Si Pools Research Articles

Silicon balance in an integrated multi-tropical aquaculture ecosystem, Sanggou Bay, China

Farmed aquaculture species play an important role in regulating nutrient cycles in farming systems. Compared with nitrogen and phosphorus, the role of farmed species in the silicon (Si) cycle remains poorly understood. To help reduce this uncertainty, we clarified the sources and sinks of silicate and quantified the Si pools in an aquaculture system in Sanggou Bay (SGB). The results showed that dissolved inorganic nutrient levels were significantly lower during the dry season than during the wet. Dissolved silicate (DSi) is a potential limiting factor for phytoplankton growth during spring, and phosphorus limitation occurs during summer. The budget results indicated that large amounts of nitrogen, phosphate (DIP), and DSi were buried in the sediment or transformed into other forms during both the wet and dry seasons. The nitrogen and DIP cycles were strongly influenced by bivalve excretion and farmed species harvesting; however, these processes had little impact on the Si cycle. Si availability depends on both external inputs and internal recycling. DSi was primarily supplied from the Yellow Sea, with a minor contribution from the river due to river discharge during spring. However, during summer, riverine inflow (accounting for 83% of the total influx) was the major DSi source followed by benthic flux (12%). Biogenic silica (BSi) burial efficiency in the sediment was estimated to be 78% during spring and 23% during summer. The BSi preservation efficiency in bivalves during spring was high (53%), leading to a higher Si retention than in river discharge. Bivalves biodeposition plays an important role in the Si burial process. We suggest that this high retention is essentially controlled by the biodeposition mechanism, which is directly controlled by the exotic suspension feeders. Bivalves have the potential to alter Si retention in the bay by producing large amounts of biodeposits and accelerating the silica cycle, which may lead to more carbon dioxide being absorbed by diatoms.

Forms of Silicon in Rice Soils of Kerala, India

Aim: The plant-available silicon (PAS) content is highly influenced by different forms/fractions of silicon (Si) in which it exists in the soil. The solubility of these forms varies and affects the Si concentration in soil solution. Quantification of Si pools in soils is needed for a better understanding of biogeochemical processes that govern Si dynamics in soils, the magnitude of Si release, and Si-cycling between soil and plant systems. In this context, an attempt was made to elucidate the status and distribution of Si pools in major rice-growing soils of Kerala.
 Methodology: The soil samples were collected from five different locations representing the major rice growing tracts (Kuttand, Kole, Pokkali, sandy and lateritic soils) of Kerala, India. The processed soil samples were analyzed to estimate various physicochemical properties. Then, using standard procedure, different fractions of Si in these soils were estimated and their correlation with physico-chemical properties of soils were worked out.
 Results: The forms of Si estimated in these soils were mobile Si, adsorbed Si, organic Si, amorphous Si, and residual Si. The percentage distributions of Si pools in these soils were mainly in the order; of residual Si > amorphous Si > occluded Si > organic Si > mobile Si > adsorbed Si. Mobile and adsorbed Si were the smallest fractions and amorphous Si was the largest fraction in these soils.
 Conclusion: Mobile and occluded Si were found to be the major contributors of PAS in major rice soils of Kerala.

An investigation into the characteristics of reactive silicon pools of coastal marine sediments

Many studies use traditional alkaline leach digestions to quantify biogenic silica (bSiO2) in sediments. Studies from river-plume systems demonstrate this methodology precludes information on other operational Si reactive pools, missing potentially valuable information on early diagenetic products. We build on previous studies by broadly examining the reactive Si pools in Mississippi River plume-influenced coastal sediments. The magnitude of Si released among operational pools across the region revealed large spatial variations in the conservation of detrital materials. Contrary to other analyses using different approaches, we suggest the availability of detrital sourced components (i.e. Al, Fe and Mn) may be a factor limiting authigenesis and resulting Si storage capacity in coastal sediments by this chemical mechanism. Overall, these data suggest authigenic precipitates may be forming as a function of bSiO2 accumulation, thereby implying that bSiO2 acts as the primary substrate for formation of authigenic products. The amount of authigenic Si products, relative to bSiO2, appears to be larger in this region than other systems studied to date, and this may be due to regional eutrophication which has led to an increase in sediment bSiO2 compounding the substrate available for authigenic precipitates. Thus, regions which have all experienced increased sediment bSiO2 accumulation coincident with eutrophication, may be primed for substantial authigenic product formation and increased Si sequestration.

