Biogeochemical Cycle Of Silicon Research Articles

Location, speciation, and quantification of carbon in silica phytoliths using synchrotron scanning transmission X-ray microspectroscopy.

Phytoliths of biogenic silica play a vital role in the silicon biogeochemical cycle and occlude a fraction of organic carbon. The location, chemical speciation, and quantification of this carbon within phytoliths have remained elusive due to limited direct experimental evidence. In this work, phytoliths (bilobate morphotype) from the sugarcane stalk epidermis are sectioned with a focused ion beam to produce lamellas (≈10 × 10 μm2 size, <500 nm thickness) and probed by synchrotron scanning transmission X-ray microspectroscopy (≈100-200 nm pixel size; energies near the silicon and carbon K-absorption edges). Analysis of the spectral image stacks reveals the complementarity of the silica and carbon spatial distributions, with carbon found at the borders of the lamellas, in islands within the silica, and dispersed in extended regions that can be described as a mixed silica-carbonaceous matrix. Carbon spectra are assigned mainly to lignin-like compounds as well as to proteins. Carbon contents of 3-14 wt.% are estimated from the spectral maps of four distinct phytolith lamellas. The results provide unprecedented spatial and chemical information on the carbon in phytoliths obtained without interference from wet-chemical digestion.

A dataset of atmospheric silicon wet deposition in China's terrestrial ecosystems (2013–2020)

As a "quasi essential" nutrient element for plants, Silicon (Si) serves as an essential nutrient for some phytoplankton. Atmospheric deposition has gradually become an important supplement way for silicon nutrients to enter the ecosystem. At present, there is no national observation data report on atmospheric silicon deposition. In 2013, China Wet Deposition Observation Network (ChinaWD) was established based on the Chinese Ecosystem Research Network (CERN) and stations across the country. Since 2013, the observation network has covered 43 stations nationwide. These stations are evenly distributed in various ecological regions, and almost cover the main types of ecosystems in China. This paper focuses on the compilation and collation of atmospheric silicon wet deposition flux data of 43 stations in ChinaWD network from 2013 to 2020. ChinaWD network boasts rigorous quality assurance and high quality control standards. We adopted standard solutions and blank samples for testing and calibration, and carried out repeated processing to ensure the accuracy of data. All the parameters indicate the high accuracy and reliability of test results. The dataset is expected to make up for the deficiency of regional silicon deposition data, provide data support for the follow-up regional, national and global silicon biogeochemical cycle research. Moreover, it can be used for reference to comprehensively evaluate the proportions and ecological effect of nutrient elements (nitrogen, phosphorus, silicon).

Well-preserved Norway spruce needle phytoliths in sediments can be a new paleoenvironmental indicator

Analysis of phytoliths (plant silica bodies) still may have an unrevealed potential in paleoenvironmental reconstruction studies. This can provide novel findings in research on environmental change as phytoliths play an important role in the silicon biogeochemical cycle. In favorable environmental conditions, Picea abies [L.] H. Karst (Norway spruce) needles develop a phytolith layer consisting of more or less cubical or cuboid (blocky) phytoliths in their transfusion tissue that becomes continuous toward the apex of the needle. This can be studied in situ in fossil (subfossil) needles under a stereomicroscope. This study reports the blocky-type phytolith preservation in fossil spruce needles in sediment sections of the lake Černé jezero (Bohemian Forest, Czech Republic). The oldest needle containing phytoliths was 7.8 cal ka BP. Despite differences in the Energy Dispersive X-ray (EDX) spectra of different age phytoliths, the studied subfossil phytoliths did not lose their globular ultrastructure in the needle tissue, proving the stability of this phytolith morphotype. As the tissue of the needle fossils can preserve phytoliths in situ, further micro-analytical measurements will make these needles promising tools for paleoenvironmental reconstructions. The most favorable period for spruce phytolith formation for the studied region appears to be the period 6.0–4.5 cal ka BP, within the Holocene Climate Optimum period. In order to use these phytoliths as a terrestrial climate proxy, the next step is to refine their sensitivity to environmental changes.

