Phytolith Dissolution Research Articles

Impact of earthworms on soil Si availability and wheat Si concentration in low- and high-Si soils

Silicon (Si) is a beneficial element known to increase growth and the stress resistance of plants, including resistance against drought. However, as with many other elements, only a small fraction of Si in soils is available for plant uptake. Plant-available Si originates either from the weathering of soil minerals or from the dissolution of phytoliths in plant litter. It is known that soil fauna plays a role in both processes that contribute to the availability of Si in the soil. However, very little is known about the interactions between the weathering of soil minerals and the dissolution of Si by earthworms from phytoliths and subsequent Si uptake by plants. In greenhouse pot experiments, this study investigated the effect of an epigeic earthworm species (Dendrobaena veneta) on Si availability and uptake by wheat (Triticum aestivum) grown in soils with high or low Si availability. In addition, Si-rich plant litter (wheat straw) was added to the topsoil in some treatments. The addition of plant litter significantly increased the available Si in the soil. Plant litter addition was positively correlated with the Si concentration in wheat in low-Si soil, but not in high-Si soil. Earthworms increased Si availability in soil and casts of the low-Si soil and partly of the high-Si soil, but not Si uptake by wheat. These results suggest that earthworms play an important role in regulating the Si status of soils and may influence Si uptake by plants, especially in soils with initially low Si content and when earthworm population densities are high.

Silicon in paddy fields: Benefits for rice production and the potential of rice phytoliths for biogeochemical carbon sequestration

Silicon (Si) biogeochemical cycling is beneficial for crop productivity and carbon (C) sequestration in agricultural ecosystem, thus offering a nonnegligible role in alleviating global warming and food crisis. Compared with other crops, rice plants have a greater quantity of phytolith production, because they are able to take up a lot of Si. However, it remains unclear on Si supply capacity of paddy soils across the world, general rice yield-increasing effect after Si fertilizer addition, and factors affecting phytolith production and potential of phytolith C sequestration in paddy fields. This study used a meta-analysis of >3500 data from 87 studies to investigate Si supply capacity of global paddy soils and elaborate the benefits of Si regarding rice productivity and phytolith C sequestration in paddy fields. Analytical results showed that the Si supply capacity of paddy soils was insufficient in the major rice producing countries/regions. Dealing with this predicament, Si fertilization was an effective strategy to supply plant-available Si to improve rice productivity. Our meta-analysis results further revealed that Si fertilization led to the average increasing rate of 36 % and 39 % in rice yield and biomass, which could reach up to 52 % and 46 % with the increasing doses of Si fertilizer, respectively. Especially, this strategy also improved the potential of phytolith C sequestration through the increased phytolith content and rice biomass, despite that this potential might have a decline in old paddy soils (≥ 7000 year) compared to in young paddy soils (≤ 1000 year) due to the slow migration and dissolution of phytoliths at millennial scale. Our findings thus indicate that a deep investigation on the benefits of Si in agroecosystem will further improve our understanding on regulating crop production and the potential of biogeochemical C sequestration within phytoliths in global cropland.

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.

Estimating oil-palm Si storage, Si return to soils, and Si losses through harvest in smallholder oil-palm plantations of Sumatra, Indonesia

Abstract. Most plant-available Si in strongly desilicated soils is provided through litter decomposition and subsequent phytolith dissolution. The importance of silicon (Si) cycling in tropical soil–plant systems raised the question of whether oil-palm (Elaeis guineensis Jacq.) cultivation alters Si cycling. As oil palms are considered Si hyper-accumulators, we hypothesized that much Si is stored in the above-ground biomass of oil palms with time. Furthermore, the system might lose considerable amounts of Si every year through fruit-bunch harvest. To test these hypotheses, we analysed Si concentrations in fruit-bunch stalks, fruit pulp and kernels, leaflets, rachises, and frond bases of mature oil palms on eight smallholder oil-palm plantations in Sumatra, Indonesia. We estimated Si storage in the total above-ground biomass of oil palms, Si return to soils through decomposing pruned palm fronds, and Si losses from the system through harvest. Leaflets of oil-palm fronds had a mean Si concentration of > 1 wt %. All other analysed plant parts had < 0.5 wt % Si. According to our estimates, a single palm tree stored about 4–5 kg Si in its total above-ground biomass. A smallholder oil-palm plantation stored at least 550 kg Si ha−1 in the palm trees' above-ground biomass. Pruned palm fronds returned 111–131 kg of Si ha−1 to topsoils each year. Fruit-bunch harvest corresponded to an annual Si export of 32–72 kg Si ha−1 in 2015 and 2018. Greater Si losses (of at least 550 kg Si ha−1) would occur from the system if oil-palm stems were removed from plantations prior to replanting. Therefore, it is advisable to leave oil-palm stems on the plantations, e.g. by distributing chipped stem parts across the plantation at the end of a plantation cycle (∼ 25 years).

