Elemental and isotope fingerprint of metal deficiency in soils

Abstract

Metal deficiency in soils is a frequently observed phenomenon in (sub)-tropical regions, and both natural and anthropogenic processes contribute to the depletion of soil reservoirs. Advanced chemical weathering, in old regolith, can lead to very severe natural depletion of metals in certain soil types, while soil over-cropping represents the human impact that increases the net offtake of metals from many soils. In both cases, the health of humans can suffer since many trace metals have vital functions in the human body. For example, nearly half the world population is affected by Zn deficiency symptoms caused by over-dependence on local crops that are grown on Zn deficient soils. This is all the more concerning in view of the anticipated rise in world population that will put additional demands on crop production with likely knock-on effects for essential metal depletion in soil. This thesis presents petrographic, mineralogical and geochemical as well as isotopic investigations of two contrasting weathering profiles from the Deccan Traps, India to investigate the natural causes of metal deficiency in soils. Major- and trace elements (including essential metals), organic carbon (C) content, chemical weathering indices, uranium-series (U-series) systematics as well as zinc (Zn)- and chromium (Cr)- isotopes were analysed to quantify the mineralogical transformations, biogeochemical processes and environmental factors that lead to the loss of metals during soil formation. In addition, agricultural topsoil and animal manure samples were investigated from Zn deficient regions in India to compare and contrast their biogeochemical fingerprints to the findings from the weathering profiles. The work revealed that actively weathering soils are strongly depleted in the isotope 234U, hosted in labile sites in minerals. The chemical breakdown of metal-rich primary silicates and clay minerals in saprolite results in the mobilisation of substantial amounts of metals to aqueous solutions and plants. Ultimately, the breakdown of these minerals contributes to oceanic metal inventories, exemplified by the excess of 234U in seawater. By contrast, strongly weathered soils (i.e. laterites) have lost their natural capacity to supply bio-available forms of metals to the environment and concentrations of residual essential metals such as Zn or redistributed Cr are mainly hosted in weathering resistant iron (Fe)-oxides. The isotopic fractionation of Zn and Cr is strongly affected by the formation of Fe-oxides, whereas the metal stable isotope ratios in actively weathering saprolite remain in the range of unweathered bedrocks. Hence, Zn isotopes can be used to distinguish between fertile and Zn deficient soils. Importantly, the geochemical fingerprints in the agricultural topsoils reveal that high concentrations of essential metals (Zn, copper (Cu), cobalt (Co), nickel (Ni), Cr) are also hosted in bio-unavailable oxide phases, whereas the bio-available pool in the inorganic soil constituents is much lower. Since the bio-available concentrations of metals in animal manure are too low to counterbalance essential metal deficiencies in Indian soils, their fertility status remains delicate without application of industrial metal fertilisers. Overall, the findings of this thesis suggest that geochemical and isotopic data can contribute to a deeper understanding of metal deficiency in soils and help in the development of more sustainable farming practices in (sub)-tropical regions