Production of phytolith and PhytOC and distribution of extractable Si Pools in aerobic rice as influenced by different Si sources.

Phytoliths are composed of 66 to 91% SiO2 and 1 to 6% organic carbon (C) known as phytolith occluded carbon (PhytOC). PhytOC is critical for long-term C storage in the agroecosystem. A field experiment was carried out to investigate the effect of three different sources of exogenous Si, i.e., diatomaceous earth (DE), silicic acid (SA) and rice husk biochar (RHB) on 1) plant phytolith, C content in phytolith and PhytOC content in different rice organs; 2) relationship between plant phytolith, C content in phytolith, PhytOC content, and soil properties (soil physicochemical properties and readily soluble silicon pools). Different Si sources produced significantly higher phytolith, PhytOC content, and readily soluble Si pools (CCSi, AASi, and ASi) than the control (RDF), with treatment receiving 4t RHB ha-1 outperforming the other treatments. Phytolith and PhytOC production were found to be significantly correlated to soil organic carbon (OC), available nitrogen (N) and potassium (K), 0.01 M CaCl2 extractable Si (CCSi) and amorphous Si (ASi) content in the soil. Redundancy analysis showed that treatments receiving 4t RHB ha-1 have a stronger relationship with the CCSi and ASi which majorly contributed to the higher phytolith and PhytOC production. Thus, practices such as Si fertilizers and RHB application have a high potential for phytolith production and PhytOC sequestration, a critical mechanism of the global biogeochemical C sink.

Effects of turning rainforest into oil-palm plantations on silicon pools in soils within the first 20 years after the transformation

Potential effects of land-use/land-cover (LULC) transformation from lowland rainforest into oil-palm plantations on silicon (Si) pools in tropical soils remain poorly understood, although appropriate levels of plant-available Si in soils may contribute to maintain high crop yields and increase the vitality and drought resistance of oil palms. Therefore, the aim of our study was to identify possible effects of such LULC change on soil Si pools. For this purpose, we compared soil Si pools under lowland rainforest and ca. 20 year-old oil-palm plantations in Jambi Province, Indonesia. The investigated soils were Acrisols and Stagnosols, in which we quantified six different soil Si pools following a sequential extraction procedure to evaluate, whether 20 years of oil-palm cultivation has led to a depletion of these soil Si pools. The considered pools included mobile Si, adsorbed Si, Si bound in soil organic matter (SOM), Si included in pedogenic oxides and hydroxides, and Si in amorphous silica of biogenic and pedogenic origin. Finally, we also determined total Si. All oil-palm plantations established on sloping terrain and Acrisols only showed decreased Si stocks of mobile Si, adsorbed Si and SOM-bound Si; those established in floodplains and Stagnosols had decreased stocks of SOM-bound Si and biogenic-amorphous silica. Lower Si stocks were mostly attributed to a missing “stable” phytolith pool in the subsoil and less organic matter in topsoils under oil-palm plantations. When comparing well-drained and riparian areas, flooding seemed to increase phytolith dissolution. We conclude that 20 years of oil-palm cultivation has not yet led to a significant depletion of soil Si pools. As topsoils comprise the highest concentrations of SOM-bound Si and Si in amorphous silica of biogenic origin but are susceptible to erosion and surface runoff under managed oil-palm plantations, it would be advisable to instate specific management practices that maintain organic-rich and well-aired topsoils on oil-palm plantations.