Hyalolithus didymus sp. nov. (Haptophyta): a late Miocene siliceous microfossil from Sicily, Italy

ABSTRACT Siliceous scales observed in an upper Miocene outcrop sample from the Caltanissetta Basin, Sicily, are similar to those of the extant haptophyte Hyalolithus neolepis and the fossil species H. tumescens. The newly discovered scales, examined by light and scanning electron microscopy, are sufficiently different to justify their assignment to H. didymus sp. nov. Like the two previously described species, the scales of H. didymus are elliptical, with a prominent marginal rim and numerous central openings (pores), separated by a submarginal hyaline area. However, the three species can be distinguished by slight (but visually recognizable) differences in the size and number of their central openings (pores). In distal view, the central area of the new species has two mounds, which are smaller and more distinct than in H. neolepis. Unlike the other two species, a swelling on the proximal side (and its corresponding depression on the distal side) has not been seen yet in H. didymus. This finding increases the knowledge of the fossil record of siliceous haptophytes, a poorly known phytoplankton group involved in the biogeochemical silicon cycle.

Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26–0.52 wt% or 92–185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

Spatiotemporal dynamics of coupled dissolved silica and bicarbonate in a dam-induced urban lake system in the Three Gorges Reservoir Area

Dissolved silica (DSi) and bicarbonate (HCO3–) stoichiometric coupling is collectively regulated by chemical weathering and algal activity, and potentially indicates the aquatic silicon retention and carbon emission in aquatic systems. Damming markedly remodels the hydrological conditions and aquatic biological activity, however, its effects on the spatiotemporal dynamics of coupled DSi and HCO3– remain poorly understood. Here we deciphered a 2-year hydrological and water quality dataset collected at multiple sites within an urban lake, namely Hanfeng Lake, the largest pre-dam of the Three Gorges Reservoir, to unravel the spatiotemporal dynamics of DSi and HCO3– and selected hydrological and water quality variables using cluster analysis, and to explore the best predictors for DSi:HCO3– using partial least squares regression. The changes of DSi, HCO3– and selected water variables could be characterized by spatial east and west zones, and further by temporal periods 1, 2 and 3 corresponding respectively to fluctuation stage, fluvial stage and lacustrine stage in each zone based on the hydrographic features of Hanfeng Lake. Wet season presented higher flow velocity, permanganate index, concentrations of DSi, HCO3–, nitrate-nitrogen and total suspended solids, and lower water level, secchi depth and chlorophyll-a, but equal DSi:HCO3– relative to dry season. DSi was overall negatively correlated with HCO3–, highlighting the dominant role of chemical weathering rather algal activity. DSi:HCO3– exhibited significant relationships with various variables, but was the best predictable using water level, permanganate index and nitrogen-based variables, suggesting the strong effects of water level fluctuation and pollution discharge on the coupling of silicon and carbon. Our results have great significance for understanding and modeling the biogeochemical cycles of silicon and carbon in the dam-induced lake ecosystems worldwide.

Reviews and syntheses: The biogeochemical cycle of silicon in the modern ocean

Abstract. The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 37 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr−1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr−1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs =14.8(±2.6) Tmol Si yr−1 and outputs =15.6(±2.4) Tmol Si yr−1. Potential impacts of global change on the marine Si cycle are discussed.

