Consistent controls on trace metal micronutrient speciation in wetland soils and stream sediments

Abstract

Trace metal are essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally at or below geological background levels, which generally followed the trend Zn > Cu ≈ Ni > Co and showed no correlation with major element compositions. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. X-ray absorption near-edge structure (XANES) spectroscopy shows that sulfur speciation in both wetlands is dominated by organic sulfur. Elemental sulfur and iron sulfides together made up <25% of the sulfur in the wetland soils, but the distribution between inorganic and organic forms was reversed in the stream sediments. Ferrous and ferric iron in clay minerals were common species identified by both XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies at all sites. Iron(III) oxides were substantial components in all but the marsh wetland soils. Quantitative analysis of copper, nickel, and zinc XANES spectra revealed similar metal speciation across all sites. Copper speciation was dominated by sulfides, adsorbed species, and minor amounts of copper bound to organic matter; no metallic copper was detected. Nickel speciation also varied little and was dominated by nickel in clay mineral octahedral sheets and nickel sulfide, with adsorbed species also present. Zinc speciation was slightly more varied, with the marsh wetland soils and stream bed sediments containing adsorbed species, zinc associated with clay mineral structures, and zinc bound to reduced sulfur groups on organic matter, whereas the riparian wetland soils lacked clay-associated zinc but contained zinc sulfide. Trace metals bound to reduced sulfur occurred at every site, with a greater sulfur-bound fraction for copper. The fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. This may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g⁻¹) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc.

Introduction

Subsurface aquatic systems in freshwater environments, such as wetland soils and stream sediments, are foci of diverse anaerobic biogeochemical processes (Arora et al., 2016, Bowden, 1987, Kocar and Fendorf, 2009, McClain et al., 2003, Neumann et al., 2016, Zarnetske et al., 2011). Denitrification and methanogenesis in freshwater aquatic systems serve as important sources to the atmosphere of the greenhouses gases N₂O and CH₄, respectively (Groffman et al., 1998, Glatzel et al., 2008, Bouwman et al., 2013, Kirschke et al., 2013, Tian et al., 2015, Zhang et al., 2017). Substantial mercury methylation and subsequent emission also occur in these settings (St Louis et al., 1994, Skyllberg, 2008, Schaefer et al., 2014, Riscassi et al., 2016, Singer et al., 2016, Yang et al., 2016). Many of these processes in subsurface aquatic systems also couple to the biogeochemical cycling of other elements, such as iron and sulfur (Koretsky et al., 2003, Hlaváčová et al., 2005, Hansel et al., 2015, Segarra et al., 2015, Glodowska et al., 2020).

While the biogeochemistry of aquatic systems has been widely studied from the perspective of redox conditions, substrate availability, and thermodynamic controls on metabolic processes (Falkowski et al., 1998, LaRowe and Van Cappellen, 2011, Flynn et al., 2014, Arora et al., 2016, Danczak et al., 2016, Janot et al., 2016), there is a growing recognition of the importance of trace metal availability in affecting biogeochemical processes (Basiliko and Yavitt, 2001, Glass and Orphan, 2012, Jacquot et al., 2014). Elements that include cobalt, nickel, copper, and zinc serve as key reaction centers in metalloenzymes (Gärtner et al., 1993, Ermler et al., 1997, Thauer, 1998, Brown et al., 2000, Parks et al., 2013, Zheng et al., 2016) and their low availability has been documented in laboratory studies to inhibit methanogenesis, nitrous oxide reduction to nitrogen, and mercury methylation (Schönheit et al., 1979, Granger and Ward, 2003, Ekstrom and Morel, 2008, Glass and Orphan, 2012, Lu et al., 2018). The geochemistry of trace metals in aquatic systems may thus have a direct impact on biogeochemical cycling of carbon, nutrients, and contaminants.

The availability of trace metals in wetland soils and stream sediments is expected to be controlled by their chemical speciation rather than bulk concentration (Worms et al., 2006, Harmsen, 2007, Zhao et al., 2016). Past studies have identified an array of chemical forms of trace metals in similar types of anoxic settings, although these have often focused on locations containing elevated metal concentrations because of environmental contamination or geogenic enrichments. Of the trace metals, copper is the most widely studied in anoxic subsurface aquatic systems. Copper sulfides and metallic copper are generated in contaminated soils, often in nanoparticulate form, upon flooding (Weber et al., 2009a, Weber et al., 2009b, Fulda et al., 2013a, Hofacker et al., 2013b, Xia et al., 2018, Cervi et al., 2021), with organic matter then potentially stabilizing these phases under oxic conditions (Fulda et al., 2013b, Mantha et al., 2019). Copper sulfides were also observed in contaminated paddy soils (Yang et al., 2015, Sun et al., 2016) and a copper-rich bog (Lett and Fletcher, 1980). Binding to organic matter, including as Cu(I) species, may be important in contaminated, flooded soils with limited sulfate (Fulda et al., 2013a, Fulda et al., 2013b). A recent study examined uncontaminated transitional and anoxic soils and found copper binds extensively to organic matter but also forms copper sulfide and minor metallic copper in the anoxic zone (Mehlhorn et al., 2018).

