Soil processes drive the biological silicon feedback loop

Remote Sensing of Depth-Induced Variations in Soil Organic Carbon Stocks Distribution Within Different Vegetated Landscapes

The preservation and augmentation of soil organic carbon (SOC) stocks is critical to designing climate change mitigation strategies and alleviating global warming. However, due to the susceptibility of SOC stocks to environmental and topo-climatic variability and changes, it is essential to obtain a comprehensive understanding of the state of current SOC stocks both spatially and vertically. Consequently, to effectively assess SOC storage and sequestration capacity, precise evaluations at multiple soil depths are required. Hence, this study implemented an advanced Deep Neural Network (DNN) model incorporating Sentinel-1 Synthetic Aperture Radar (SAR) data, topo-climatic features, and soil physical properties to predict SOC stocks at multiple depths (0-30cm, 30-60cm, 60-100cm, and 100-200cm) across diverse land-use categories in the KwaZulu-Natal province, South Africa. There was a general decline in the accuracy of the DNN model's prediction with increasing soil depth, with the root mean square error (RMSE) ranging from 8.34 t/h to 11.97 t/h for the four depths. These findings imply that the link between environmental covariates and SOC stocks weakens with soil depth. Additionally, distinct factors driving SOC stocks were discovered in both topsoil and deep-soil, with vegetation having the strongest effect in topsoil, and topo-climate factors and soil physical properties becoming more important as depth increases. This underscores the importance of incorporating depth-related soil properties in SOC modelling. Grasslands had the largest SOC stocks, while commercial forests have the highest SOC sequestration rates per unit area. This study offers valuable insights to policymakers and provides a basis for devising regional management strategies that can be used to effectively mitigate climate change.

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.

Mapping Sub-surface Distribution of Soil Organic Carbon Stocks in South Africa's Arid and Semi-Arid Landscapes: Implications for Land Management and Climate Change Mitigation

Soil organic carbon (SOC) stocks are critical for land management strategies and climate change mitigation. However, understanding SOC distribution in South Africa's arid and semi-arid regions remains a challenge due to data limitations, and the complex spatial and sub-surface variability in SOC stocks driven by desertification and land degradation. Thus, to support soil and land-use management practices as well as advance climate change mitigation efforts, there is an urgent need to provide more precise SOC stock estimates within South Africa's arid and semi-arid regions. Hence, this study adopted remote-sensing approaches to determine the spatial sub-surface distribution of SOC stocks and the influence of environmental co-variates at four soil depths (i.e., 0-30 cm, 30-60 cm, 60-100 cm, and 100-200 cm). Using two regression-based algorithms, i.e., Extreme Gradient Boosting (XGBoost) and Random Forest (RF), the study found the former (RMSE values ranging from 7.12 t/ha to 29.55 t/ha) to be a superior predictor of SOC in comparison to the latter (RMSE values ranging from 7.36 t/ha to 31.10 t/ha). Nonetheless, both models achieved satisfactory accuracy (R2 ≥ 0.52) for regional-scale SOC predictions at the studied soil depths. Thereafter, using a variable importance analysis, the study demonstrated the influence of climatic variables like rainfall and temperature on SOC stocks at different depths. Furthermore, the study revealed significant spatial variability in SOC stocks, and an increase in SOC stocks with soil depth. Overall, these findings enhance the understanding of SOC dynamics in South Africa's arid and semi-arid landscapes and emphasizes the importance of considering site specific topo-climatic characteristics for sustainable land management and climate change mitigation. Furthermore, the study offers valuable insights into sub-surface SOC distribution, crucial for informing carbon sequestration strategies, guiding land management practices, and informing environmental policies within arid and semi-arid environments.

Equisetum arvense as a silica fertilizer

The aim was to use the agricultural weed and silica (Si) hyperaccumulator Equisetum arvense as Si fertilizer in plant cultivation. We investigated (1) the Si uptake in various Equisetum species, (2) where Si accumulates in the Equisetum plant, (3) processing methods to release as much Si as possible from dried, ground E. arvense plants and (4) which treatment yields gives the highest uptake of Si in young wheat plants cultivated in soil containing ground E. arvense. The results showed that E. arvense containes 22% Si and was among the best Si accumulators. Equisetum arvense accumulates Si as both soluble and firmly bound fractions. Amorphous silica (SiO2) accumulates in the outer cell walls of epidermis of the entire plant. Regarding the processing method, a longer treatment time, greater concentration of Equisetum, boiling, and the addition of sodium bicarbonate increased the Si availability in ground, dried E. arvense. The addition of untreated, ground, dried E. arvense to the soil, corresponding to 160 kg Si ha-1, increased the available Si in the soil and the Si uptake in wheat plants by five-fold, compared with the control. Boiling the ground E. arvense increased the Si uptake by 10 times, and the of sodium bicarbonate increased the availability and uptake by 40 times, compared with the control. In conclusion, dried, ground E. arvense can be used as a Si fertilizer as is, after boiling for a slightly better effect, or with sodium bicarbonate (up to a similar amount as the ground material) for best effect.