典型喀斯特流域旱雨季交替下溶解硅的输送特征

河流溶解硅（DSi）作为营养物质对维持陆地、河流及海洋生态系统稳定性起到至关重要的作用。选取贵州典型喀斯特流域为研究对象，通过对DSi湿沉降过程，基流过程及降雨径流过程的动态变化进行全年监测分析，探讨DSi在旱雨季交替下的输送特征及河流DSi浓度变化引起的环境效应。结果表明：①湿沉降过程降雨量越大，DSi浓度越小，河流DSi浓度变化有明显的旱、雨季特征，雨季DSi浓度较高，旱季较低，地表水径流量及DSi浓度对降雨径流过程的响应比地下水明显。②DSi沉降通量及输出通量呈明显的旱、雨季差异，雨季DSi湿沉降通量占全年的69.5%，地表水雨季DSi输出负荷占全年的98.1%，地下水占51.4%。③流域硅酸盐岩风化过程不强烈，主要受到碳酸盐岩及蒸发岩控制。流域DSi浓度受人为水库影响明显，经过水库后河流中DSi浓度旱季下降29.0%、雨季下降70.9%。研究为全面认识硅在陆地生态系统中的生物地球化学循环提供科学依据。;Dissolved silicon (DSi) in natural waters plays important roles in sustaining continent, riverine and ocean ecosystems. It is a necessary nutrient for the growth and reproduction of aquatic plants and phytoplankton, especially diatoms. The composition of dissolved silicon in river systems is complex and depends on physical, chemical, and biological processes that occur in drainage basins and rivers. In this study, a typical karst watershed located in Guizhou Province, Southwest China was selected as the study area to reveal the characteristics of DSi transport and discuss seriously environmental effects on the variation of DSi concentration. The objects of this research were to monitor the dynamic characteristics of dissolved silicon in wet deposition, rainfall-runoff processes and baseflow process in the dry and wet seasons during the whole year and to discuss about serious environmental effects on the variation of DSi concentration. Results showed that (1) The greater the rainfall, the smaller the DSi concentration during the wet deposition. DSi concentration in the wet season was higher than that in the dry season. DSi concentration and discharge in surface water were more easily responded by rainfall-runoff process than in groundwater. (2) The deposition flux and export flux of DSi showed evident differences in the wet and dry seasons. DSi deposition flux in the wet season accounted for 69.5%, and export load of surface water and groundwater accounted for 98.1% and 51.4% of the whole year, respectively. (3) The study area had a slow weathering rate of silicate minerals which was mainly controlled by carbonates and evaporite rates. DSi concentration was significantly affected by man-made reservoir as the results showed that the DSi concentration dropped by 29% and 70% in the dry and wet seasons, respectively. The study is conducive to a more comprehensive understanding of the silicon biogeochemical cycle in terrestrial ecosystems. In future studies, long-term studies on the effects of agricultural activities and land use patterns on the supply of nutrients in the watershed are needed to predict the change of phytoplankton community with the dynamic change of DSi concentration from a long-term perspective. In addition to DSi monitoring, BSi monitoring should be added to further study the silicon cycling process and related environmental effects in terrestrial ecosystems.

Terrestrial biogeochemistry of silicon

Biogeochemistry Silicon is an important element in plant tissues and contributes to structural defenses against herbivores and other stresses. However, the terrestrial biogeochemical cycling of silicon is poorly understood, particularly the relative importance of geochemical and biological mechanisms in its regulation. de Tombeur et al. studied this question in 2-million-year chronosequences of soil and vegetation in Western Australia. Sites became progressively more weathered and infertile as they aged, indicating that the silicon cycle shifts from geochemical to biological control as the ecosystem develops (see the Perspective by Carey). They found that foliar silicon concentrations increase continuously during ecosystem development, even though rock-derived silicon is depleted in the older soils. By contrast, other major rock-derived nutrients showed decreasing concentrations in plants. Hence, biological silicon cycling allows plants to maintain concentrations even under conditions of extreme soil infertility. Science , this issue p. [1245][1]; see also p. [1161][2] [1]: /lookup/doi/10.1126/science.abc0393 [2]: /lookup/doi/10.1126/science.abd9425

Plants sustain the terrestrial silicon cycle during ecosystem retrogression.

The biogeochemical silicon cycle influences global primary productivity and carbon cycling, yet changes in silicon sources and cycling during long-term development of terrestrial ecosystems remain poorly understood. Here, we show that terrestrial silicon cycling shifts from pedological to biological control during long-term ecosystem development along 2-million-year soil chronosequences in Western Australia. Silicon availability is determined by pedogenic silicon in young soils and recycling of plant-derived silicon in old soils as pedogenic pools become depleted. Unlike concentrations of major nutrients, which decline markedly in strongly weathered soils, foliar silicon concentrations increase continuously as soils age. Our findings show that the retention of silicon by plants during ecosystem retrogression sustains its terrestrial cycling, suggesting important plant benefits associated with this element in nutrient-poor environments.

Impact of human disturbance on the biogeochemical silicon cycle in a coastal sea revealed by silicon isotopes

AbstractBiogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. Besides the well‐known processes, diatom production and dissolution regulating δ30SiDSi values in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased δ30SiDSi values in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the δ30SiDSi distribution. An atypical vertical distribution of δ30SiDSi with higher values in deep water (1.57–1.95‰) relative to those in surface water (1.24–1.68‰) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore‐water δ30SiDSi values, which then leads to pore‐water DSi flux carrying higher δ30SiDSi compositions into deep water. Our findings provide new quantitative information on how the isotope‐based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean.