Release of labile Si from forest and agricultural soils

Despite the importance of silicon (Si) as benefecial nutrient for many plants, including economically-important cereals, the reactivity of various Si pools in soils (silicate minerals, amorphous compounds, phytoliths, organic litter) is not fully quantified which does not allow predicting the capacity of agricultural or forested soil to provide soluble Si to soil porewaters where it can be used by plants. Towards better understanding of factors controlling bioavailable Si in soils, here we quantified the release rate of Si from several pairs of French forest and agricultural topsoils, developed on calcareous or loess parent material. We used mixed-flow and column reactors, in acetate and carbonate buffers and distilled water, at various pH (4–8) and time of reaction (day to month).The rate of Si release from soil (RSi) exceeded that of crystalline clay minerals by 1–2 orders of magnitude, being 5–10 times lower than that of various allophanes. The rates were weakly dependent on pH (compared to clays or phytoliths) and varied from 5 × 10−7 to 2 × 10−6 mol/gsoil/day at 4 < pH ≤ 8. In terms of Si reactivity, four studied soil groups followed the order: “calcaric cambisols ≈ hypereutric cambisols ≥ luvisols ≥ albeluvisol”. Calcaric and hypereutric cambisols as well as luvisols exhibited a weak decrease of RSi with pH increase. The rate of Si release from albeluvisol increased 2 times with a pH increase from 4 to 8. There was no measurable difference in RSi between agricultural and forest soils.The pool of labile Si in forest soils was quantified via soil column flow-through experiments. The breakthrough curves of Si demonstrated high concentrations (1.6–5.7 mg/L) over first several hours of reaction in the soil column. The water leachable Si pool ranged from 0.04 to 0.08 mg Si gsoil−1, corresponding to labile Si stock in the 0–20 cm soil of 80–160 kg ha−1. These values can meet the annual requirements of plants in forests and cultivated soils. The pool of labile Si correlated (RPearson > 0.90; p < 0.05) with Si associated with amorphous Fe and Al compounds and soluble bioavailable Si extracted using CaCl2 method, but negatively correlated with total Si content in soils and Si of phytoliths. The main pools of soil labile Si could be allophanes, organo-Fe-Al-Si compounds and adsorbed forms of Si onto Fe and Al hydroxides. We hypothesize that, because of low sensibility of RSi to type of soil and fluid pH, the majority of soils regulate Si release at some ‘universal’ rate which is further reflected in relatively narrow range of riverine Si concentrations and export fluxes across the world. Therefore, the modeling of chemical weathering and element export flux in the watersheds should incorporate the experimentally measured reactivity of the whole soil rather than individual constituting primary or secondary minerals.

Oil-palm management alters the spatial distribution of amorphous silica and mobile silicon in topsoils

Abstract. Effects of oil-palm (Elaeis guineensis Jacq.) management on silicon (Si) cycling under smallholder oil-palm plantations have hardly been investigated. As oil palms are Si accumulators, we hypothesized that management practices and topsoil erosion may cause Si losses and changes in spatial Si concentration patterns in topsoils under oil-palm cultivation. To test this hypothesis, we took topsoil samples under mature oil-palm plantations in well-drained and riparian areas of Jambi Province, Indonesia. The samples were taken from four different management zones within each oil-palm plot: palm circles, oil-palm rows, interrows, and below frond piles. We quantified mobile Si (SiM) and Si in amorphous silica (SiAm) by the extraction of CaCl2 and NaCO3, respectively. Both fractions are important Si pools in soils and are essential for plant–soil Si cycling. We further installed sediment traps on sloping, well-drained oil-palm plantations to estimate the annual loss of soil and SiAm caused by erosion. In well-drained areas, mean topsoil SiAm concentrations were significantly higher below frond piles (3.97 ± 1.54 mg g−1) compared to palm circles (1.71 ± 0.35 mg g−1), oil-palm rows (1.87 ± 0.51 mg g−1), and interrows (1.88 ± 0.39 mg g−1). In riparian areas, the highest mean topsoil SiAm concentrations were also found below frond piles (2.96 ± 0.36 mg g−1) and in grass-covered interrows (2.71 ± 0.13 mg g−1), whereas topsoil SiAm concentrations of palm circles were much lower (1.44 ± 0.55 mg g−1). We attributed the high SiAm concentrations in topsoils under frond piles and in grass-covered interrows to phytolith release from decaying oil-palm fronds, grasses, and sedges. The significantly lower SiAm concentrations in palm circles (in both well-drained and riparian areas), oil-palm rows, and unvegetated interrows (only in well-drained areas) were explained by a lack of litter return to these management zones. Mean topsoil SiM concentrations were in the range of ∼ 10–20 µg g−1. They tended to be higher in riparian areas, but the differences between well-drained and riparian sites were not statistically significant. Soil-loss calculations based on erosion traps confirmed that topsoil erosion was considerable in oil-palm interrows on slopes. Erosion estimates were in the range of 4–6 Mg ha−1 yr−1, involving SiAm losses in a range of 5–9 kg−1 ha−1 yr−1. Based on the observed spatial Si patterns, we concluded that smallholders could efficiently reduce erosion and support Si cycling within the system by (1) maintaining a grass cover in oil-palm rows and interrows, (2) incorporating oil-palm litter into plantation management, and (3) preventing soil compaction and surface-crust formation.