Silicon fractionations in coastal wetland sediments: Implications for biogeochemical silicon cycling

Coastal wetland sediment is important reservoir for silicon (Si), and plays an essential role in controlling its biogeochemical cycling. However, little is known about Si fractionations and the associated factors driving their transformations in coastal wetland sediments. In this study, we applied an optimized sequential Si extraction method to separate six sub-fractions of non-crystalline Si (Sinoncry) in sediments from two coastal wetlands, including Si in dissolved silicate (Sidis), Si in the adsorbed silicate (Siad), Si bound to organic matter (Siorg), Si occluded in pedogenic oxides and hydroxides (Siocc), Si in biogenic amorphous silica (Siba), and Si in pedogenic amorphous silica (Sipa). The results showed that the highest proportion of Si in the Sinoncry fraction was Siba (up to 6.6 % of total Si (Sitot)), followed by the Sipa (up to 1.8 % of Sitot). The smallest proportion of Si was found in the Sidis and Siad fractions with the sum of both being <0.1 % of the Sitot. We found a lower Siocc content (188 ± 96.1 mg kg-1) when compared to terrestrial soils. The Sidis was at the center of the inter-transformation among Si fractions, regulating the biogeochemical Si cycling of coastal wetland sediments. Redundancy analysis (RDA) combined with Pearson's correlations further showed that the basic biogenic elements (total organic carbon and total nitrogen), pH, and sediment salinity collectively controlled the Si fractionations in coastal wetland sediments. Our research optimizes sediment Si fractionation procedure and provides insights into the role of sedimentary Si fractions in controlling Si dynamics and knowledge for unraveling the biogeochemical Si cycling in coastal ecosystems.

Moso bamboo (Phyllostachys edulis) expansion enhances soil pH and alters soil nutrients and microbial communities

Amid global environmental concerns, the issue of bamboo expansion has garnered significant attention due to its extensive and profound impacts on the ecosystems. Bamboo expansion occurs in native and introduced habitats worldwide, particularly in Asia. However, the effects of bamboo expansion on soil pH, nutrient levels, and microbial communities are complex and vary across different environments. To address this knowledge gap, we conducted a meta-analysis with 2037 paired observations from 81 studies. The results showed that soil pH increased by 6.99 % (0-20 cm) and 4.49 % (20-40 cm) after bamboo expansion. Notably, soil pH increased more in the coniferous forest with bamboo expansion than in the broadleaf forest. Soil pH progressively increased over time since the establishment of bamboo stands. The extent of soil pH elevation was significantly positively correlated with the proportion of bamboo within the forest stand and mean annual solar radiation. In contrast, it was significantly negatively correlated with the mean annual temperature. The elevation of pH is closely related to expansion stage and expanded forest type rather than primarily shaped by climatic factors across a large scale. We also found that bamboo expansion into coniferous forests brought about a notable 14.14 % reduction in total nitrogen (TN). Varied expansion stages resulted in TN reductions of 6.88 % and 7.99 % for mixed forests and bamboo stands, respectively, compared to native forests. Pure bamboo stands exhibited a remarkable 30.39 % increase in ammonium nitrogen and a significant 21.12 % decrease in nitrate nitrogen compared to their native counterparts. Furthermore, bamboo expansion contributed to heightened soil fungal diversity. Taken together, our findings highlight that bamboo expansion leads to an increase in soil pH and alters soil N components and fungal microbial communities, providing valuable insights for future ecological conservation and resource management.

Soil nutrient limitation and natural enemies promote the establishment of alien species in native communities

The invasion of alien plant species threatens the composition and diversity of native communities. However, the invasiveness of alien plants and the resilience of native communities are dependent on the interactions between biotic and abiotic factors, such as natural enemies and nutrient availability. In our study, we simulated the invasion of nine invasive plant species into native plant communities using two levels of nutrient availability and suppression of natural enemies. We evaluated the effect of biotic and abiotic factors on the response of alien target species and the resistance of native communities to invasion. The results showed that the presence of enemies (enemy release) increased the biomass proportion of alien plants while decreasing that of native communities in the absence of nutrient addition. Furthermore, we also found that the negative effect of enemy suppression on the evenness of the native community and the root-to-shoot ratio of alien target species was greatest under nutrient addition. Therefore, nutrient-poor and natural enemies might promote the invasive success of alien species in native communities, whereas nutrient addition and enemy suppression can better enhance the resistance of native plant communities to invasion.

