Elevated Si Concentrations Research Articles

Effects of silicon supplementation on growth and silicon accumulation in tall fescue (Festuca arundinacea) and bentgrass (Agrostis stolonifera)

Previous studies of the benefits of silicon (Si) supplementation in turfgrass have had inconsistent results. This study assessed the effect of Si supplementation on biomass production and Si accumulation of tall fescue (Festuca arundinacea) and bentgrass (Agrostis stolonifera). Plants were grown in buffered Hoagland’s media (pH 6) with four sodium silicate (Na2SiO4) treatments (0, 0.5, 1, 2, and 4 mM Si). The two species responded differently to Si supplementation in terms of biomass. For bentgrass, biomass was enhanced by Si supplementation, but only significantly (p < 0.05) at the highest concentration (4 mM Si). Lower Si concentrations (0.5, 1, and 2 mM Si) significantly increased biomass in tall fescue compared to control, but there was no increase in biomass at 4 mM Si. There was a positive linear relationship between Si concentrations in media and Si concentrations in tissue. At 4 mM Si, the Si concentrations in aboveground tissue of tall fescue and bentgrass averaged 24 and 30.5 g kg−1 dry mass, respectively. Elevated Si concentrations in growth media promoted formation of silica bodies. The highest Si rates (4 mM Si) resulted in the highest silica body areal coverage in leaves of both species. We observed two silica body morphologies (which we termed long and barbed). Our findings indicate that tall fescue is likely to benefit from Si supplementation when soil pore water dissolved Si is below 0.5 mM, but increases in soil pore water dissolved Si above 2 mM are unlikely to increase biomass. In contrast, benefits in bentgrass may only be realized if dissolved Si in pore water is increased to 4 mM.

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

In this work, the influence of selected alloying variations on Zn-assisted liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) is investigated. Cold-rolled AHSS alloys of different carbon (C), manganese (Mn), silicon (Si), and aluminum (Al) concentrations were continuous-annealed to generate a third generation AHSS microstructure (composed of martensite and retained austenite) via quenching and partitioning. High temperature tension tests using simulated spot-weld thermomechanical cycles revealed no significant influence of C and Mn variations on the Zn-LME susceptibility of AHSS. On the other hand, Zn-LME susceptibility was strongly correlated with the Si content of AHSS. A direct comparison of the (reacted) coating microstructures of the Si-alloyed and Low-Si AHSS variants revealed that Si in the AHSS substrate suppresses Fe–Zn alloying reactions and retards the nucleation and growth of Fe-Zn intermetallic phases at the coating-substrate interface in these spot weld simulations. The suppressed intermetallic formation at elevated Si concentrations is consistent with phase equilibria considerations in the Fe-Zn-Si ternary system. In the context of Zn-assisted LME, therefore, Si is hypothesized to aggravate LME behavior by increasing the liquid Zn availability for embrittlement and promoting direct contact between liquid Zn and the AHSS steel substrate.

Hydrogeochemical factors controlling the occurrence of chronic kidney disease of unknown etiology (CKDu).

Chronic kidney disease of unknown etiology (CKDu) has posed a serious threat to human health around the world. The link between the prevalence of CKDu and groundwater geochemistry is not well understood. To identify the potential geogenic risk factors, we collected 52 groundwater samples related to CKDu (CKDu groundwater) and 18 groundwater samples related to non-CKDu (non-CKDu groundwater) from the typical CKDu prevailing areas in Sri Lanka. Results demonstrated that CKDu groundwater had significantly higher Si (average 30.1mg/L, p < 0.05) and F- (average 0.80mg/L, p < 0.05) concentrations than those of non-CKDu groundwater (average 21.0 and 0.45mg/L, respectively), indicating that Si and F- were the potential risk factors causing CKDu. The principal hydrogeochemical process controlling local groundwater chemistry was chemical weathering of silicates in Precambrian metamorphic rocks. Groundwater samples were mostly undersaturated with respect to amorphous silica and clay minerals such as talc and sepiolite, which was conducive to silicate weathering and elevated Si concentrations in groundwater. Decreased Ca2+ being facilitated by calcite precipitation and cation exchange between Ca2+ and Na+ favored fluorite dissolution and thus led to high groundwater F- concentrations. Competitive adsorption between [Formula: see text] and F- also enhanced the release of F- from solid surfaces. This study highlights the CKDu potential risk factors regarding groundwater geochemistry and their enrichment factors, which helps in preventing the prevalence of CKDu.