A Large Silicon Pool in Small Picophytoplankton.

Marine picophytoplankton (<2 μm) play a key role in supporting food web and energy flow in the ocean, and are major contributors to the global marine carbon (C) cycle. In recent years, picophytoplankton have been found to have significant silica (Si) accumulation, a finding which provides a new sight into the interaction of marine C and Si cycles and questions the overwhelming role of large diatoms (>2 μm) in the Si cycle. As picophytoplankton have high cell abundance and wide distribution in the open ocean, exploring their influences on the C and Si cycles as well as other element cycles are becoming new scientific hotspots. However, there are still few studies on the physiology and ecology of picophytoplankton, especially their potential roles in the biogeochemical Si cycle at present. Thus, it is necessary to accurately evaluate and quantify the contributions of picophytoplankton to the C and Si cycles, and to further understand their C and Si sinking mechanisms. In this review, we expect to have a novel understanding of picophytoplankton Si pool and regulation mechanism by conducting targeted studies on these scientific issues. This also provides a premise foundation and theoretical framework for further study of the role of small cells in the global ocean Si cycle and the coupling of C and Si cycles.

Variation of soil silicon fractions and bioavailability during centennial‐scale evolution of ecosystem after glacier retreat

AbstractThe biogeochemical cycle of Si plays an important role in maintaining the function and stability of ecosystems. Although there have been many studies on long‐term silicate weathering, there are few reports on Si fractions and cycling in early ecosystem development. Using optimized sequential chemical extraction processes, this study quantified soil Si fractions of the eluvial horizon and deposition layers in Hailuogou Glacier Retreat Area of Gongga Mountain, Southwest China. The results showed that plant‐available Si (NaAc–Si) content decreased significantly from 33.40 to 2.16 mg kg−1 with the increase of soil age. There was a significantly positive correlation (p &lt; .01) between soil pH and NaAc–Si. Within the labile Si fractions, the largest fraction was amorphous Si (Na2CO3–Si) (165.35 ± 81.50 mg kg−1), followed by pedogenic oxides/hydroxides chemisorbed Si (Oxalate–Si) (135.13 ± 66.83 mg kg−1), Si adsorbed on the surface of inorganic soil particles (Acetic–Si) (9.66 ± 4.27 mg kg−1), and soluble Si (CaCl2–Si) (4.89 ± 1.68 mg kg−1). Compared with Stage 1 (i.e., a variety of pioneer species symbiosis stage) after glacier retreat, the storage of these four labile Si fractions in Stage 3 (i.e., climax species stage) decreased by 45.95% (CaCl2–Si), 42.98% (Acetic–Si), 67.34% (Oxalate–Si), and 75.24% (Na2CO3–Si), respectively, due to the absorption and utilization of Si by plants. Our study implies that the multiple transformations of different Si fractions in soils and the absorption and return of Si by plants can keep the CaCl2–Si pool in dynamic equilibrium, which provides a basis for the stability of ecosystem functions during the vegetation evolution after glacier retreat.