Trade-offs between ligninase and cellulase and their effects on soil organic carbon in abandoned Moso bamboo forests in southeast China

A vast expanse of Moso bamboo (Phyllostachys pubescens J.Houz.) forests in subtropical areas was once intensively managed but has been abandoned in recent years. However, the response of soil organic carbon (SOC) to abandonment management remains unclear, partly because how carbon-degrading enzymes vary with abandonment management and the role of this change in the soil carbon cycle are still poorly understood, which restricts the scientific evaluation of carbon sink benefits of these abandoned Moso bamboo forests. The results of the survey, based on 40 Moso bamboo forests, showed that compared with intensive management, abandonment management for 7-10 and 11-14 years exhibited a significant decrease in ligninase activities (a reduction of 12.14 % and 44.41 %, respectively) and a significant increase in SOC content (an increase of 49.39 % and 52.64 %, respectively). However, abandonment management did not affect cellulase activities or easily oxidizable organic carbon content (p > 0.05), but significantl increased non-easily oxidizable organic carbon (p < 0.05). Furthermore, the total nitrogen (TN) content and pH value increased with prolonged abandonment, and these trade-offs between ligninase and cellulase were primarily driven by pH and TN. The ligninase-to-cellulase activities ratio is the most key factor affecting NEOC and SOC changes in abandoned Moso bamboo forests. Together, these findings demonstrate the response of carbon-degrading enzyme trade-offs to abandonment management and highlight the role of these trade-offs in controlling SOC accumulation. In addition, the different responses of different SOC fractions to abandonment management deserve attention in future studies.

Abiogenic silicon: Interaction with potentially toxic elements and its ecological significance in soil and plant systems

Abiogenic silicon (Si), though deemed a quasi-nutrient, remains largely inaccessible to plants due to its prevalence within mineral ores. Nevertheless, the influence of Si extends across a spectrum of pivotal plant processes. Si emerges as a versatile boon for plants, conferring a plethora of advantages. Notably, it engenders substantial enhancements in biomass, yield, and overall plant developmental attributes. Beyond these effects, Si augments the activities of vital antioxidant enzymes, encompassing glutathione (GSH), catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), among others. It achieves through the augmentation of reactive oxygen species (ROS) scavenging gene expression, thus curbing the injurious impact of free radicals. In addition to its effects on plants, Si profoundly ameliorates soil health indicators. Si tangibly enhances soil vitality by elevating soil pH and fostering microbial community proliferation. Furthermore, it exerts inhibitory control over ions that could inflict harm upon delicate plant cells. During interactions within the soil matrix, Si readily forms complexes with potentially toxic metals (PTEs), encapsulating them through Si-PTEs interactions, precipitative mechanisms, and integration within colloidal Si and mineral strata. The amalgamation of Si with other soil amendments, such as biochar, nanoparticles, zeolites, and composts, extends its capacity to thwart PTEs. This synergistic approach enhances soil organic matter content and bolsters overall soil quality parameters. The utilization of Si-based fertilizers and nanomaterials holds promise for further increasing food production and fortifying global food security. Besides, gaps in our scientific discourse persist concerning Si speciation and fractionation within soils, as well as its intricate interplay with PTEs. Nonetheless, future investigations must delve into the precise functions of abiogenic Si within the physiological and biochemical realms of both soil and plants, especially at the critical juncture of the soil-plant interface. This review seeks to comprehensively address the multifaceted roles of Si in plant and soil systems during interactions with PTEs.

Origin, formation, and transformation of different forms of silica in Xuanwei Formation coal, China, and its' emerging environmental problem

The study on the origin of quartz and silica in Xuanwei Formation coal in Northwest Yunnan, China, is helpful to understand the relationship between quartz and silica and the high incidence of lung cancer from the root. To address these questions, the mineralogy and microscopic studies of silica in Xuanwei Formation coal were performed. The following results were obtained: (1) silica in the late Permian Xuanwei Formation coal seams originated from detrital input, early diagenesis, and late diagenesis. (2) A more significant contribution comes from early diagenesis, which contains abundant authigenic quartz and amorphous silica. (3) Quartz and silica from inorganic silicon are more symbiotic with kaolinite and from biogenic silicon with chamosite. (4) Three silica polymorphs in coal samples have been identified: opal-A (amorphous silica), opal-CT/-C (cristobalite/tridymite), and α quartz. (5) Opal-A is ubiquitous, while opal-CT/-C and α quartz are rare. (5) Opal-A is an amorphous and nontoxic ordinary silica. (6) Since the toxicity of amorphous silica and its presence in coal is an emerging topic, it should be continuously monitored.

Forest buffer-strips mitigate the negative impact of oil palm plantations on stream communities

The global area cultivated with oil palm has doubled in the past two decades, causing deforestation, land-use change, freshwater pollution, and species loss in tropical ecosystems worldwide. Despite the palm-oil industry been linked to severe deterioration of freshwater ecosystems, most studies have focused on terrestrial environments, while freshwaters have been significantly less studied. We evaluated these impacts by contrasting freshwater macroinvertebrate communities and habitat conditions in 19 streams from primary forests (7), grazing lands (6), and oil palm plantations (6). In each stream, we measured environmental characteristics, e.g., habitat composition, canopy cover, substrate, water temperature, and water quality; and we identified and quantified the assemblage of macroinvertebrates. Streams in oil palm plantations lacking riparian forest strips showed warmer and more variable temperatures, higher turbidity, lower silica content, and poorer macroinvertebrate taxon richness than primary forests. Grazing lands showed higher conductivity and temperature, and lower dissolved oxygen and macroinvertebrate taxon richness than primary forests. In contrast, streams in oil palm plantations that conserved a riparian forest, showed a substrate composition, temperature, and canopy cover more similar to the ones in primary forests. These habitat improvements by riparian forests in the plantations increased macroinvertebrate taxon richness and maintained a community resembling more the one in primary forests. Therefore, the conversion of grazing lands (instead of primary forests) to oil palm plantations can increase freshwater taxon richness only if riparian native forests are safeguarded.