High silicon concentrations in grasses are linked to environmental conditions and not associated with C4 photosynthesis.

The uptake and deposition of silicon (Si) as silica phytoliths is common among land plants and is associated with a variety of functions. Among these, herbivore defense has received significant attention, particularly with regard to grasses and grasslands. Grasses are well known for their high silica content, a trait which has important implications ranging from defense to global Si cycling. Here, we test the classic hypothesis that C4 grasses evolved stronger mechanical defenses than C3 grasses through increased phytolith deposition, in response to extensive ungulate herbivory ("C4 -grazer hypothesis"). Despite mixed support, this hypothesis has received broad attention, even outside the realm of plant biology. Because C3 and C4 grasses typically dominate in different climates, with the latter more abundant in hot, dry regions, we also investigated the effects of water availability and temperature on Si deposition. We compiled a large dataset of grasses grown under controlled environmental conditions. Using phylogenetically informed generalized linear mixed models and character evolution models, we evaluated whether photosynthetic pathway or growth condition influenced Si concentration. We found that C4 grasses did not show consistently elevated Si concentrations compared with C3 grasses. High temperature treatments were associated with increased concentration, especially in taxa adapted to warm regions. Although the effect was less pronounced, reduced water treatment also promoted silica deposition, with slightly stronger response in dry habitat species. The evidence presented here rejects the "C4 -grazer hypothesis." Instead, we propose that the tendency for C4 grasses to outcompete C3 species under hot, dry conditions explains previous observations supporting this hypothesis. These findings also suggest a mechanism via which anthropogenic climate change may influence silica deposition in grasses and, by extension, alter the important ecological and geochemical processes it affects.

Serpentinization as a reactive transport process: The brucite silicification reaction

Serpentinization plays a fundamental role in the biogeochemical and tectonic evolution of the Earth and perhaps many other rocky planetary bodies. Yet, geochemical models still fail to produce accurate predictions of the various modes of serpentinization, which limits our ability to predict a variety of related geological phenomena over many spatial and temporal scales. Here, we use kinetic and reactive transport experiments to parameterize the brucite silicification reaction and provide fundamental constraints on SiO2 transport during serpentinization. We show that, at temperatures characteristic of the sub-seafloor at the serpentinite-hosted Lost City Hydrothermal Field (150 °C), the assembly of Si tetrahedra onto MgOH2 (i.e., brucite) surfaces is a rate-limiting elementary reaction in the production of serpentine and/or talc from olivine. Moreover, this reaction is exponentially dependent on the activity of aqueous silica (aSiO2(aq)), such that it can be calculated according to the rate law:Rate=2.3×10−4aSiO2(aq)1.5(mol/m2/s). Calculations performed with this rate law demonstrate that both brucite and Si are surprisingly persistent in serpentinizing environments, leading to elevated Si concentrations in fluids that can be transported over comparatively large distances without equilibrating with brucite. Moreover, applying this rate law to an open-system reactive transport experiment indicates that advection, preferential flow pathways, and reactive surface area armoring can diminish the net rate of Si uptake resulting from this reaction even further. Because brucite silicification is a fundamentally rate-limiting elementary reaction for the production of both serpentine and talc from forsterite, our new constraints are applicable across the many environments where serpentinization occurs. The unexpected but highly consequential behavior of this simple reaction emphasizes the need for considering serpentinization and many other hydrothermal processes in a reactive transport framework whereby fluid, solute, and heat transport are intimately coupled to kinetically-controlled reactions.