Root silicification of grasses and crops from the Pampean region and its relevance to silica and silicophytolith content of soils

AimsSilicophytoliths contribute to the pool of amorphous silica that is incorporated into the soils acting as intermediary in the biogeochemical cycle of silicon. Most studies focus their attention in the production of aerial parts of plants but not in roots. In order to advance into the knowledge about the relevance of root amorphous silica in soil Si cycle, we studied the silicophytolith content of roots from grasses and crops, and from soil’s roots of different sites from Pampean region, Argentina, one of the most fertile and used for agricultural practices.MethodsRoots from three crops and seven grasses and from soils under three different uses were studied. Silicophytoliths were extracted by a calcination technique.ResultsAll the species produced silicophytoliths derived from vascular and/or endodermal tissues. Silicophytolith content was higher in Pampean grasses (6.34–15.38%) than in crops (0.01–1.58%). Root silicophytolith content in the pasture was 10 times higher than in the other sites.ConclusionsThe potential input of root silica to soils is related to the type and cover of vegetation, the amount of roots in soils and the silicophytoliths accumulated in roots. Results suggested that root silicophytoliths should be consider in future studies of biogeochemical cycle of Si.

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

Competition is a central part of the evolutionary process, and silicification is no exception: between biomineralized and non-biomineralized organisms, between siliceous and non-siliceous biomineralizing organisms, and between different silicifying groups. Here we discuss evolutionary competition at various scales, and how this has affected biogeochemical cycles of silicon, carbon, and other nutrients. Across geological time we examine how fossils, sediments, and isotopic geochemistry can provide evidence for the emergence and expansion of silica biomineralization in the ocean, and competition between silicifying organisms for silicic acid. Metagenomic data from marine environments can be used to illustrate evolutionary competition between groups of silicifying and non-silicifying marine organisms. Modern ecosystems also provide examples of arms races between silicifiers as predators and prey, and how silicification can be used to provide a competitive advantage for obtaining resources. Through studying the molecular biology of silicifying and non-silicifying species we can relate how they have responded to the competitive interactions that are observed, and how solutions have evolved through convergent evolutionary dynamics.

Silicon cycle in Indian estuaries and its control by biogeochemical and anthropogenic processes

We study the silicon biogeochemical cycle and its associated parameters in 24 and 18 Indian estuaries during dry and wet periods respectively. We focus more specifically on dissolved Si (DSi), amorphous Si (ASi,) lithogenic Si (LSi), Particulate Organic Carbon (POC), Total Suspended Material (TSM), Dissolved Inorganic Nitrogen (DIN), salinity and fucoxanthin, a marker pigment for diatoms. Overall, we show that the estuaries have strong inter and intra variability of their biogeochemical parameters both seasonally and along salinity gradients. Based on Principal Component Analysis and clustering of categorised (upper and lower) estuaries, we discuss the four major processes controlling the Si variability of Indian estuaries: 1) lithogenic supply, 2) diatom uptake, 3) mixing of sea water and, 4) land use. The influence of lithogenic control is significantly higher during the wet period than during the dry period, due to a higher particle supply through monsoonal discharge. A significant diatom uptake is only identified in the estuaries during dry period. By taking into account the non-conservative nature of Si and by extrapolating our results, we estimate the fluxes from the Indian subcontinent of DSi, ASi, LSi to the Bay of Bengal (211 ± 32, 10 ± 4.7, 2028 ± 317Gmol) and Arabian Sea (80 ± 15, 7 ± 1.1, 1717 ± 932Gmol). We show the impact of land use in watersheds with higher levels of agricultural activity amplifies the supply of Si to the coastal Bay of Bengal during the wet season. In contrast, forest cover and steep slopes cause less Si supply to the Arabian Sea by restricting erosion when entering the estuary. Finally, Si:N ratios show that nitrogen is always in deficit relative to silicon for diatom growth, these high Si:N ratios likely contribute to the prevention of eutrophication in the Indian estuaries and coastal sea.

Identification, distribution, and quantification of biominerals in a deciduous forest.