Closing the Nutrient Loop-The New Approaches to Recovering Biomass Minerals during the Biorefinery Processes

The recovery of plant mineral nutrients from the bio-based value chains is essential for a sustainable, circular bioeconomy, wherein resources are (re)used sustainably. The widest used approach is to recover plant nutrients on the last stage of biomass utilization processes-e.g., from ash, wastewater, or anaerobic digestate. The best approach is to recover mineral nutrients from the initial stages of biomass biorefinery, especially during biomass pre-treatments. Our paper aims to evaluate the nutrient recovery solutions from a trans-sectorial perspective, including biomass processing and the agricultural use of recovered nutrients. Several solutions integrated with the biomass pre-treatment stage, such as leaching/bioleaching, recovery from pre-treatment neoteric solvents, ionic liquids (ILs), and deep eutectic solvents (DESs) or integrated with hydrothermal treatments are discussed. Reducing mineral contents on silicon, phosphorus, and nitrogen biomass before the core biorefinery processes improves processability and yield and reduces corrosion and fouling effects. The recovered minerals are used as bio-based fertilizers or as silica-based plant biostimulants, with economic and environmental benefits.

Impact of Silicon on Plant Nutrition and Significance of Silicon Mobilizing Bacteria in Agronomic Practices

Globally, rejuvenation of soil health is a major concern due to the continuous loss of soil fertility and productivity. Soil degradation decreases crop yields and threatens global food security. Improper use of chemical fertilizers coupled with intensive cultivation further reduces both soil health and crop yields. Plants require several nutrients in varying ratios that are essential for the plant to complete a healthy growth and development cycle. Soil, water, and air are the sources of these essential macro- and micro-nutrients needed to complete plant vegetative and reproductive cycles. Among the essential macro-nutrients, nitrogen (N) plays a significant in non-legume species and without sufficient plant access to N lower yields result. While silicon (Si) is the 2nd most abundant element in the Earth’s crust and is the backbone of soil silicate minerals, it is an essential micro-nutrient for some plants. Silicon is just beginning to be recognized as an important micronutrient to some plant species and, while it is quite abundant, Si is often not readily available for plant uptake. The manufacturing cost of synthetic silica-based fertilizers is high, while absorption of silica is quite slow in soil for many plants. Rhizosphere biological weathering processes includes microbial solubilization processes that increase the dissolution of minerals and increases Si availability for plant uptake. Therefore, an important strategy to improve plant silicon uptake could be field application of Si-solubilizing bacteria. In this review, we evaluate the role of Si in seed germination, growth, and morphological development and crop yield under various biotic and abiotic stresses, different pools and fluxes of silicon (Si) in soil, and the bacterial genera of the silicon solubilizing microorganisms. We also elaborate on the detailed mechanisms of Si-solubilizing/mobilizing bacteria involved in silicate dissolution and uptake by a plant in soil. Last, we discuss the potential of silicon and silicon solubilizing/mobilizing to achieve environmentally friendly and sustainable crop production.

If All Else Fails: Impact of Silicon Accumulation in Maize Leaves on Volatile Emissions and Oviposition Site Selection of Spodoptera exigua Hübner

Silicon (Si) fertilization alleviates biotic stresses in plants. Si enhances plant resistance against phytophagous insects through physical and biochemical mechanisms. In particular, Si modifies jasmonic acid levels and the emissions of herbivore-induced plant volatiles (HIPVs). Here, we investigated whether Si accumulation in the tissues of maize leaves modifies the emissions of constitutive and herbivore-induced plant volatiles, with cascade deterrent effects on oviposition site selection by Spodoptera exigua Hübner (Lepidoptera: Noctuidae). Maize plants were cultivated in a hydroponic system under three Si concentrations, resulting in three groups of plants expressing different Si concentrations in their tissues (0.31 ± 0.04, 4.69 ± 0.49, and 9.56 ± 0.30 g Si. Kg^- 1 DW). We collected volatiles from undamaged and caterpillar-infested plants, and found that Si concentration in plant tissues had no significant impact. Jasmonic acid content was high in insect-infested plants, but was similar across all Si treatments. Oviposition site selection bioassays using fertilized S. exigua females showed that Si concentration in plant tissues did not affect the number of eggs laid on Si-treated plants. In conclusion, our study shows that the Si content in maize tissues does not impact the semiochemical interactions with S. exigua.