Assessment of the influence of bamboo expansion on Si pools and fluxes in a disturbed subtropical evergreen broadleaved forest

Silicon (Si) pools and fluxes are affected by plant community shifts in forest ecosystems. Moso bamboo (Phyllostachys pubescens) is a Si accumulating plant, and its expansion exerts various ecological effects in colonized forest ecosystems. However, it is unclear how bamboo expansion might alter the biogeochemical Si cycle. To estimate the consequences of bamboo expansion on Si availability and biochemical cycling in subtropical areas, we selected bamboo-pure forest (BPF), bamboo-broadleaved mixed forest (BMF), and adjacent secondary evergreen broadleaved forest (EBF) stands in an area of bamboo expansion and compared Si pools and fluxes of plants in each forest type. We found that the content of the soluble and exchangeable Si in soils of BPF was lower than EBF (0.18 g kg−1 versus 0.25 g kg−1), that is, bamboo expansion decreased the soluble and exchangeable Si fraction, whereas there was no significant difference among BPF, BMF and EBF. In addition, the plant Si pool in BPF (285 g Si m−2) was 32.6 % lower than EBF (428 g Si m−2), and there was a shift in Si allocation from aboveground to belowground with conversion from EBF to BPF, largely attributed to abundant roots in BPF. We found that bamboo expansion accelerated the biogeochemical Si cycle to some extent, with higher production of phytoliths and quicker turnover of fine roots in BPF than EBF. We also highlight that bamboo expansion accelerated the uptake and return of Si, and promoted the release and dissolution of phytoliths. These results have implications for assessing the impacts of vegetation shifts on the biogeochemical Si cycle.

Allophanes, a significant soil pool of silicon for plants

While in tropical soils recycling of plant phytoliths has been shown to represent a major pool of Si available for plants, the main Si pools available for plants in temperate soils are still poorly constrained. We characterised the various Si pools of temperate forested and cultivated soils in France by selecting seven paired sites (adjacent wheat and forest plots) for four soil groups: Luvisols, Albeluvisols, calcaric and hypereutric Cambisols. We showed that CaCl2-extracted Si (bioavailable) is mostly controlled by pH and allophanes (short-range ordered aluminosilicates) but not by phytoliths. Cultivation, by decreasing the soil organic carbon content increases the allophane content and the SiCaCl2 concentration. Our work highlights the importance of allophanes as the missing link between the soil solution and the clay minerals that could explain the reported correlation between the clay mineral content and bioavailable Si.

Silicon isotope fractionation with a low‐silica rice mutant reflects plant uptake strategy in response to different silicon supply levels

AbstractPlants may modulate uptake pathways when the nutrient concentration varies. The identified transporters Lsi1 and Lsi2 belong to low‐affinity systems; however, it is unknown whether a high‐affinity system responsible for silicon (Si) transport exists or how plants modulate Si uptake pathways. We aimed at developing isotope fractionation as a novel tool for nutrient uptake and cycling in plants. We grew the wild‐type rice ‘Oochikara’ and its mutant lsi1 under low (Si‐L: 0.17 mM) and high (Si‐H: 1.70 mM) Si supply levels and evaluated Si isotope fractionation between different Si pools. The isotope fractionations between Oochikara and nutrient solution were –0.15 and –0.63‰ at Si‐L and Si‐H, respectively; between lsi1 and nutrient solution they were –0.18 and –1.33‰ at Si‐L and Si‐H, respectively. The qualitative isotope fractionation caused by Lsi1 were –0.09 and –0.49‰ at Si‐L and Si‐H, respectively. We thus verified the hypothesis that Lsi1 preferred 28Si transport over 30Si and that this preference was independent of Si supply levels. Changes in Δ30Siplant‐solution suggested that a high‐affinity system exists and that plants may regulate Si uptake strategy by changing the participation of low‐affinity and high‐affinity pathways. Another root–shoot translocation that favors 28Si has been found in Si‐H‐lsi1 that previously occurred in banana (Musa spp.), which suggested that Si xylem loading is related to the different Si transport abilities of Lsi1 and Lsi2. Our results demonstrated that Si isotope fractionation can shed light on plants’ response to variable nutrient conditions.