Goethite affects phytolith dissolution through clay particle aggregation and pH regulation

Formed in plant tissues as fine silica particles, phytoliths are deposited within plant debris in soils where they can dissolve and feed silicon (Si) fluxes to the biosphere and hydrosphere. Yet, soil phytoliths can be protected from dissolution by entrapment in aggregates, and thus be withdrawn from the global silica cycle. Among clay-sized minerals interacting in stable aggregates, goethite and kaolinite are ubiquitous in soils. We analyzed the impact of goethite-mediated aggregation on the release of aqueous Si from phytoliths entrapped in microaggregates containing organic matter and kaolinite, with quartz and goethite at variable concentrations, simulating a soil sequence with increasing contents of clay and iron oxide, as it can commonly occur in terrestrial ecosystems. The microaggregates stored sizable amount of organic carbon, the release of which decreased with increasing goethite concentration. Aqueous Si was assessed through a kinetic extraction with dilute CaCl2, while pH, aluminum (Al) and germanium (Ge) concentrations were measured in the CaCl2 extracts. Our experimental data showed that phytoliths were the source of aqueous Si. They also showed that the process of phytolith dissolution prevailed over Si adsorption on goethite. As mediated by goethite, aggregation protected phytoliths from dissolution at goethite concentrations above 20 g kg−1. Increasing goethite concentration enhanced aggregation on the one hand, but increased pH from 5.5 to 7.5 on the other. Thus, while aggregation significantly reduced the release of aqueous Si by 1.7- to 3-fold at a given pH, the increase in pH enhanced it. Overall, at common soil solution pH (5–6), aggregation reduced Si release by 2–3 times. Thus, preservation of phytoliths in aggregates can be widespread in well-aerated soils and more effective than Si adsorption on secondary oxides in retaining Si.

Dissolution does not affect grass phytolith assemblages

Dissolution is one among several taphonomical processes that may bias paleoenvironmental, paleoclimatic or taxonomic interpretation of phytolith assemblages. To improve our understanding of dissolution on grass phytoliths, we studied systematic changes of surface features, morphotype assemblages, and dissolution rates of phytoliths extracted from two grass species Hyparrhenia involucrata (Panicoideae), Nastus borbonicus (Bambusoideae), one soil from La Réunion Island (approximate mean age < 800 yr), and three paleosols from Ethiopia (approximate age of 4.4 million years). We used heavy-liquid to extract phytoliths, and 1% Na2CO3 to perform partial dissolution experiments. Physicochemical surface properties, morphotypes, and assemblages were analyzed using optical and scanning electron microscopy, laser diffraction, and X-ray diffractometry. Our results show that 1) phytoliths from different grass species may have different dissolution rates: phytoliths from the leaves of Hyparrhenia involucrata (Panicoideae) are more prone to dissolution than those from Nastus borbonicus (Bambusoideae). 2) Silicon (Si) released by phytolith assemblages (i.e., phytolith dissolution rate) decreases as follows: plant > soil > paleosol. 3) Dissolution leads to cavity formation on phytolith surfaces and disappearance of fragile silica particles. 4) Partial dissolution does not significantly change percentages of common grass phytolith morphotypes in a given assemblage. These results provide a benchmark for assessing the reliability of paleoenvironmental reconstructions using grass phytolith assemblages from buried soils and sediments.