Understanding the Relationship between Water Availability and Biosilica Accumulation in Selected C4 Crop Leaves: An Experimental Approach

Biosilica accumulation in plant tissues is related to the transpiration stream, which in turn depends on water availability. Nevertheless, the debate on whether genetically and environmentally controlled mechanisms of biosilica deposition are directly connected to water availability is still open. We aim at clarifying the system which leads to the deposition of biosilica in Sorghum bicolor, Pennisetum glaucum, and Eleusine coracana, expanding our understanding of the physiological role of silicon in crops well-adapted to arid environments, and simultaneously advancing the research in archaeological and paleoenvironmental studies. We cultivated ten traditional landraces for each crop in lysimeters, simulating irrigated and rain-fed scenarios in arid contexts. The percentage of biosilica accumulated in leaves indicates that both well-watered millet species deposited more biosilica than the water-stressed ones. By contrast, sorghum accumulated more biosilica with respect to the other two species, and biosilica accumulation was independent of the water regime. The water treatment alone did not explain either the variability of the assemblage or the differences in the biosilica accumulation. Hence, we hypothesize that genetics influence the variability substantially. These results demonstrate that biosilica accumulation differs among and within C4 species and that water availability is not the only driver in this process.

Silicon mobilisation by root-released carboxylates

Plants have evolved numerous strategies to acquire poorly available nutrients from soil, including the release of carboxylates from their roots. Silicon (Si) release from mineral dissolution increases in the presence of chelating substances, and recent evidence shows that leaf [Si] increases markedly in old phosphorus (P)-depleted soils, where many species exhibit carboxylate-releasing strategies, compared with younger P-richer soils. Here, we propose that root-released carboxylates, and more generally rhizosphere processes, play an overlooked role in plant Si accumulation by increasing soil Si mobilisation from minerals. We suggest that Si mobilisation is costly in terms of carbon but becomes cheaper if those costs are already met to acquire poorly available P. Uptake of the mobilised Si by roots will then depend on whether they express Si transporters.

The role of soils in the regulation of ocean acidification

Soils play an important role in mediating chemical weathering reactions and carbon transfer from the land to the ocean. Proposals to increase the contribution of alkalinity to the oceans through 'enhanced weathering' as a means to help prevent climate change are gaining increasing attention. This would augment the existing connection between the biogeochemical function of soils and alkalinity levels in the ocean. The feasibility of enhanced weathering depends on the combined influence of what minerals are added to soils, the formation of secondary minerals in soils and the drainage regime, and the partial pressure of respired CO2 around the dissolving mineral. Increasing the alkalinity levels in the ocean through enhanced weathering could help to ameliorate the effects of ocean acidification in two ways. First, enhanced weathering would slightly elevate the pH of drainage waters, and the receiving coastal waters. The elevated pH would result in an increase in carbonate mineral saturation states, and a partial reversal in the effects of elevated CO2. Second, the increase in alkalinity would help to replenish the ocean's buffering capacity by maintaining the 'Revelle Factor', making the oceans more resilient to further CO2 emissions. However, there is limited research on the downstream and oceanic impacts of enhanced weathering on which to base deployment decisions. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Exploration of silicate solubilizing bacteria for sustainable agriculture and silicon biogeochemical cycle

Silicon (Si), a quasi-essential element for plants, is abundant in the soil typically as insoluble silicate forms. However, plants can uptake Si only in the soluble form of monosilicic acid. Production of monosilicic acid by rock-weathering mostly depends on temperature, pH, redox-potential, water-content, and microbial activities. In the present review, approaches involved in the efficient exploration of silicate solubilizing bacteria (SSB), its potential applications, and available technological advances are discussed. Present understanding of Si uptake, deposition, and subsequent benefits to plants has also been discussed. In agricultural soils, pH is found to be one of the most critical factors deciding silicate solubilization and the formation of different Si compounds. Numerous studies have predicted the role of Indole-3-Acetic Acid (IAA) and organic acids produced by SSB in silicate solubilization. In this regard, approaches for the isolation and characterization of SSB, quantification of IAA, and subsequent Si solubilization mechanisms are addressed. Phylogenetic evaluation of previously reported SSB showed a highly diverse origin which provides an opportunity to study different mechanisms involved in Si solubilization. Soil biochemistry in concern of silicon availability, microbial activity and silicon mediated changes in plant physiology are addressed. In addition, SSB's role in Si-biogeochemical cycling is summarized. The information presented here will be helpful to explore the potential of SSB more efficiently to promote sustainable agriculture.

Silicon in the Soil-Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

Plants' ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon's roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

Silicon Cycling in Soils Revisited

Silicon (Si) speciation and availability in soils is highly important for ecosystem functioning, because Si is a beneficial element for plant growth. Si chemistry is highly complex compared to other elements in soils, because Si reaction rates are relatively slow and dependent on Si species. Consequently, we review the occurrence of different Si species in soil solution and their changes by polymerization, depolymerization, and condensation in relation to important soil processes. We show that an argumentation based on thermodynamic endmembers of Si dependent processes, as currently done, is often difficult, because some reactions such as mineral crystallization require months to years (sometimes even centuries or millennia). Furthermore, we give an overview of Si reactions in soil solution and the predominance of certain solid compounds, which is a neglected but important parameter controlling the availability, reactivity, and function of Si in soils. We further discuss the drivers of soil Si cycling and how humans interfere with these processes. The soil Si cycle is of major importance for ecosystem functioning; therefore, a deeper understanding of drivers of Si cycling (e.g., predominant speciation), human disturbances and the implication for important soil properties (water storage, nutrient availability, and micro aggregate stability) is of fundamental relevance.