Biomineralization is a common process in most vascular plants, but poorly investigated for trees. Although the presence of calcium oxalate and silica accumulation has been reported for some tree species, the chemical composition, abundance, and quantification of biominerals remain poorly documented. However, biominerals may play important physiological and structural roles in trees, especially in forest ecosystems, which are characterized by nutrient-poor soils. In this context, our study aimed at investigating the morphology, distribution, and relative abundance of biominerals in the different vegetative compartments (foliage, branch, trunk, and root) of Fagus sylvatica L. and Acer pseudoplatanus L. using a combination of scanning electron microscopy and tomography analyses. Biomineral crystallochemistry was assessed by X-ray diffraction and energy-dispersive X-ray analyses, while calcium, silicon, and oxalic acid were quantified in the compartments and at the forest scale. Our analyses revealed that biominerals occurred as crystals or coating layers mostly in bark and leaves and were identified as opal, whewellite, and complex biominerals. In both tree species, opal was mostly found in the external tissues of trunk, branch, and leaves, but also in the roots of beech. In the stand, opal represents around 170kg/ha. Whewellite was found to suit to conductive tissues (i.e., axial phloem parenchyma, vascular bundles, vessel element) in all investigated compartments of the two tree species. The shape of whewellite was prismatic and druses in beech, and almost all described shapes were seen in sycamore maple. Notably, the amount of whewellite was strongly correlated with the total calcium in all investigated compartments whatever the tree species is, suggesting a biologic control of whewellite precipitation. The amount of whewellite in the aboveground biomass of Montiers forest was more important than that of opal and was around 1170 kg/ha. Therefore, biominerals contribute in a substantial way to the biogeochemical cycles of silicon and calcium.

Silicon isotope fractionation in rice and cucumber plants over a life cycle: Laboratory studies at different external silicon concentrations

AbstractUnderstanding the variations of silicon isotopes in terrestrial higher plants can be helpful toward elucidating the global biogeochemical silicon cycle. We studied silicon isotope fractionation in rice and cucumber plants over their entire life cycles. These two different silicon‐absorbing plants were grown hydroponically at different external silicon concentrations. The ranges of δ30Si values in rice were −1.89‰ to 1.69‰, −1.81‰ to 1.96‰, and −2.08‰ to 2.02‰ at 0.17 mM, 1.70 mM, and 8.50 mM silicon concentrations, respectively. The ranges of δ30Si values in cucumber were −1.38‰ to 1.21‰, −1.33‰ to 1.26‰, and −1.62‰ to 1.40‰ at 0.085 mM, 0.17 mM, and 1.70 mM external silicon concentrations, respectively. A general increasing trend in δ30Si values from lower to upper plant parts reflected the preferential incorporation of lighter silicon isotopes from transpired water to biogenic opal. Furthermore, the active uptake mechanism regulated by several transporters might have also played an important role in the preferential transport of heavy silicon isotopes into aboveground plant parts. This suggested that silicon isotope fractionation in both rice and cucumber was a Rayleigh‐like process. The data on δ30Si values for the whole plants and nutrient solutions indicated that biologically mediated silicon isotope fractionation occurred during silicon uptake by roots. At lower external silicon concentrations, heavy silicon isotopes entered plants more readily than light silicon isotopes. Conversely, at higher external silicon concentrations, light silicon isotopes entered plants more readily than heavy silicon isotopes.

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation.

Alkaline‐extractable silicon from land to ocean: A challenge for biogenic silicon determination

AbstractThe biogeochemical cycling of silicon (Si) along the land‐to‐ocean continuum is studied by a variety of research fields and for a variety of scientific reasons. However, there is an increasing need to refine the methodology and the underlying assumptions used to determine biogenic silica (BSi) concentrations. Recent evidence suggests that contributions of nonbiogenic sources of Si dissolving during alkaline extractions, not corrected by standard silicate mineral dissolution correction protocols, can be substantial. The ratio between dissolved Si and aluminum (Al) monitored continuously during the alkaline extraction can be used to infer the origin of the Si fractions present. In this study, we applied both a continuous analysis method (0.5 M NaOH) and a traditional 0.1 M Na2CO3 extraction to a wide array of samples: (1) terrestrial vegetation, (2) soils from forest, cropland and pasture, (3) lake sediments, (4) suspended particulate matter and sediments from rivers, (5) sediments from estuaries and salt marshes and (6) ocean sediments. Our results indicate that the 0.1 M Na2CO3 extraction protocol can overestimate the BSi content, by simultaneously dissolving Si fractions of nonbiogenic origin that may represent up to 100% of the Si traditionally considered as biogenic, hampering interpretation especially in some deeper soil horizons, rivers and coastal oceanic sediments. Moreover, although the term amorphous Si was coined to reflect a growing awareness of nonbiogenic phases we show it is actually inappropriate in samples where silicate minerals may account for a large part of the extracted Si even after linear mineral correction.