Evaluation of phosphate-uptake mechanisms by Fe(III) (oxyhydr)oxides in Early Proterozoic oceanic conditions

Environmental contextReconstructing the Precambrian oceanic P cycle, in conjunction with the As cycle, is critical for understanding the rise of atmospheric O2 in Earth’s history. Bioavailable phosphorus (P) has been found to regulate photosynthetic activity, whereas dissolved arsenic (As) maxima correlate with photosynthetic minima. New data on empirical adsorption and coprecipitation models with Fe(III) (oxyhydr)oxides suggest coprecipitation is a more efficient method of P sorption than is adsorption in Precambrian surface ocean conditions. AbstractBanded iron formations (BIF) are proxies of global dissolved inorganic phosphate (DIP) content in Precambrian marine waters. Estimates of Precambrian DIP rely on constraining the mechanisms by which Fe(III) (oxyhydr)oxides scavenge DIP in NaCl solutions mimicking elevated Precambrian marine Si and Fe(II) concentrations. The two DIP binding modes suggested for Early Proterozoic marine waters are (1) surface attachment on pre-formed Fe(III) (oxyhydr)oxides (adsorption), and (2) incorporation of P into actively growing Fe(III) (oxyhydr)oxides (coprecipitation) during the oxidation of Fe(II) to Fe(III) (oxyhydr)oxides in the presence of DIP. It has been suggested that elevated Si concentrations, such as those suggested for Precambrian seawater, strongly inhibit adsorption of DIP in Fe(III) (oxyhydr)oxides; however, recent coprecipitation experiments show that DIP is scavenged by Fe(III) (oxyhydr)oxides in the presence of Si, seawater cations and hydrothermal As. In the present study, we show that the DIP uptake onto Fe(III) (oxyhydr)oxides by adsorption is less than 5 % of DIP uptake by coprecipitation. Differences in surface attachment and the possibility of structural capture within the Fe(III) (oxyhydr)oxides are inferred from the robust influence Si has on DIP binding during adsorption, meanwhile the influence of Si on DIP binding is inhibited during coprecipitation when As(III) and As(V) are present. In the Early Proterozoic open oceans, Fe(III) (oxyhydr)oxides precipitated when deep anoxic Fe(II)-rich waters rose and mixed with the first permanently oxygenated ocean surface waters. Our data imply that, DIP was removed from surface waters through coprecipitation with those Fe(III) (oxyhydr)oxides, rather than adsorption. Local variations in DIP and perhaps even stratification of DIP in the oceans were likely created from the continuous removal of DIP from surface waters by Fe(III) (oxyhydr)oxides, and by the partial release of DIP into the anoxic bottom waters and buried sediments. In addition to a DIP famine, the selectivity for DIP over As(V) may have led to As enrichment in surface waters, both of which would have most likely decreased the productivity of cyanobacteria and O2 production.

Silica uptake by Spartina-evidence of multiple modes of accumulation from salt marshes around the world.

Silicon (Si) plays a critical role in plant functional ecology, protecting plants from multiple environmental stressors. While all terrestrial plants contain some Si, wetland grasses are frequently found to have the highest concentrations, although the mechanisms driving Si accumulation in wetland grasses remain in large part uncertain. For example, active Si accumulation is often assumed to be responsible for elevated Si concentrations found in wetland grasses. However, life stage and differences in Si availability in the surrounding environment also appear to be important variables controlling the Si concentrations of wetland grasses. Here we used original data from five North American salt marshes, as well as all known published literature values, to examine the primary drivers of Si accumulation in Spartina, a genus of prolific salt marsh grasses found worldwide. We found evidence of multiple modes of Si accumulation in Spartina, with passive accumulation observed in non-degraded marshes where Spartina was native, while rejective accumulation was found in regions where Spartina was invasive. Evidence of active accumulation was found in only one marsh where Spartina was native, but was also subjected to nutrient over-enrichment. We developed a conceptual model which hypothesizes that the mode of Si uptake by Spartina is dependent on local environmental factors and genetic origin, supporting the idea that plant species should be placed along a spectrum of Si accumulation. We hypothesize that Spartina exhibits previously unrecognized phenotypic plasticity with regard to Si accumulation, allowing these plants to respond to changes in marsh condition. These results provide new insight regarding how salt marsh ecosystems regulate Si exchange at the land-sea interface.

