from the beginning, suggesting that these elements mainly originated from silicate weathering. The stable Mg isotope analysis revealed that the entire Mg loss from the whole ecosystems originated from the A horizons, because the δ26Mg values of the C horizons remained unchanged at -0.55‰, while isotopically lighter Mg (-1.20‰) was released during H-induced weathering at the young study sites. This Mg seemed to be leached through the C horizons without changing their δ26Mg values. The δ26Mg values of roots, leaves and organic layers were higher than those of the C horizons assumed to represent the δ26Mg values of the parent material, because plants prefer to take up the heavy Mg isotopes. The δ26Mg values of the roots increased with time in agreement with the increase of the δ26Mg values of the fast-reacting Mg pool in soil. The needles had markedly lower δ26Mg values (-1.40‰ to -0.80‰) than the leaves and the C horizons, which increased with age similar to the roots. We attribute the different δ26Mg values between needles and leaves to Mg retranslocation before needle abscission, which preferentially retains the light Mg isotopes. Our findings support the view that the well-synchronized interplay between carbonate and silicate weathering facilitated the fast vegetation succession. The additional consideration of stable Mg isotope ratios allowed us to elucidate the Mg redistribution processes along the chronosequence in more detail than Mg concentrations, stocks and fluxes alone.
Publications
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(2019) Ca and Mg mass balances along a 120 years-old glacial retreat chronosequence in the subtropical Hailuogou region (China). Congress of the German Soil Science Society (DBG), 24-29 Aug. 2019, Berne, Switzerland
Basdediós N, Wang JP, Wu YH, & Wilcke W
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(2022) Initial carbonate weathering is linked with vegetation development along a 127-year glacial retreat chronosequence in the subtropical high mountainous Hailuogou region (SW China). Plant Soil 471:609–628
Basdediós N, Zhong ZL, Wu YH, & Wilcke W