Oil-palm and Rainforest Phytoliths Dissolve at Different Rates - with Implications for Silicon Cycling After Transformation of Rainforest Into Oil-palm Plantation

Phytoliths make up the predominant fraction of biogenic silica in plant litter and soils. Thus, they represent a major source of dissolved silicon (Si) in soil-plant systems. Dissolution of phytoliths from Si-accumulating crops such as rice has been well studied in recent years. However, phytolith dissolution in oil-palm plantations remains largely understudied. In this study, we compared dissolution rates of phytoliths isolated from oil-palm fronds, oil-palm litter, and rainforest litter. Our results showed that phytoliths from oil-palm fronds represent an important reservoir of easily dissolvable Si with high dissolution rates (0.44 - 0.69 mg g^{-1} d^{-1}). Compared to fresh phytoliths from oil-palm fronds, phytoliths isolated from litter showed up to 18 times lower dissolution rates, reflecting silica aging over time. The dissolution rate of phytoliths isolated from rainforest litter (0.067 mg g^{-1} d^{-1}) was significantly higher than that of phytoliths from oil-palm litter (0.038 mg g^{-1} d^{-1}). These results demonstrate that transformation of rainforest into oil-palm plantation involves a major change in phytolith production and Si release from litter, considerably altering Si cycling in the soil-plant system. We identified cut-off palm fronds that are usually piled up between the palm rows as most important Si sources maintaining biogeochemical Si cycling in oil-palm plantations.

Effects of microsized rice straw on soil clay dispersibility

AbstractThe incorporation of straw into soil has been encouraged as an alternative straw‐disposal strategy to the on‐site burning that occurs in many paddy regions. However, this practice may lead to changes in the colloidal properties of soil clays that potentially intensify clay and nutrient losses. This study aimed to demonstrate the effect of engineered microsized straw powder on the colloidal properties of two soil clays, that is, illitic and kaolinitic soil clays. Dynamic light scattering was coupled with the test tube method to evaluate time‐resolved changes in the particle size, zeta potential and dispersibility for the suspensions of straw powder and its mixtures with illitic and kaolinitic soil clays. Data from kinetic experiments over a time span of 20 days revealed that straw powder remarkably increased the dispersibility of soil clays. It was found that straw particles carried negative charges; thus, the introduction of straw powder into the clay suspensions increased the number of negative charges in the system, thereby increasing internal repulsive forces and eventually favouring clay dispersion. Moreover, certain mutual processes, that is, the biodegradation and dissolution of phytoliths (silica in straw), resulted in the release of dissolved organic matter and silicon, which aggravated clay dispersibility. In addition to the “traditional” awareness of the impacts of straw incorporation, for example, creating toxic, reduced environments or increasing CH4 emissions, the changes in the colloidal properties of soil clays should also be highlighted. We propose that the incorporation of straw requires additional solutions for the prevention of clay loss.Highlights Straw powder was examined for possible effect on colloidal properties of soil clays It was found that straw particles carried negative charges Introduction of straw powder increase repulsive forces and favor clay dispersion Microorganism, B. amyloliquefaciens, enhances the effect of straw powder

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.

Phytolith morphology and assemblage variations in a bamboo forest plant–soil system

The investigation of phytolith variations in plant–soil systems provides a good opportunity to quantitatively study taphonomic effects on phytoliths in soil, and evaluate their representation of the aboveground plant community. In this study, the assemblage and size variations of phytolith morphotypes in a bamboo (Dendrocalamus ronganensis) forest plant–soil system were analyzed to study how well soil phytoliths represent the overlying plant community and the taphonomic processes acting on phytoliths. Phytolith content and morphotype assemblages differed between organs of D. ronganensis. There were no apparent differences in phytolith morphotypes of components of the plant–soil system (fallen leaf, mold and soil). The phytolith assemblage of raw leaves showed a high positive correlation with soil-system samples. Compared to (June and January) raw leaves, fallen leaves exhibited decreased percentages of ACUTE BULBOSUS, STOMA (Representativeness coefficient less than 1), and increased BULLIFORM FLABELLATE, RONDEL and ELONGATE (R-values higher than 1). Soil phytolith morphologies were different due to phytolith dissolution in the soil. Paleoenvironmental reconstruction based on soil phytolith morphologies and assemblages should consider phytolith preservation and taphonomy in soils as well as sediments.