Paleoproterozoic A- and S-granites in the eastern Voronezh Crystalline Massif: Geochronology, petrogenesis, and tectonic setting of origin

The eastern part of the Voronezh Crystalline Massif hosts coeval S- and A-granitoids. The biotite-muscovite S-granites contain elevated concentrations of Si, Al, and alkalis (with K predominance) and relatively low concentrations of Ca, Mg, Ti, Sr, and Ba, show pronounced negative Eu anomalies, and have low concentrations of Y and HREE. The biotite A-granitoids are enriched in Fe, Ti, P, HFSE, REE and have strongly fractionated REE patterns with deep Eu minima. According to their Rb/Nb and Y/Nb ratios, these rocks are classified with group A2 of postcollisional granites. The SIMS zircon crystallization age of the granitoids lies within the range of 2050–2070 Ma. Both the A- and the S-granitoids have positive ɛNd(T) values, which suggests that they should have had brief crustal prehistories and were derived from juvenile Paleoproterozoic sources. The simultaneous derivation of the A- and S-granites was caused by the melting of the lower crust in response to the emplacement of large volumes of mafic magma in an environment of postcollisional collapse and lithospheric delamination with the simultaneous metamorphism of the host rocks at high temperatures and low pressures. The S-granites are thought to be derived via the melting of acid crustal material in the middle and lower crust. The A2 granites can possibly be differentiation products of mafic magmas that were emplaced into the lower crust and were intensely contaminated with crustal material.

Mineralogical variation of silica induced by Al and Na in hydrothermal solutions

Various silica minerals form in hydrothermal settings; however, the main controls on mineralogical variations in such environments remain poorly understood. In this study, we investigate the effects of Al and Na on the mineralogy of silica precipitates by hydrothermal flow-through experiments for silica precipitation at 430 °C and 31 MPa. We used solutions with elevated Si concentrations, exceeding the solubility of amorphous silica. As the Al and Na concentrations in the input solution were raised from 0 to 7 ppm, the dominant silica minerals systematically changed from amorphous silica to cristobalite to quartz. The positive correlation between Al and Na contents in precipitated quartz indicates that Al coupled with Na substitutes for Si. Our results suggest a possible mechanism for quartz vein formation in which quartz nucleates directly without precursor amorphous silica or cristobalite due to the presence of feldspar-derived Al and Na in hydrothermal fluids.

Use of multi-element tracers to source apportion mercury in south Florida aerosols

Abstract The relative importance of local sources of mercury (Hg) in aerosols from urban areas in south Florida in relation to regional or global sources transported to the Everglades was investigated using a multi-element tracer approach. The sources of metals and Hg within aerosols were determined by integrating the collection of aerosols at seven locations with meteorology, source sampling, and statistical analysis. Sources include sea spray, soil dust from local carbonate bedrock and long range Saharan dust transport, regional scale transport of sulfate aerosols, and local point sources including oil-fired power plants, medical and waste incineration, and cement kilns. Using a principal components analysis-multiple linear regression (PCA-MLR) approach, 80% of the Hg in particulate form at the Thompson Park Everglades receptor site (THP) could be attributed to local sources. The key to the success of the source attribution at THP was collection of samples on a 12-h sampling basis in order to account for diurnal changes in meteorological conditions in south Florida associated with land–sea breeze development. Fifty-six±7% of the particulate Hg at THP was associated with elevated Zn concentrations which source sampling and surface meteorology indicate as emissions from municipal waste incineration located southeast of THP. Another 14±5% of the particulate Hg was associated with elevated Cu and Pb concentrations from sources SSE of THP. Eleven±1% of the particulate Hg originated from medical waste incineration sources and was associated with elevated levels of Cl and rapid SE to NW transport. Elevated concentrations of Si, Al, Fe, Mn, and K occurred on the same days at all sites, following passage of tropical storms over south Florida. PCA grouped these elements within a factor that is likely Saharan dust in origin, only 12±2% of the particulate Hg at THP could be attributed to this non-local source. Because the majority of the particulate Hg at THP can be attributed to emission sources located in nearby urban areas, regulations resulting in a reduction in particulate material emitted by industrial sources in south Florida will likely have a significant effect on reducing the flux of Hg and other trace metals in aerosol form to the Everglades.

