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000889995 1001_ $$0P:(DE-HGF)0$$aWang, Yi$$b0$$eCorresponding author
000889995 245__ $$aMagnesiumstabile Isotope als Proxy für biogeochemische Prozesse in terrestrischer UmgebungMagnesium stable isotopes as a proxy for biogeochemical processes in terrestrial environment$$f2015-10-12 - 2019-11-12
000889995 260__ $$aAachen$$bRWTH Aachen University$$c2019
000889995 300__ $$a115 pages
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000889995 502__ $$aDissertation, RWTH Aachen, 2019$$bDissertation$$cRWTH Aachen$$d2019$$o2019-11-12
000889995 520__ $$aMagnesium (Mg) is the fourth most abundant element in the Earth and plays a significant role in biological activities. Processes involved in Mg cycle can fractionate Mg stable isotopes, which makes Mg isotope systematics as a novel and promising proxy in biogeochemistry. To improve our understanding of Mg isotope fractionations in different ecosystems, studies performed in this doctoral work include: i) Mg isotope fractionation of plants under Mg deficient environment, ii) Mg isotope fractionation in agricultural system impacted by long-term anthropogenic soil practices, and iii) Soil Mg isotope signatures in the Atacama Desert as extreme environment of hyper-aridity. Magnesium deficiency is detrimental to plant growth. However, an integrative tracer revealing plant responses to Mg deficiency is still lack. Here, Mg isotopes were used as an indicator with the hypothesis that Mg deficiency could promote increased Mg isotope fractionation in plants during uptake and subsequent translocation processes. To test this hypothesis, wheat plants (Triticum aestivum) were grown in a greenhouse under Mg-sufficient and deficient conditions, and Mg concentrations as well as their δ26Mg isotope compositions in roots, stems, leaves and spikes at different growth stages were analyzed. The results confirmed previous studies that plants were systematically enriched in heavy isotopes relative to the nutrient solution. This enrichment, however, was more pronounced under low-Mg supply, which was attributed to the increased contribution from active transport system for Mg. With crop growth, the δ26Mg of shoots shifted towards higher values under control but not under low-Mg supply, reflecting a reduced root-shoot upward translocation under low-Mg supply. At reproduction state, light Mg was redistributed into the stems. Overall, initial Mg supply can impact Mg isotope fractionation in plants and assessing the Mg isotope compositions of plant organs provides a useful indicator for different plant responses to Mg supply from external environment. Liming is widely used to alleviate soil acidity in western and central Europe. However, its role on the cycling of Mg in arable soil-plant systems is still not fully clarified. In the second part of work, Mg concentrations and natural isotope compositions with soil profiles and its vegetation (winter rye) from a long-term agricultural experimental field with and without liming practice were systematically analyzed. The results showed that the δ26Mg signatures of the bulk Mg pool in soil in the studied Albic Luvisol displayed limited variation with depth and between trials. In contrast, the exchangeable Mg pool in soil exhibited an apparent increase of δ26Mg values along profile down to 50 cm depth, especially in limed field with more negative shift of δ26Mg than non-limed field. The observed enrichment of light Mg isotopes in upper layers mainly resulted from the lime deposition and plant uptake. An isotope-mixing model was used to assess the respective contribution from lime application and plant uptake to the Mg isotope compositions in the exchangeable Mg pool in soil. The results indicated that decades of liming practice presumably enhanced Mg uptake by vegetation when comparing limed and non-limed fields. However, no evidenced effects of liming on the isotope compositions of plant Mg were observed. Winter ryes grown on both limed and non-limed fields displayed identical Mg isotope compositions with roots and spikes enriched isotopically heavy Mg most. Silicate weathering is confirmed to fractionate Mg isotopes in nature. However, how Mg isotopes are fractionated under extreme climate is still not reported. In the third part of this work, Mg isotope compositions of surface soil layer with altitudinal gradient (from 1300 to 2700 m a.s.l.) in the Aroma transect of the Atacama Desert were analyzed. The surface soil Mg in the Aroma transects is considered to stem from the mixing deposition of both oceanic aerosols and the Andeans inputs. With elevation decreased, soils Mg became isotopically lighter. Enrichment of 26Mg at the site 2000 and 1300m a.s.l indicated another Mg input or loss process. The δ26Mg values in the pit soils from both the Aroma transect and well-known Yungay site were positively related to the weathering degree. Climatic aridity is assumed to change the Mg isotope signatures by influencing weathering degree. The present study for the first time reported soil Mg isotope composition in such hyper-arid environment and suggested a potential use of Mg isotopes to reconstruct the paleoclimatic changes. To conclude, studies in this doctoral work provided novel and comprehensive insights into Mg isotope signatures and fractionations in biogeochemistry, and improved the knowledge of Mg stable isotopes as a proxy for biogeochemical processes in terrestrial environment.
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