Silica fertilization improved wheat performance and increased phosphorus concentrations during drought at the field scale

Drought and the availability of mineable phosphorus minerals used for fertilization are two of the important issues agriculture is facing in the future. High phosphorus availability in soils is necessary to maintain high agricultural yields. Drought is one of the major threats for terrestrial ecosystem performance and crop production in future. Among the measures proposed to cope with the upcoming challenges of intensifying drought stress and to decrease the need for phosphorus fertilizer application is the fertilization with silica (Si). Here we tested the importance of soil Si fertilization on wheat phosphorus concentration as well as wheat performance during drought at the field scale. Our data clearly showed a higher soil moisture for the Si fertilized plots. This higher soil moisture contributes to a better plant performance in terms of higher photosynthetic activity and later senescence as well as faster stomata responses ensuring higher productivity during drought periods. The plant phosphorus concentration was also higher in Si fertilized compared to control plots. Overall, Si fertilization or management of the soil Si pools seem to be a promising tool to maintain crop production under predicted longer and more serve droughts in the future and reduces phosphorus fertilizer requirements.

Spatial patterns of aboveground phytogenic Si stocks in a grass-dominated catchment – results from UAS-based high-resolution remote sensing

Abstract. Various studies have been performed to quantify silicon (Si) stocks in plant biomass and related Si fluxes in terrestrial biogeosystems. Most studies are deliberately designed on the plot scale to ensure low heterogeneity in soils and plant composition, hence similar environmental conditions. Due to the immanent spatial soil variability, the transferability of results to larger areas, such as catchments, is therefore limited. However, the emergence of new technical features and increasing knowledge on details in Si cycling lead to a more complex picture at landscape and catchment scales. Dynamic and static soil properties change along the soil continuum and might influence not only the species composition of natural vegetation but also its biomass distribution and related Si stocks. Maximum likelihood (ML) classification was applied to multispectral imagery captured by an unmanned aerial system (UAS) aiming at the identification of land cover classes (LCCs). Subsequently, the normalized difference vegetation index (NDVI) and ground-based measurements of biomass were used to quantify aboveground Si stocks in two Si-accumulating plants (Calamagrostis epigejos and Phragmites australis) in a heterogeneous catchment and related corresponding spatial patterns of these stocks to soil properties. We found aboveground Si stocks of C. epigejos and P. australis to be surprisingly high (maxima of Si stocks reach values up to 98 g Si m−2), i.e. comparable to or markedly exceeding reported values for the Si storage in aboveground vegetation of various terrestrial ecosystems. We further found spatial patterns of plant aboveground Si stocks to reflect spatial heterogeneities in soil properties. From our results, we concluded that (i) aboveground biomass of plants seems to be the main factor of corresponding phytogenic Si stock quantities, and (ii) a detection of biomass heterogeneities via UAS-based remote sensing represents a promising tool for the quantification of lifelike phytogenic Si pools at landscape scales.

Rainfall is the major driver of plant Si availability in perudic gibbsitic Andosols