Fewer studies have examined the speciation of other essential trace metals in subsurface aquatic systems. In diverse coastal sediments that drain ultramafic laterite deposits highly elevated in nickel, this metal occurs coprecipitated with iron sulfides and in the octahedral sheets of clay minerals (Noël et al., 2015, Noël et al., 2017, Merrot et al., 2019). While nickel speciation in other anoxic systems has seen limited study, contaminated or enriched oxic soils contain nickel in phyllosilicate and iron oxide structures, associated with gibbsite phases in clay interlayers, and bound to organic matter (Dublet et al., 2012, Manceau et al., 2005, McNear et al., 2007, Siebecker et al., 2017, Siebecker et al., 2018). Zinc sulfide forms under anoxic conditions in metalliferous peatlands (Yoon et al., 2012), contaminated lake sediments (Webb and Gaillard, 2015), and in a contaminated wetland, along with zinc carbonate, adsorbed zinc, and possibly zinc oxide (Bostick et al., 2001). Similar to nickel, exploration of zinc in other anoxic soil systems is generally lacking, but in contaminated oxic soils zinc sulfide, zinc in phyllosilicate octahedral sheets, and adsorbed species are common (Manceau et al., 2004, Voegelin et al., 2005, Jacquat et al., 2009, Voegelin et al., 2011, Williams et al., 2011). Less is known regarding cobalt speciation because of the difficulty probing this element using available methods in the presence of iron. Binding studies suggest that adsorbed species and complexes with organic matter are important in aerobic soils (Woodward et al., 2018) and that cobalt readily incorporates into pyrite under anoxic conditions (Swanner et al., 2019).

Prior studies provide important insight into trace metal species in anoxic subsurface systems that are contaminated with or naturally elevated in metals. All metals show occurrence as sulfide phases, although some results, such as nickel associating with pyrite in coastal sediments, may not be transferable to freshwater systems that are generally low in sulfur (Brown, 1985, Wieder et al., 1985, Spratt and Morgan, 1990, Prietzel et al., 2009). Similarly, nickel and zinc often occur in clay structures, likely produced through a neoformation process (Manceau et al., 1999, Ford and Sparks, 2000, Schlegel et al., 2001, Dähn et al., 2002), but it is unclear whether such species form when metals are not elevated in concentration from contamination or natural enrichment. The solid-phase trace metal concentrations in most prior studies of metal speciation in subsurface aquatic systems (Lett and Fletcher, 1980, Bostick et al., 2001, Weber et al., 2006, Weber et al., 2009b, Weber et al., 2009a, Yoon et al., 2012, Fulda et al., 2013b, Fulda et al., 2013a, Hofacker et al., 2015, Noël et al., 2015, Yang et al., 2015, Sun et al., 2016, Noël et al., 2017, Xia et al., 2018, Mantha et al., 2019, Merrot et al., 2019, Cervi et al., 2021) far exceed geological background levels (Rudnick and Gao, 2003): 280 to 23,000 μg g⁻¹ Cu versus 28 ± 4 μg g⁻¹ background, 255–6044 μg g⁻¹ Ni versus 47 ± 11 μg g⁻¹ background, 1900–71,000 μg g⁻¹ Zn versus 67 ± 6 μg g⁻¹ background. Investigations of speciation in wetland soils and stream sediments with metal concentrations similar to geological background levels are rare (Webb and Gaillard, 2015, Mehlhorn et al., 2018). However, such metal concentrations are widespread in nature (de Caritat et al., 2018) and likely representative of the large majority of freshwater wetlands and streambeds where biogeochemical carbon, nitrogen, and mercury cycling occur in terrestrial ecosystems. Anoxic subsurface systems displaying background metal levels should more frequently exhibit metal limitations on biogeochemical processes than contaminated or naturally-enriched systems However, the dominant controls on metal speciation in environments with low metal contents are unclear. A central challenge to investigating metal speciation in relevant anoxic subsurface systems is the difficulty in applying X-ray spectroscopic techniques to soils and sediments having low (<100 μg g⁻¹) element concentrations.

In this work, we assess the variability in trace metal speciation in the subsurface of diverse freshwater aquatic systems. Three field sites are investigated that are distributed over a ∼1000 km transect, each occurring in geologically-distinct regions representing different types of subsurface aquatic system: marsh wetland soils, riparian wetland soils, and stream bed sediments. Duplicate soil or sediment cores and overlying surface waters were collected in two locations at each of the three study areas. Major element and trace metal concentrations, dissolved and extractable nutrients, and soil and sediment mineralogy were evaluated at these sites. X-ray absorption spectroscopy evaluated bulk sulfur and iron speciation and quantified the solid-phase speciation of nickel, copper, and zinc. The latter measurements overcame the low concentrations of trace metals by utilizing a synchrotron beamline configuration optimized for high-sensitivity measurements. Metal speciation was compared across sites to identify the dominant forms of trace metals occurring in subsurface aquatic systems.