Arsenic in rice straw phytoliths: Encapsulation and release properties

Rice plants assimilate both silicon (Si) and arsenic (As) from the soil solution and accumulate these elements in their tissues. While Si has been known as a major material that builds up a so-called phytolith structure throughout the plant body, the fate of As in rice plants is not fully understood. This study aims to clarify the possible encapsulation of As into the phytolith structure in rice straw or rice straw ash and to evaluate the dissolution properties of the encapsulated As. Rice straw contains up to ~12 mg As kg−1, which highlights this material as a sink and source of As that needs to be taken into account. The intensified heat treatments of rice straw could lead to an enrichment of As in the burned products, from which As is likely associated more tightly with the phytolith structure. We observed strong correlations of released As and Si, demonstrating that the release of As is tightly controlled by the dissolution of phytoliths. This means that phytoliths can contribute to the soil As pool, but this As source might be obscured because it is recalcitrantly lodged within the phytolith structure. This finding serves as a basis for further assessing the role of phytolith-encapsulated As rice paddy systems and for developing proactive practices to manage As-tainted straw.

Effects of CO2 and temperature on phytolith dissolution

Phytoliths, silica structures derived from plant residues in silicon (Si)-accumulating plant species, have recently been recognized as a sink and source of nutrients and a hosting phase for carbon sequestration in soil. While the solubility of phytoliths in relation to their respective nature and solution chemistry has been intensively studied, the combined effects of CO2 and temperature, two highly variable parameters in soil, have not been fully understood. We hypothesized that changes in CO2 and temperature may affect the dissolution rate, thereby resizing the soil phytolith pool. Rice straw phytoliths were obtained from either open burning or controlled heating of straw from 300 to 900 °C and used to determine their batch incubation kinetics in a closed chamber at CO2 concentrations of 0 to 15% vol. and a temperature range of 20 to 50 °C for six days. The results revealed a contrasting effect in which temperature and CO2 were correspondingly found to accelerate or decelerate the dissolution rate of phytoliths. Under the most dissimilar conditions, i.e., 0% vol. CO2 and 50 °C and 15% vol. CO2 and 20 °C, the discrepancy in solubility was approximately six-fold, indicating a high vulnerability of phytoliths to CO2 and temperature changes. This finding also suggests that the soil phytolith pool can be diminished in the case of either increasing soil temperature or decreasing CO2 flux. Calculations based on these data revealed that the dissolution rate of phytoliths could be increased by an average of 4.5 to 7.3% for each 1 °C increase in temperature. This finding suggests a possible impact of current global warming on the global biogenic silica pool, and more insight into the relationship between this pool and climate change is, therefore, necessary to maintain the function of the phytolith phase in soil.

Neoformed aluminosilicate and phytogenic silica are competitive sinks in the silicon soil–plant cycle

In soils, mineral weathering and phytolith dissolution release aqueous monosilicic acid that can be taken up by plant, adsorbed on mineral surfaces, entrapped in neoformed clay minerals, or exported to watersheds. The balance between biotic and abiotic processes determines the fluxes of bioavailable silicon (Si), impacting plant growth and health, and diatoms biomass, hence the oceanic capacity to fix carbon dioxide. Here we quantified this balance in an experimental system using rice and an albic soil material (quartz grains) containing no weatherable silicate minerals. Materials representing a soil weathering gradient were prepared by adding variable amounts of silicate minerals within fresh powdered tephrite (<83 µm) as a source of weatherable minerals whereas phytogenic Si was supplied within phytolith-rich biochar. We quantified the distribution of Si in leachate, soil and plant reservoirs. Weatherable minerals rapidly released Si that flowed into two main sinks: allophanic substances neoformed in soil from released Si and aluminum (Al), and phytoliths formed by precipitation of absorbed silica in plant tissues. The leaching of Si was negligible. Allophane rapidly formed at a maximum rate of 0.85 g allophane kg−1 soil day−1. Of the Si stock of weatherable minerals, 7–14% contributed to allophanic Si and less than 1% to phytogenic Si. The contribution of supplied phytoliths to plant Si accumulation markedly increased from 26 to 80% with the decrease in weatherable mineral reserve from 405 to 5 cmolc kg−1. Weatherable minerals primarily controlled bioavailable Si while phytoliths increasingly contributed to Si plant uptake with increasing mineral depletion. Returning phytoliths to soil thus boosts the biological recycling of Si in proportion to the increase in soil weathering stage.