The amount of water available to leach solutes from soil is one of the major features determining mineral weathering, mineral neoformation and soil properties. It affects the fate of dissolved silicon (Si), which may follow four routes: leaching, mineral synthesis, adsorption, uptake by plants forming phytoliths. Here, we quantify the reservoirs of bioavailable Si and phytolithic Si in topsoils (0–20 cm) from wet tropical Andosols along a climosequence where mean annual rainfall (MAR) increases from 2650 to 4400 mm with increasing altitude (65–375 m asl) in Basse-Terre, Guadeloupe. We assessed bioavailable Si through CaCl2 extraction in soil, and foliar analysis in banana plants. We evaluated the pool of phytolithic Si through Na2CO3 extraction and heavy liquid separation. The Na2CO3 extraction was performed on both the bulk soil and oxalate–treated soil (ox-Na2CO3) cleared of its amorphous aluminosilicates. The Andosols have reached an advanced weathering stage. Their secondary products included (Al, Fe)–humus complexes, ferrihydrite, gibbsite, poorly crystalline Al hydroxide, and aluminous allophanic substances. The contents of organic C, metal-humus, ferrihydrite and gibbsite increased in soils in the wettest conditions (>3000 mm) whereas allophane content concomitantly decreased. Silicon leaf content varied little (3.8–5.3 g kg−1), but slightly decreased with increasing MAR (r = −0.48). Bioavailable Si content in soil decreased from 63 to 12 mg kg−1 with increasing MAR (r = −0.92), and was strongly correlated (r = +0.95) to that of phytolithic Si as assessed after ox-Na2CO3 extraction, linked to fresh, labile phytoliths. The Si/Al atomic ratio of the ox-Na2CO3 extract regularly decreased from 1.06 to 0.37 with increasing MAR, hence corroborating strongest desilication in soils in the wettest conditions. In these highly leached, gibbsitic Andosols, rainfall is thus the major driver of plant Si availability.

How management practices affect silicon uptake by Hordeum vulgare grown in a highly calcareous soil

AbstractThere are many studies on the beneficial effects of silicon (Si) in plants related to a decrease of stress (biotic/abiotic). The objective of the present work was to study how plant‐available Si in highly calcareous soil with different soil chemical properties, amendments and growing conditions can affect Si absorption by Hordeum vulgare L. (Barley) and its biomass. Barley was cultivated under field conditions in dryland agriculture. Three treatments were compared: phytolithic biochar, compost pellets and no amendment. Greenhouse trials with the same soil and treatments were also carried out to achieve a better understanding of the dynamics in the soil–plant system under controlled growing conditions. In both experiments, physical and chemical soil and plant parameters were determined from collected samples. Results showed that the Si uptake and biomass in soils with the highest available Si (33 ± 1 mg kg−1) was 4.7‐fold and 2.4‐fold higher, respectively, than in soils with the lowest available Si (22 ± 1 mg kg−1). Also, with greater Si uptake there was an increase in N uptake (τ = 0.68, p &lt; 0.01). Therefore, Si could improve N use efficiency within the plant. A strategy to improve Si values would be to use organic amendments that are enriched with Si and supply N in a form other than ammonium (). A high pH in this type of soil produces dissolution of the biogenic Si pool and low electrical conductivity values improve plant‐available Si values. It is important to increase plant‐available Si in highly calcareous soils as this has important implications for improving barley production.

How Does Sphagnum Growing Affect Testate Amoeba Communities and Corresponding Protozoic Si Pools? Results from Field Analyses in SW China.

The policy and practice of ecological restoration and conservation in China obtained some remarkable results. For example, Sphagnum moss growing on abandoned farmland, which was peatland before agricultural use, has rapidly expanded the wetland area in SW China. Microorganisms such as testate amoebae are sensitive to environmental change and thus have been widely used as ecological indicators in various habitats. We analyzed differently aged Sphagnum growing plots on a Sphagnum growing farmland and natural Sphagnum plots in SW China to examine how Sphagnum-dwelling testate amoeba communities and corresponding protozoic silicon (Si) pools respond to ecological restoration practice. We found that abundance, taxon richness, and diversity of testate amoebae were higher in Sphagnum growing farmland plots compared to natural Sphagnum plots. Protozoic Si pools showed an increase with Sphagnum growing time representing increased Si accumulation by idiosomic testate amoeba shells. However, protozoic Si pools were negatively correlated with taxon richness and diversity of testate amoebae. Our results showed that (i) natural Sphagnum plots were not characterized by the expected higher biodiversity of testate amoebae compared to Sphagnum growing plots and (ii) consequently protozoic Si pool quantity in natural Sphagnum plots was less driven by biodiversity of testate amoebae than expected. We concluded our results to underline the value of (i) environmental restoration policy in general and (ii) testate amoeba communities and corresponding protozoic Si pools for Si cycling in restoration areas of peatlands in particular. Based on our results, we recommend a sustainable cultivation of Sphagnum moss and an additional establishment of protected areas, where no Sphagnum harvesting occurs. These protected Sphagnum areas might represent hot spots of undisturbed testate amoeba communities and corresponding protozoic Si pools and thus of microbial Si cycling.