Phytolith production in wetland plants of the Hangzhou Xixi Wetlands ecosystem

Phytoliths，also referred to as plant opal，are silicified features that form as a result of biomineralization within plants and plays an important role in terrestrial biogeochemical cycles of carbon and silicon. This study selected 18 different plant species at the Xixi Wetlands in Zhejiang Province，to determine the phytolith content and its production flux using microwave，Walkley-Black digestion，and a correlation analysis. The main purpose of this study is to provide scientific reference for understanding the role of phytoliths in biogeochemical cycles of silicon and carbon. Results showed strong variation in phytolith content among the 18 plants：Cortaderia selloana（7.7%），Setaria viridis（7.6%），Digitaria ternata（6.9%），and Phragmites australis （6.6%）were higher than Salvinia natans（1.3%），Halerpestes cymbalaria（1.1%），and Canna indica L.（1.0%）. A positive correlation between phytolith content and total SiO2 in the 18 plants（R2=0.663 3，P＜0.01）was also found. Phytolith production flux was 4.48-129.92 gm-2a-1；phytolith sequestration of atmospheric CO2 was 0.16-1.03 gm-2a-1；and the phytolith sequestration rate of atmospheric CO2 was 8.29 ta-1. So，it is very important to improve the flux of aboveground plant phytolith production in wetlands ecosystems by selecting a plant with high-phytolith（silicon）content and high-ANPP（aboveground net primary production）yields for the plant Phragmites australis.［Ch，1 fig. 1 tab. 35 ref.］

Controls on riverine δ30Si signatures in a temperate watershed under high anthropogenic pressure (Scheldt — Belgium)

We analysed Si isotopic signatures (δ30Si signature) of dissolved silicon (DSi) in the Scheldt (Belgium) tidal river (four stations) and in its main tributaries (Zenne, Dender, Grote Nete and Kleine Nete). For each station, we analysed four samples representative of the four seasons. These are the first δ30Si data at the basin and seasonal scales measured in a watershed under high anthropogenic pressure. In the Scheldt main stem, we show that the role of diatoms on the silicon biogeochemical cycle and δ30Si signatures is dominant in summer and sometimes in fall. Out of the diatoms' growing season, the Scheldt main stem exhibits unexpected large δ30Si signatures variations, especially in winter. Although these isotopic variations are not reflected in DSi concentrations, they are consistent with δ30Si signatures recorded in the main tributaries. This indicates that except for summer diatom blooms, δ30Si signatures in the tidal river result from a conservative mixing with tributaries. Overall we suggest that the variations are mainly set by land use and lithology at the sub-basin level. Forest cover (inducing clay dissolution favoured by the presence of humic acids) and wetland cover (yielding to dissolution of biogenic silica) are highly and significantly correlated with δ30Si (R2>0.85). Both environments tend to render the tributaries δ30Si signatures lighter while agriculture cover has the reverse effect on δ30Si values (R2=0.70, p-value<0.08). In contrast, urban zone cover does not have any noticeable effect on Si isotopic signatures of the Scheldt tributaries. Despite this effect of land use, we estimate a mean annual δ30Si signature output from the Scheldt tidal river to the saline estuary at 0.95‰, well comparable to other rivers. This results from the mixing of tributaries within the tidal river which levels off at the outlet the Si isotopic differences of the sub-basins. In the Scheldt watershed, it thus seems that Si isotopes could trace anthropogenic activities at the sub-basin scale. In contrast, at the outlet, the variety of land-use and lithology found over the whole watershed are merged into a mean isotopic signature similar to the world river average. As it is now recognised that change in land use alter the silicon biogeochemical continental cycle, our data show that use of Si isotopic signatures might constitute a useful proxy to better trace these modifications on contrasted (sub-)basins.