Agriculture increases the bioavailability of silicon, a beneficial element for crop, in temperate soils

Crops may take benefits from silicon (Si) uptake in soil. Plant available Si (PAS) can be affected by natural weathering processes or by anthropogenic forces such as agriculture. The soil parameters that control the pool of PAS are still poorly documented, particularly in temperate climates. In this study, we documented PAS in France, based on statistical analysis of Si extracted by CaCl2 (SiCaCl2) and topsoil characteristics from an extensive dataset. We showed that cultivation increased SiCaCl2 for soils developed on sediments, that cover 73% of France. This increase is due to liming for non-carbonated soils on sediments that are slightly acidic to acidic when non-cultivated. The analysis performed on non-cultivated soils confirmed that SiCaCl2 increased with the < 2 µm fraction and pH but only for soils with a < 2 µm fraction ranging from 50 to 325 g kg-1. This increase may be explained by the < 2 µm fraction mineralogy, i.e. nature of the clay minerals and iron oxide content. Finally, we suggest that 4% of French soils used for wheat cultivation could be deficient in SiCaCl2.

Sorption competition with natural organic matter as mechanism controlling silicon mobility in soil

Growing evidence of silicon (Si) playing an important role in plant health and the global carbon cycle triggered research on its biogeochemistry. In terrestrial soil ecosystems, sorption of silicic acid (H4SiO4) to mineral surfaces is a main control on Si mobility. We examined the competitive sorption of Si, dissolved organic matter, and phosphorus in forest floor leachates (pH 4.1-4.7) to goethite, in order to assess its effects on Si mobility at weathering fronts in acidic topsoil, a decisive zone of nutrient turnover in soil. In batch sorption experiments, we varied the extent of competition between solutes by varying the amount of added goethite (α-FeOOH) and the Si pre-loading of the goethite surfaces. Results suggest weaker competitive strength of Si than of dissolved organic matter and ortho-phosphate. Under highly competitive conditions, hardly any dissolved Si (< 2%) but much of the dissolved organic carbon (48-80%) was sorbed. Pre-loading the goethite surfaces with monomeric Si hardly decreased the sorption of organic carbon and phosphate, whereas up to about 50% of the Si was released from surfaces into solutions, indicating competitive displacement from sorption sites. We conclude sorption competition with dissolved organic matter and other strongly sorbing solutes can promote Si leaching in soil. Such effects should thus be considered in conceptual models on soil Si transport, distribution, and phytoavailability.

Comparative analysis of borate fusion versus sodium carbonate extraction for quantification of silicon contents in plants

Studies of plant-silicon (Si) interaction benefit from safe, affordable and accurate methods to measure acid-insoluble silica (phytoliths) for a large number of plant samples. This study aimed to evaluate the comparability between two chemical methods to dissolve leaf silica, borate fusion and 1% sodium carbonate (Na₂CO₃) extraction, in combination of two detection methods (ICP, molybdenum-blue colorimetry).We compared the results obtained by these methods, using dried leaf samples of five tropical tree species that differ widely in Si concentrations (4 to 100 mg g DW^-1). Leaf Si concentration values determined after the two extraction methods were highly correlated (y = 0.79x, R² = 0.998). However, compared to the extraction with borate fusion, the 1% Na₂CO₃ method resulted in lower Si concentration per unit dry mass by 16% to 32% (mean of 24.2%). We also found that molybdenum-blue colorimetry method may interfere with certain extraction methods. A simple equation can be used to correct for systematic underestimation of Si contents determined after extraction with 1% Na₂CO₃, which is the least expensive and safest among commonly used methods for extraction of Si from land plants.

Combined Silicon-Phosphorus Fertilization Affects the Biomass and Phytolith Stock of Rice Plants

Phytoliths are silica bodies formed in living plant tissues. Once deposited in soils through plant debris, they can readily dissolve and then increase the fluxes of silicon (Si) toward plants and/or watersheds. These fluxes enhance Si ecological services in agricultural and marine ecosystems through their impact on plant health and carbon fixation by diatoms, respectively. Fertilization increases crop biomass through the supply of plant nutrients, and thus may enhance Si accumulation in plant biomass. Si and phosphorus (P) fertilization enhance rice crop biomass, but their combined impact on Si accumulation in plants is poorly known. Here, we study the impact of combined Si-P fertilization on the production of phytoliths in rice plants. The combination of the respective supplies of 0.52 g Si kg^-1 and 0.20 g P kg^-1 generated the largest increase in plant shoot biomass (leaf, flag leaf, stem, and sheath), resulting in a 1.3-fold increase compared the control group. Applying combined Si-P fertilizer did not affect the content of organic carbon (OC) in phytoliths. However, it increased plant available Si in soil, plant phytolith content and its total stock (mg phytolith pot^-1) in dry plant matter, leading to the increase of the total amount of OC within plants. In addition, P supply increased rice biomass and grain yield. Through these positive effects, combined Si-P fertilization may thus address agronomic (e.g., sustainable ecosystem development) and environmental (e.g., climate change) issues through the increase in crop yield and phytolith production as well as the promotion of Si ecological services and OC accumulation within phytoliths.