Coastal silicon cycling amplified by oyster aquaculture

Filter-feeders play an important role in regulating nutrient availability in coastal systems, with important implications for phytoplankton community composition, primary production, and food web structure. The role of filter-feeding bivalves in the nitrogen and phosphorus cycles is relatively well established, but their impact on coastal silicon (Si) cycling remains poorly understood. To help reduce this uncertainty, we quantified rates of Si recycling and the size of various Si pools at an oyster (Crassostrea virginica) farm. We found that oysters drive rapid recycling of dissolved Si (DSi) to the water column, primarily by altering rates of sediment Si flux. Sediments beneath oyster aquaculture recycled DSi to the water column at more than twice the rate (2476.06 µmol DSi m-2h-1) of nearby bare sediments (998.75 µmol DSi m-2h-1). Oysters consume DSi at a low rate (-0.06 µmol DSi ind.-1h-1), and, while we were unable to determine the fate of that Si, we hypothesize that at least some of it may be stored in the shell and tissue, which are both small Si pools (0.55 and 0.13% Si by mass respectively). Si held in oysters is removed from the system when oysters are harvested, but this removal is small compared to oyster-mediated enhancements in sediment Si recycling. In a broader context, coastal systems with larger oyster populations are likely to have a more rapid Si cycle, with more Si available to primary producers in the water column than those with no oysters.

Using Stable Isotopes to Disentangle Marine Sedimentary Signals in Reactive Silicon Pools

AbstractMany studies use sedimentary biogenic silica (bSiO2) stable isotopes (e.g., δ30Si) as paleoproxies but neglect signals from other sedimentary reactive SiO2 phases. We quantified δ30Si for multiple reactive Si pools in coastal river‐plume sediments, revealing up to −5‰ difference between acid‐leachable and alkaline‐digestible amorphous SiO2. Thus, previous studies have missed valuable information on early diagenetic products and, in cases where sediments were not cleaned, potentially biased bSiO2 δ30Si values. Acid‐leachable δ30Si, that is, from authigenic products, are the result of either multistep fractionation from a bSiO2 source or an ~2‰ fractionation (consistent with metal hydroxide formation) from slowly dissolving lithogenic SiO2. This analysis also suggests that sedimentary diatom bSiO2, which has increased regionally in the last half‐century, is the critical substrate of early authigenic Si precipitates. Regional eutrophication, which has stimulated sedimentary bSiO2 accumulation, may have facilitated additional sequestration of both sedimentary Si and cations from early diagenetic products.

Land-use change effects on protozoic silicon pools in the Dajiuhu National Wetland Park, China

Many peatlands have been damaged by human activities like Sphagnum harvesting and draining reflected in reduced peatland areas and ecosystem functioning. Peatlands generally play an important role in global silicon (Si) cycling and maintain huge numbers of testate amoebae (TA). However, there is a lack of knowledge on (i) the quantity of protozoic Si pools (represented by siliceous shells of TA) in peatlands and (ii) how these pools respond to land use changes caused by human influence. In our study, we quantified protozoic Si pools in the Dajiuhu National Wetland Park, China, with a focus on the comparison between peatland and cropland areas (i.e., former peatland drained for agriculture). We found protozoic Si pools to (i) represent an important biogenic silica fraction in peatlands and (ii) significantly differ between peatlands and croplands with soil moisture as a key factor for protozoic Si pool quantity showing a significant decrease of these pools in drained areas (i.e., in croplands). From our results we concluded the decrease of protozoic Si pools due to land-use change to represent another example (apart from the loss of phytogenic Si) for anthropogenic desilication in agricultural biogeosystems in general.