Variations and controls of iron oxides and isotope compositions during paddy soil evolution over a millennial time scale
Introduction
Iron (Fe) is the fourth most abundant element in the Earth's crust (6.7 wt%) (Rudnick and Gao, 2004), serving as an essential nutrient for almost all living organisms (Bernuzzi and Recalcati, 2006). Its dynamic changes in valence state in response to shifting redox conditions trigger many processes in terrestrial ecosystems, such as mineral weathering, nutrient cycling, and contaminant mobility (Cornell and Schwertmann, 2003). Fe also plays an important role in the preservation of organic carbon in both soils and sediments (Kaiser and Guggenberger, 2000, Lalonde et al., 2012), which contributes to the global carbon cycle and affects climate change (Jickells et al., 2005).
In soils, Fe occurs in a variety of different phases, such as primary silicate minerals, pedogenic clay minerals, Fe (oxyhydr)oxides with different degrees of crystallinity, as well as in organic complexes (Stucki et al., 1988). The weathering of primary silicate minerals in soils releases FeII, which is rapidly oxidized in oxic environments and precipitated as poorly crystalline Fe (oxyhydr)oxides such as ferrihydrite or lepidocrocite. The Ostwald ripening of these short-range-ordered (SRO) Fe phases to crystalline Fe (oxyhydr)oxides, such as goethite and hematite, takes place during further soil development. Under oxic conditions in well-drained soils, FeIII is the thermodynamic stable oxidation state and characterized by a very low solubility (Cornell and Schwertmann, 2003). Under anoxic conditions in water-saturated soils, however, FeIII provides a terminal electron acceptor for anaerobic respiration of dissimilatory Fe-reducing microorganisms (Lovley et al., 2004), resulting in the formation of the highly soluble FeII aquoion. In addition to reductive dissolution, mobilization of Fe also occurs via proton-promoted dissolution at pH < pznpc (point of zero net proton charge) and to a greater extent via ligand-promoted dissolution when strong binding organic acids are present (Jansen et al., 2003). The mobile FeII can be transported within soils by both lateral and vertical diffusion and it is rapidly oxidized and re-precipitated as FeIII in the presence of O2. The re-precipitation process happens depending on the temporal and spatial variability of soil moisture regimes, resulting in the formation of relatively “Fe-depleted” and “Fe-enriched” micro-sites with distinct redoximorphic features. Thus, the Fe mobilization, translocation, and redistribution as well as the associated Fe mineral transformation are key processes in soil formation influencing the morphological and physico-chemical properties of soils (van Breemen and Buurman, 2004). Understanding the mechanisms and processes that control the behavior and dynamics of Fe in soils is among the fundamental questions in pedology and geochemistry, and will be conducive to assess the function and ecosystem service of Critical Zone responding to the ever-increasing natural and anthropogenic changes.
Previous studies have shown that the natural soil formation involves significant changes in species, amounts, and stability of Fe (oxyhydr)oxides (e.g., Torrent et al., 1980, McFadden and Hendricks, 1985, Diaz and Torrent, 1989, Aniku and Singer, 1990, Cornell and Schwertmann, 2003, Vodyanitskii, 2010) and measureable Fe isotope fractionations (e.g., Fantle and DePaolo, 2004, Emmanuel et al., 2005, Thompson et al., 2007, Wiederhold et al., 2007a, Wiederhold et al., 2007b, Yamaguchi et al., 2007, Buss et al., 2010, Kiczka et al., 2011, Yesavage et al., 2012, Mansfeldt et al., 2012, Fekiacova et al., 2013, Akerman et al., 2014, Schuth et al., 2015, Garnier et al., 2017, Li et al., 2017). The ratio of dithionite-citrate-bicarbonate extractable Fe to total Fe generally increases while the ratio of oxalate extractable Fe to total Fe decreases with increasing pedogenic age as indicated by the selective chemical extractions (Torrent et al., 1980, McFadden and Hendricks, 1985, Diaz and Torrent, 1989, Aniku and Singer, 1990). In addition, the crystallinity of Fe (oxyhydr)oxides and the amount of Al that substitutes Fe in goethite often increase with increasing soil development (Cornell and Schwertmann, 2003, Vodyanitskii, 2010). These changes are caused by weathering of silicate minerals, redox reactions, and the lattice replacement of other metals with Fe in Fe-bearing minerals. Recent analytical advances in MC-ICP-MS (multiple collector inductively plasma mass spectrometry) technology have shown significant deviations of δ56Fe values in soils from that of igneous rocks (e.g., Fantle and DePaolo, 2004, Thompson et al., 2007, Wiederhold et al., 2007a, Fekiacova et al., 2013, Akerman et al., 2014, Schuth et al., 2015, Garnier et al., 2017). The fractionation of Fe isotope in soils can be mediated by abiotic processes (e.g., proton-promoted or ligand-controlled Fe dissolution and mobilization, Fe adsorption and precipitation, as well as mineral transformation), biotic processes (microbial reduction or oxidation of Fe), and a combination of both pathways, which favor the preferential release of light isotope (54Fe) to solution leaving an isotopically heavy solid (enriched in 56Fe) (Johnson et al., 2002, Johnson et al., 2008). Contrasting to the well-documented Fe dynamics during natural pedogenesis, a comprehensive understanding of the variations and controls of Fe oxides and Fe isotope compositions during anthropedogenesis of paddy soils strongly affected by human activities is poorly constrained. The natural pedogenic controls on Fe evolution may be superseded by human activities (Dudal, 2005) that alter the rate and trajectory of net Fe dynamics either directly (e.g., Fe additions by irrigation) or indirectly (e.g., Fe transformations by artificial flooding and draining). The combined use of different approaches, such as selective extraction and Fe isotope analysis, for characterizing Fe dynamics would provide a more comprehensive understanding of the mechanisms and processes that control Fe biogeochemical cycling.