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.

Silicon and Plant Natural Defenses against Insect Pests: Impact on Plant Volatile Organic Compounds and Cascade Effects on Multitrophic Interactions

Environmental factors controlling silicon (Si) accumulation in terrestrial plant are key drivers to alleviate plant biotic stresses, including insect herbivory. While there is a general agreement on the ability of Si-enriched plant to better resist insect feeding, recent studies suggest that Si also primes biochemical defense pathways in various plant families. In this review, we first summarize how soil parameters and climate variables influence Si assimilation in plants. Then, we describe recent evidences on the ability of Si to modulate plant volatile emissions, with potential cascade effects on phytophagous insects and higher trophic levels. Even though the mechanisms still need to be elucidated, Si accumulation in plants leads to contrasting effects on the levels of the three major phytohormones, namely jasmonic acid, salicylic acid and ethylene, resulting in modified emissions of plant volatile organic compounds. Herbivore-induced plant volatiles would be particularly impacted by Si concentration in plant tissues, resulting in a cascade effect on the attraction of natural enemies of pests, known to locate their prey or hosts based on plant volatile cues. Since seven of the top 10 most important crops in the world are Si-accumulating Poaceae species, it is important to discuss the potential of Si mobility in soil-plant systems as a novel component of an integrated pest management.

Phytolith Formation in Plants: From Soil to Cell

Silica is deposited extra- and intracellularly in plants in solid form, as phytoliths. Phytoliths have emerged as accepted taxonomic tools and proxies for reconstructing ancient flora, agricultural economies, environment, and climate. The discovery of silicon transporter genes has aided in the understanding of the mechanism of silicon transport and deposition within the plant body and reconstructing plant phylogeny that is based on the ability of plants to accumulate silica. However, a precise understanding of the process of silica deposition and the formation of phytoliths is still an enigma and the information regarding the proteins that are involved in plant biosilicification is still scarce. With the observation of various shapes and morphologies of phytoliths, it is essential to understand which factors control this mechanism. During the last two decades, significant research has been done in this regard and silicon research has expanded as an Earth-life science superdiscipline. We review and integrate the recent knowledge and concepts on the uptake and transport of silica and its deposition as phytoliths in plants. We also discuss how different factors define the shape, size, and chemistry of the phytoliths and how biosilicification evolved in plants. The role of channel-type and efflux silicon transporters, proline-rich proteins, and siliplant1 protein in transport and deposition of silica is presented. The role of phytoliths against biotic and abiotic stress, as mechanical barriers, and their use as taxonomic tools and proxies, is highlighted.

Silicon Availability from Chemically Diverse Fertilizers and Secondary Raw Materials

Crops may require Si fertilization to sustain yields. Potential Si fertilizers include industrial byproducts (e.g., steel slags), mined minerals (CaSiO₃), fused Ca-Mg-phosphates, biochar, ash, diatomaceous earth, and municipal sewage sludge. To date, no extraction method was shown to accurately predict plant availability of Si from such chemically diverse Si fertilizers. We tested a wide range of products in greenhouse experiments and related the plant Si content to Si extracted by several common Si fertilizer tests: 5-day extraction in Na₂CO₃-NH₄NO₃, 0.5 mol L^-1 HCl, and Resin extraction. In addition, we tested a novel sink extraction approach for Si(OH)₄⁰ that utilizes a dialysis membrane filled with ferrihydrite ("Iron Bag"). Wheat straw biochars and ash exhibited equivalent or marginally higher Si solubility and availability compared to wheat straw. Thermo-chemically treated municipal sewage sludge, as well as diatomaceous earth, did not release substantial amounts of Si. The Resin and the Iron Bag extraction methods gave the best results to predict plant availability of Si. These methods better reproduce the conditions of fertilizer dissolution in soil and around the root by (1) buffering the pH close to neutral and (2) extracting the dissolved Si(OH)₄⁰ with ferrihydrite (Iron Bag method) for maximum quantitative extraction.

Effects of biochar amendment on the soil silicon cycle in a soil-rice ecosystem

For the soil-plant ecosystem, knowledge about the effects of biochars on the soil silicon (Si) cycle is still tenuous. In this study, the effect of biochars on the yield, Si uptake and Si distribution within different tissues of rice plants and the soil Si cycles in a soil-plant system were investigated. Si-rich (RH300-700) and Si-deficient (WB300-700) biochars prepared from rice husk and wood sawdust were applied to high-Si soil (HSS) and low-Si soil (LSS). Biochar addition increased the yield of grain and straw and had no effect on the yield of root, and the increase in the yield with Si-rich biochars was obvious; this effect had a high response to LSS. Si-rich biochars increased the plant Si content of grain and root and had no effect on straw. RH300 amendment increased the Si concentration in grains, compared to RH500 and RH700. The addition of Si-deficient biochar to HSS had little effect on the Si content, while Si-deficient biochar-amended LSS had a great impact on the reduced Si content in rice straw and root, and WB700 decreased the Si concentration in grains, compared to WB300 and WB500. Finally, the Si-rich biochars increased the total Si uptake within rice, while Si-deficient biochars decreased the total Si uptake in LSS. According to the FTIR and SEM-EDX spectra of biochars before and after rice harvest, a new band of SiOSi at 471 cm^-1 was found after aged WB700, and the minerals of iron and Si were found on the surface of aged WB700; biochars can fix the dissolved Si on its surface as a temporary store to prevent Si loss. Therefore, biochars can be considered reservoirs of soil Si, which is a slow release source of available Si, to impact the speed of biogeochemical cycling of soil Si in agricultural paddy soil.

