Evidence of Zn isotopic fractionation in a soil–plant system of a pristine tropical watershed (Nsimi, Cameroon)

Abstract

The objective of this study is to determine for the first time the zinc (Zn) isotopic composition of different soil horizons, parent rocks, litter, and plants from a single pristine watershed. Three soil profiles from Nsimi–Zoétélé site (South Cameroon, Africa) have been investigated. The δ 66Zn (δ 66Zn = [( 66Zn/ 64Zn) sample / ( 66Zn/ 64Zn) JMC 3–0749L) − 1] ⁎ 10 3) measured in the soils and rocks range from − 0.05 to 0.64‰. In a typical hillslope soil profile developed in situ from a granitic parent rock, we observe two important and systematic trends: i) a weathering regime in the saprolite horizon that involves strong Zn depletion, with no or slight fractionation compared to the fresh igneous rock, and ii) a shallower weathering regime with significant depletion in heavy Zn isotopes in the most superficial soil horizons. In contrast, the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock. Six plants and two litter samples were analyzed for their Zn isotopic compositions. δ 66Zn varied from 0.75 to − 0.91‰ among all samples. Plant roots and shoots are generally enriched in heavy isotopes (δ 66Zn of + 0.42 to + 0.76‰) relative to the litter layer (0.12 to 0.25‰) and most superficial soils. Similar to the controlled plant growth study of Weiss et al. [Weiss, D.J., Mason, T.F.D., Zhao, F.J., Kirk, G.J.D., Coles, B.J. and Horstwood, M.S.A. (2005) Isotopic discrimination of zinc in higher plants. New Phytologist 165, 703-710.], negative δ 66Zn values were obtained for tree leaves (the most aerial part of the tree). Based on biomass data for the tropical forest, vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs. Due to the age of these soils and the complexity of pedogenetic processes, it is difficult to explain the isotopic composition of the soils at present. However, the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light Zn isotopes. This observation may be due to the adsorption of Zn onto clays surfaces. The uptake of Zn by roots in the soils leads to an enrichment in heavy isotopes. Once the Zn is in the xylem, it will be transported within the plant. During this transport, both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants. The difference in Zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that Zn isotopes can be used to discriminate the source of Zn and other metals leaving the catchment by river water.