PH-dependent silicon release from phytoliths of Norway spruce (Picea abies)

Accurate evaluation of the preservation state of fossil phytoliths in glacial lake sediments is important, as these microfossils are often used in paleoecological and archaeological studies. The characteristic phytolith type of the Norway spruce (Picea abies [L.] Karst.) needle is a potential keystone in paleoecological studies. In this laboratory study, we investigated dissolution of Picea abies blocky type phytoliths, to simulate dissolution processes in sediments and soils and create reference material to compare with fossil phytoliths. Intact needles, needle ash, diatomite and silica gel were treated with Britton–Robinson buffer solutions at pH values from 2 to 12 for 22 days. Silicon was measured by microwave plasma atomic emission spectrometry. Treatment effects were evaluated on longitudinal cuts of needles under a stereomicroscope and on phytolith assemblages from needles using a light microscope. Surfaces of treated phytoliths were investigated by scanning electron microscope and elemental analysis of phytoliths was determined by energy dispersive X-ray fluorescence. Dissolution of silicon in spruce needles was inhibited between pH 8.0 and 11.1. Needle tissue protects phytoliths from erosion processes at this alkaline pH range. Most dissolved silicon appeared to originate from the phytolith surfaces and the silica matrix of the apoplast in the tissues, with less from complete dissolution of phytoliths. Our experiment suggests that extraneous metal elements are incorporated into the silica structure during the dissolution process. Thus, higher element content is an effect of partial dissolution rather than a cause of dissolution. Ultrastructure of the surface of Picea-blocky type phytoliths, namely disappearance of the globular structure, may be useful to assess the intensity of destructive processes in sediments. Our experimental treatments indicate that characteristic Picea-blocky phytoliths in needles can be well-preserved, depending on circumstances in sediments. Further micro-analytical measurements will make these needles promising tools for paleoenvironmental reconstructions.

Copper encapsulated in grass-derived phytoliths: Characterization, dissolution properties and the relation of content to soil properties

The formation of phytoliths as a result of the precipitation of Si in many Si-rich plant species is known to encapsulate organic matter. This work aims to examine the possible encapsulation of Cu in grass phytoliths in an orange growing area, where Cu-rich fungicides have been excessively applied. Batch experiments, in combination with SEM-EDS and microscopy, were conducted for the grass-derived phytoliths and phytoliths separated from soil, thus revealing their dissolution properties, morphotypes and contents, in relation to soil properties. By measuring the Cu release accompanying the dissolution of phytoliths by different extractants, especially an Na2CO3/HNO3 solution, it was revealed that Cu was encapsulated within the silica body of the phytolith. This sink of Cu in the grass can be cycled to serve as a new Cu source in soils. Phytolith contents in the soil were up to 17.7 g kg−1 and tended to accumulate in soil depths from 0 to 20 cm. A positive correlation was found for soil phytolith and phytCu contents and may be indicative of the role of phytoliths as an enhancer of Cu accumulation in soil. It would be worth developing suitable techniques for the determination of phytCu, because common extraction/digestion methods are not suited for evaluating this Cu pool.

The Relative Importance of Cell Wall and Lumen Phytoliths in Carbon Sequestration in Soil: A Hypothesis

There has been much interest in the possibility that phytoliths might sequester substantial amounts of carbon and might continue to do so in soils and sediments after the death of the plant. This may contribute to mitigating climate change. However, this idea is controversial and it is unclear how much carbon is sequestered in phytoliths. High values would suggest that sequestration on a global scale could be significant, but low values would indicate insignificant sequestration. Different methodologies in preparing phytoliths give different carbon concentrations. Little interest has been shown in determining which types of phytoliths are most important for carbon sequestration. There are two main types of phytolith in plants, the cell wall types which are formed on a carbohydrate matrix, and the cell lumen types which are not. A literature survey of transmission and scanning electron microscopy studies to determine which phytoliths are cell wall phytoliths was carried out. Cell wall silicification was common in most plant organs and throughout the plant kingdom. Macrohairs, prickle hairs, and the wall protrusion of papillae are certainly cell wall types. The primary cell walls of many epidermal cells types are often silicified. Cell wall phytoliths have considerably higher carbon concentrations than lumen types. An attempt is made to model mixtures of cell wall and lumen phytoliths, containing different carbon concentrations. Literature data on carbon and nitrogen concentrations in phytoliths was used to produce C/N ratios. These showed that cell wall phytoliths had higher C/N ratios than lumen phytoliths, and that over-extraction of phytolith mixtures removes carbon preferentially from the cell wall types and leads to low C/N ratios. The dissolution of phytoliths in soils and sediments is considered, and it is unknown whether cell wall or lumen phytoliths break down faster. However, it is clear from the literature that cell wall phytoliths persist in soils and sediments for hundreds or thousands of years. The paper is brought to a climax with two hypotheses, one to explain what happens to carbon in phytoliths as they undergo preparatory procedures in the laboratory, and the other looking at dissolution and breakdown in the soil.