Paddy soils make up the largest anthropogenic wetlands on earth and play critical roles in ecosystem functions (Huang et al., 2015). They may originate from many types of soils in pedological terms or different parent materials, but are highly modified by anthropogenic management during paddy cultivation. The periodic artificial flooding and draining as well as groundwater fluctuations during paddy soil evolution result in significant changes in soil moisture regime and redox conditions with both time and depth, which come to govern Fe mobilization, translocation and redistribution (Gong, 1983, Gong, 1986, Zhang and Gong, 1993, Zhang and Gong, 2003, Huang et al., 2015). Given the widespread cultivation of rice, paddy soils represent a key component of the Fe geochemical cycle at the Earth's surface. Previous studies have extensively investigated the changing status of Fe oxides (Yu, 1985, Gong, 1986) and Fe isotopes (Garnier et al., 2017) at a given stage of paddy soil evolution through a comparison with the initial parent material. However, little is known about the successive changes of Fe oxides and Fe isotopic composition during paddy soil evolution that is required to identify process rates and thresholds of Fe dynamics.
Paddy soil chronosequence provides a valuable tool for investigating the rates and directions of property changes and the associated environmental thresholds (Huang et al., 2015). In this study, we measured different forms of Fe oxides and the stable Fe isotope compositions in a paddy soil chronosequence consisting of five profiles derived from calcareous marine sediments with cultivation history ranging from 0 to 1000 years (Chen et al., 2011, Huang et al., 2013) (Fig. 1). Our objectives were to (i) investigate the dynamic changes in Fe oxides and Fe isotope compositions during anthropedogenesis of paddy soil; (ii) identify the underlying mechanisms and processes controlling millennial scale Fe evolution; (iii) establish a conceptual model characterizing Fe transfer and redistribution in paddy soils and assess their impacts on Fe isotope fractionation; and (iv) compare Fe isotopic compositions in the worldwide soils and evaluate the potential of using Fe isotopes to record information about Fe transfer and soil formation.
Section snippets
Study area and sampling sites
The study area is located on a coastal plain in Cixi County, Zhejiang Province, facing the East China Sea, between 121°2′–121°36′ E and 30°2′–30°19′ N (Fig. 1). This region belongs to the southern fringe of northern subtropics and has a mean annual air temperature of 16 °C, with yearly extremes ranging from − 5 °C to 37 °C, and a mean annual precipitation of 1325 mm of which 73% is concentrated in the rice paddy flooding season (i.e., April to October). The coastal plain ranges from 2.6 m to 5.7 m
Fe concentrations
Total Fe concentration and distribution was uniform throughout the soil profile in the uncultivated pedon (P0), ranging from 28.22 to 30.34 g kg− 1 (Fig. 2, and Table S3 in Supplementary File S1). A measurable profile differentiation of total Fe was observed in all of the paddy soils (Fig. 2), suggesting a transport and redistribution of Fe during paddy soil evolution. The standard deviation of total Fe concentration within 120 cm profile increased rapidly from 0.92 in the uncultivated pedon (P0)
Fe redistribution and accumulation during paddy soil evolution
Fe is mobilized and translocated within profile during paddy soil evolution as evidenced by the increasing differentiation of Fe mass and speciation within different selective extractions across the paddy soil chronosequence (Fig. 2). Our results are consistent with prior observations that rice cultivation influences Fe differentiation within paddy soils, irrespective of parent material (Gong, 1983, Gong, 1986, Yu, 1985, Zhang and Gong, 1993, Zhang and Gong, 2003, Han and Zhang, 2013). The
Conclusions
The calcareous paddy soil evolution under the influence of periodic flooding and groundwater fluctuation results in variations of soil moisture regimes and redox conditions with both time and depth that control Fe mobilization, translocation and redistribution, leading to enhanced profile differentiation of Fe oxides and measurable Fe isotope fractionation. The gradual accumulations of profile-scale Fe and Fe oxides in our calcareous paddy soil chronosequence contrasts markedly with the rapid
Acknowledgements
We are grateful to Decheng Li, Institute of Soil Science, Chinese Academy of Sciences, and Hong Lu, Cixi Agriculture Bureau, Zhejiang Province, for their help during the field work. We also thank Alan Matthews, Ami Nishri, Jan Wiederhold, Nadya Teutsch, Stephan Kraemer, and Yigal Erel for their patient training on Fe column chemistry and Fe isotope measurements during FIMIN workshop held at The Hebrew University of Jerusalem, Israel. Two anonymous referees are thanked for comments that helped
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