Zn isotope fractionation in a pristine larch forest on permafrost-dominated soils in Central Siberia

Abstract

Stable Zn isotopes fractionation was studied in main biogeochemical compartments of a pristine larch forest of Central Siberia developed over continuous permafrost basalt rocks. Two north- and south-oriented watershed slopes having distinctly different vegetation biomass and active layer depth were used as natural proxy for predicting possible future climate changes occurring in this region. In addition, peat bog zone exhibiting totally different vegetation, hydrology and soil temperature regime has been studied.The isotopic composition of soil profile from Central Siberia is rather constant with a δ66Zn value around 0.2‰ close to the value of various basalts. Zn isotopic composition in mosses (Sphagnum fuscum and Pleurozium schreberi) exhibits differences between surface layers presenting values from 0.14 to 0.2‰ and bottom layers presenting significantly higher values (0.5 – 0.7‰) than the underlain mineral surface. The humification of both dead moss and larch needles leads to retain the fraction where Zn bound most strongly thus releasing the lighter isotopes in solution and preserving the heavy isotopes in the humification products, in general accord with previous experimental and modeling works [GCA 75:7632–7643, 2011].The larch (Larix gmelinii) from North and South-facing slopes is enriched in heavy isotopes compared to soil reservoir while larch from Sphagnum peatbog is enriched in light isotopes. This difference may result from stronger complexation of Zn by organic ligands and humification products in the peat bog compared to mineral surfaces in North- and South-facing slope.During the course of the growing period, Zn followed the behavior of macronutrients with a decrease of concentration from June to September. During this period, an enrichment of larch needles by heavier Zn isotopes is observed in the various habitats. We suggest that the increase of the depth of rooting zone, and the decrease of DOC and Zn concentration in soil solution from the root uptake zone with progressively thawing soil could provoke heavy isotopes to become more available for the larch roots at the end of the vegetative season compared to the beginning of the season, because the decrease of DOC will facilitate the uptake of heavy isotope as it will be less retained in strong organic complexes.

The influence of weathering and soil organic matter on Zn isotopes in soils

“…Light Zn isotopes are preferentially transported into aerial plant parts, therefore, the biological uptake of Zn by vegetation is a factor that may influence the δ 66 Zn in soils (Jouvin et al, 2012;Moynier et al, 2009;Weiss et al, 2005;Viers et al, 2007Viers et al, , 2015Smolders et al, 2013;Couder et al, 2015;Tang et al, 2016;Caldelas and Weiss, 2017). The Zn concentration in Icelandic grass reported in the literature ranges from 14 to 85 μg g − 1 (e.g., Johannesson et al, 2007;Gudmundsson and Thorsteinsson, 1980).…”

“…The stable OC pool comprises organic matter associated with mineral constituents (mineral-protected organic carbon, MP-OC), and/or organic carbon intrinsically resistant to decomposition (recalcitrant organic carbon, R-OC) (Chenu and Plante, 2006;Mikutta et al, 2006). Previous work (Viers et al, 2015) suggests that the humification of litter (which mainly results in a pool of stable organic carbon intrinsically resistant to decomposition, R-OC; Berg and McClaugherty, 2008;Mikutta et al, 2005Mikutta et al, , 2006 leads to release of the lighter Zn isotopes in solution and thus preserves the heavy Zn isotopes in the humification products such as aromatic structures. These findings are consistent with studies showing that Zn adsorption and complexation with organic compounds or biological surfaces favour the heavy Zn isotopes (Jouvin et al, 2009;Gélabert et al, 2006;Wanty et al, 2013;Kafantaris and Borrok, 2014).…”

“…The pool of stable OC is mainly dominated by MP-OC, which is thought to comprise organic materials protected against biological attack (Baldock and Skjemstad, 2000). Therefore, the enrichment in light Zn isotopes with an increasing proportion of stable OC likely results from the stabilization of plant-derived Zn-organic compounds, which are usually relatively enriched in light Zn isotopes (Jouvin et al, 2012;Moynier et al, 2009;Weiss et al, 2005;Viers et al, 2007Viers et al, , 2015Couder et al, 2015;Tang et al, 2016;Caldelas and Weiss, 2017), through association with the soil mineral matrix. This interpretation is in line with previous studies that report plant-derived Zn being mainly found in organo-metallic complexes associated with the clay fraction which are relatively resistant to biodegradation (Quenea et al, 2009).…”

“…Within the plant, heavy Zn isotopes sorb onto the root surface, and light Zn isotopes are preferentially transported into aerial plant parts (Arnold et al, 2010;Aucour et al, 2011;Jouvin et al, 2012;Moynier et al, 2009;Weiss et al, 2005;Viers et al, 2007Viers et al, , 2015Couder et al, 2015;Tang et al, 2016;Caldelas and Weiss, 2017). Plant litter at the soil surface is therefore generally isotopically lighter, and organic matter decomposition is considered to lead to the retention of heavier Zn isotopes by humification (Viers et al, 2015).…”

Cu isotopic compositions in Elsholtzia splendens: Influence of soil condition and growth period on Cu isotopic fractionation in plant tissue

“…There is now a considerable body of research on heavy metal isotopes in plants, including Fe (Arnold et al, 2015;Guelke and von Blanckenburg, 2007), Cu (Jouvin et al, 2012;Ryan et al, 2013;Weinstein et al, 2011), Zn (Couder et al, 2015;Moynier et al, 2009;Viers et al, 2015, Viers et al, 2007Weiss et al, 2005), Ca (Page et al, 2008), and Mg (Black et al, 2008;Bolou-Bi et al, 2010).…”