Characterization and implication of phytolith-associated potassium in rice straw and paddy soils

ABSTRACT Rice straw contains up to 2.3% K in dry matter, including potassium (K) subcompartmented in phytoliths, complex siliceous structures formed in plant tissue via precipitation of Si. Rice straw is usually returned to the soil as a conventional practice to sustain soil nutrients, and therefore, the K pool accompanied with rice straw phytoliths is also cycled. Based on phytoliths obtained by ashing of rice straw at 400 °C and dissolution experiments using batch extraction in combination with physical separation of phytoliths by heavy liquid, this study evaluated the phytolith K(phytK) pool in rice straw and aged phytoliths in paddy soils. Entrapped organic matter containing K within phytolith silica cells was visualized by X-ray tomographic microscopy, and releases of this phytK pool accompanying phytolith dissolution were quantified. A 1% Na2CO3 solution, which has been commonly used to extract amorphous Si and to quantify soil phytoliths, showed obvious responses for K derived from phytolith dissolution, indicating that the Na2CO3 method can be developed for measurement of phytK. In 13 soil samples, Na2CO3-dissolvable K content assignable to phytK was 0.55 ± 0.39 g kg−1 in the puddled horizon, suggesting the phytK pool is of high significance for the management of K in paddy soils.

Fern, Dicranopteris linearis, derived phytoliths in soil: Morphotypes, solubility and content in relation to soil properties

Ferns are among the most popular groups of plants in the tropics and subtropics, and their role as carbon sequestrators has been widely recognized. However, there is little understanding of the silicaceous structures (phytoliths) of ferns, rate of phytolith turnover, the consequences for organic matter sequestered in phytoliths and consequences for other soil properties. In the study reported here, high‐resolution X‐ray tomographic microscopy and chemical characterization were applied to examine the traits of phytoliths of the fern Dicranopteris linearis (Burm.f.) Underw. (D. linearis), with a focus on their dissolution properties and accumulation in northern Vietnamese soils in relation to soil properties. Tomographic images revealed an inter‐embedding structure of silica and organic matter, especially in leaf‐derived material. We propose that organic matter and silica can preserve each other against decomposition. In batch experiments, there was a relatively small rate of dissolution of phytoliths with dry ashing and subsequent H2O2 treatment. Silicon (Si) dissolution for D. linearis phytolith samples was much less than that for rice phytoliths. Despite the fact that the aluminum (Al) content was large in D. linearis leaves, batch dissolution data did not confirm a relation between Al and the slow rate of phytolith dissolution. The soil phytolith content varied from 0.9 to 7.5 g kg−1 in the topsoil across the mountainous areas in northern Vietnam, whereas it tended to be smaller in the subsoil. The data indicate a relation between phytolith and soil organic matter, clay content, oxalate‐soluble Al and electrical conductivity, suggesting that these soil properties are among the important factors affecting the size of the soil phytolith Si pool.Highlights Si tends to accumulate in leaves rather than stems of the fern D. linearis. D. linearis phytoliths showed little dissolution, suggesting their strong stability. OC, EC, clay content and Alox appear as factors ‘feeding’ D. linearis phytoliths. In contrast, Feox and pH did not correlate with D. linearis phytolith content.