Geochemical evolution of highly alkaline and saline tank waste plumes during seepage through vadose zone sediments

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.

Cadmium in the California Current System: Tracer of past and present upwelling

Over 100 samples were collected off the west coast of North America during 1991–1993 to determine the relation between wind‐driven upwelling and nearshore concentrations of dissolved silicate (Si), phosphate (P), and cadmium (Cd). Highly enriched in deep water offshore, these constituents are sensitive indicators of upwelling. Coastal water was sampled from the shore in January and June 1992 at 12 sites distributed between 36° and 48°N latitude. In January the composition of nearshore water along this transect was fairly uniform: 5–15 μmol/kg for Si, 0.5 to 1.0 μmol/kg for P, and 0.1–0.3 nmol/kg for Cd. In June, elevated concentrations of Si (30 μmol/kg), P (2.0 μmol/kg), and Cd (0.6 nmol/kg) revealed a region of intense upwelling between 38° and 40°N. The pattern is broadly consistent with meridional gradients in coastal upwelling calculated from the long‐term mean of alongshore winds compiled from ship reports. Nearshore water was also collected biweekly to monthly at two sites 3 km apart near San Francisco Bay (37.5°N) during 1991–1993. The variability seen in the time series suggests that the composition of nearshore water integrates the effect of alongshore winds over timescales of several weeks. Seasonal variations in Si (5–50 μmol/kg), P (0.5–2.5 μmol/kg), and Cd (0.1–0.8 nmol/kg) concentrations were consistent with upwelling during spring and summer. Maximum Si, P, and Cd concentrations reached in May 1991 were consistent with advection to the very nearshore region from a depth of about 300 m relative to a vertical profile at a distance of 200 km from the coast. Nearshore Si, P, and Cd concentrations were reduced relative to 1991 in 1992, and, to a lesser extent, in 1993 due to weaker upwelling linked to the warm phase of the El Niño‐Southern Oscillation. During periods of weaker upwelling or downwelling, variations in P, Si, and Cd concentrations became uncoupled. There is a good correlation between the coastal Cd time series near San Francisco Bay (37.5°N) and a second order polynomial function of the upwelling index of Bakun [1975] at 36°N, filtered with a 30‐day running mean (r2 = 0.71, n = 39). The index is a daily estimate of coastal upwelling calculated from 6‐hourly mean atmospheric pressure distributions at 36°N. From this function and a record of daily upwelling indices, we infer a range of annually averaged coastal Cd concentrations of at least 0.3–0.5 nmol/kg since 1967. Cd/Ca ratios in shells of foraminifera from San Francisco Bay suggest that average coastal Cd concentrations 3500–4500 years ago were at the upper end of this range.

Hydrothermal and microbial processes at Loihi Seamount, a mid-plate hot-spot volcano

Active hydrothermal vents have been discovered at the summit of Loihi Seamount, Hawaii, in an area referred to as Pele's Vents. The active vent field is characterized by iron-rich nontronite deposits, warm (30°C) hydrothermal fluids, extensive bacterial mats and by the conspicious absence of large benthic animals. Chemical analyses of collected water samples has revealed the presence of elevated concentrations of Si, NH 4 +, PO 4 −3, dissolved inorganic carbon (DIC), methane (CH 4) and bacterial cells in the discharged vent fluids, relative to ambient bottom seawater. The most distinctive feature of Pele's Vent fluid, relative to other hydrothermal systems previously described, is the especially high content of DIC (300 mM for the warmest waters sampled). Bacteria collected from Pele's Vents were shown to be metabollically active at temperatures up to at least 60°C. Bacterial mats collected from the vent field are comprised of long filaments coated with Fe precipitates. It is hypothesized that these tubular structures are formed by the growth and metabolism of Fe-oxidizing bacteria.