Phytolith content in Vietnamese paddy soils in relation to soil properties

Understanding of the fate of phytolith in soils is important because of its role as an agronomical nutrient resource and for carbon sequestration. Accumulation of phytolith derived silica occurs in paddy soils when phytolith input, here through straw incorporation, is higher than silica removal through crop offtake, dissolution and leaching. Phytolith dissolution is thought to be the main reason for diminishing of phytolith derived silica in soil, with this dissolution being highly dependent on soil chemical properties. In this study, phytolith content from 78 paddy soils sampled in the Red River (RRD) and Mekong River (MRD) Deltas, Vietnam, were quantified and related to pH, electrical conductivity (EC), oxalate-extractable Al and Fe (Alox and Feox), organic carbon (OC) and clay content. Average phytolith contents within the topsoil (0–20 cm) were 4.8 ± 1.98 and 7.74 ± 2.97 mg g−1 for the RRD and MRD, respectively. Positive correlation coefficients obtained for Alox, OC, clay content and EC, implying that these parameters might enhance phytolith resistance to dissolution. Soil pH had a negative correlation coefficient with phytolith content. Lower pH and higher Alox content explained the predominance of the phytolith in the MRD compared to the RRD. Soil pH adjustment can be proposed as an active management strategy to maintain phytolith added Si, source for crops and for carbon sequestration.

Processes controlling silicon isotopic fractionation in a forested tropical watershed: Mule Hole Critical Zone Observatory (Southern India)

Assessing the dynamics of the silica cycle in the critical zone remains challenging, particularly within the soil, where multiple processes are involved. To improve our understanding of this cycle in the Tropics, and more specifically the role played by vegetation, we combined elemental Si mass balance with the δ30Si signatures of the compartments involved in the water-plant-rock interactions of a tropical forested watershed, Mule Hole (Southern India). To accomplish this, we analysed (1) the δ30Si values of present-day litter phytoliths from tree leaves and grass, as well as soil amorphous silica (ASi); (2) the Si isotope fractionation induced by phytolith dissolution; (3) the silicon mass balance inferred from isotopes at the soil–plant scale; and (4) the consistency between water sources and the δ30Si signatures in the ephemeral stream.The δ30Si values of present-day litter phytoliths and soil ASi vary within a narrow range of 1.10–1.40‰ for all samples, but two deep vertisol samples which likely trapped phytoliths from different vegetation growing under more humid conditions, as indicated by pollen analysis. A homogeneous signature of litter is a minimum condition for using δ30Si as a proxy for the litter/phytolith source of Si. However, litter-ash dissolution experiments demonstrate that the incipient dissolution of phytoliths fractionates Si isotopes, with the preferential dissolution of 28Si over 30Si yielding δ30Si values as low as −1.41‰. Values close to the whole-sample signatures, i.e., above 1‰, were recovered in the solution after a few hours of water-ash interaction. At the soil–plant scale, the average δ30Si value of soil-infiltrating solutions is slightly lighter than the average phytolith signature, which suggests phytoliths as the source of soil dissolved Si. The isotopic budget of dissolved Si within the soil layer, which was obtained based on previous elemental fluxes, is imbalanced. Equilibrating the isotopic budget would imply that up to 4100 mol ha−1 yr−1 of silica is taken up by vegetation, which is almost twice as large as that initially estimated from the elemental budget. The additional Si flux taken up, and likely stored in woody stems, was estimated assuming that Si isotopes followed a steady-state model for the whole Si plant uptake and then followed a Rayleigh model once in the plants. The δ30Si value of the additional Si flux taken up should be close to 0‰, i.e., enriched in light Si isotopes compared to the litter. If steady-state conditions apply, the source could correspond to soil ASi dissolution or deep (saprolite) root uptake. At the outlet of the watershed, the stream exhibits low δ30Si values (0.28–0.71‰) during peak flows and high δ30Si values (1.29–1.61‰) during the recessions at the end of the rainy season. Heavy δ30Si signatures are consistent with the expected domination of seepage at the end of floods. The light δ30Si values during peak flow are slightly lower than the overland flow signature and reflect either a sampling bias of overland flow or a minor but significant contribution of another Si source within the stream, possibly the partial dissolution of phytoliths from the suspended load, with slight isotopic fractionation. This study confirms that vegetation controls the silicon cycle in this dry tropical forest. It also shows that silicon isotopes yield a better grasp of the mass balance and sources and potential mechanisms involved than the consideration of only silicon concentrations. However, this proxy still relies on working hypotheses, notably steady-state and/or Rayleigh fractionation models, which need to be confirmed in further studies.