Silica - A trace geogenic element with emerging nephrotoxic potential

Silica is a trace-geogenic compound with limited-bioavailability. It inflicts health-perils like pulmonary-silicosis and chronic kidney disease (CKD), when available via anthropogenic-disturbances. Amidst silica-imposed pathologies, pulmonary toxicological-mechanisms are well-described, ignoring the renal-pathophysiological mechanisms. Hence, the present-study aimed to elucidate cellular-cum-molecular toxicological-mechanisms underlying silica-induced renal-pathology in-vitro. Various toxicity-assessments were used to study effects of silica on the physiological-functions of HK-cells (human-kidney proximal-tubular cells - the toxin's prime target) on chronic (1-7 days) sub-toxic (80 mg/L) and toxic (100-120 mg/L) dosing. Results depicted that silica triggered dose-cum-time dependent cytotoxicity/cell-death (MTT-assay) that significantly increased on long-term dosing with ≥100 mg/L silica; establishing the nephrotoxic-potential of this dose. Contrarily, insignificant cell-death on sub-toxic (80 mg/L) dosing was attributed to rapid intracellular toxin-clearance at lower-doses preventing toxic-effects. The proximal-tubular (HK-cells) cytotoxicity was found to be primarily mediated by silica-triggered incessant oxidative-stress (elevated ROS).·This enhanced ROS inflicted severe inflammation and subsequent fibrosis, evident from increased pro-inflammatory-cum-fibrogenic cytokines generation (IL-1β, IL-2, IL-6, TNF-α and TGF-β). Simultaneously, ROS induced persistent DNA-damage (Comet-assay) that stimulated G2/M arrest for p53-mediated damage-repair, aided by checkpoint-promoter (Chk1) activation and mitotic-inducers (i.e. Cdc-25, Cdk1, cyclinB1) inhibition. However, DNA-injuries surpassed the cellular-repair, which provoked the p53-gene to induce mitochondrial-mediated apoptotic cell-death via activation of Bax, cytochrome-c and caspase-cascade (9/3). This persistent apoptotic cell-death and simultaneous incessant inflammation culminated in the development of tubular-atrophy and fibrosis, the major pathological-manifestations of CKD. These findings provided novel-insights into the pathological-mechanisms (cellular and molecular) of silica-induced CKD, inflicted on chronic toxic-dosing (≥100 mg/L).Thereby, encouraging the development of therapeutic-strategies (e.g. anti-oxidant treatment) for specific molecular-targets (e.g. ROS) to retard silica-induced CKD-progression, for reduction in the global-CKD burden.

Variable silicon accumulation in plants affects terrestrial carbon cycling by controlling lignin synthesis

Current climate and land-use changes affect regional and global cycles of silicon (Si), with yet uncertain consequences for ecosystems. The key role of Si in marine ecology by controlling algae growth is well recognized but research on terrestrial ecosystems neglected Si since not considered an essential plant nutrient. However, grasses and various other plants accumulate large amounts of Si, and recently it has been hypothesized that incorporation of Si as a structural plant component may substitute for the energetically more expensive biosynthesis of lignin. Herein, we provide evidence supporting this hypothesis. We demonstrate that in straw of rice (Oryza sativa) deriving from a large geographic gradient across South-East Asia, the Si concentrations (ranging from 1.6% to 10.7%) are negatively related to the concentrations of carbon (31.3% to 42.5%) and lignin-derived phenols (32 to 102 mg/g carbon). Less lignin may explain results of previous studies that Si-rich straw decomposes faster. Hence, Si seems a significant but hardly recognized factor in organic carbon cycling through grasslands and other ecosystems dominated by Si-accumulating plants.

Uptake of Silicon by Sugarcane from Applied Sources May Not Reflect Plant-Available Soil Silicon and Total Silicon Content of Sources

Soils of the tropics and sub-tropics are typically acid and depleted of soluble sources of silicon (Si) due to weathering and leaching associated with high rainfall and temperatures. Together with intensive cropping, this leads to marginal or deficient plant Si levels in Si-accumulating crops such as rice and sugarcane. Although such deficiencies can be corrected with exogenous application of Si sources, there is controversy over the effectiveness of sources in relation to their total Si content, and their capacity to raise soil and plant Si concentrations. This study tested the hypothesis that the total Si content and provision of plant-available Si from six sources directly affects subsequent plant Si uptake as reflected in leaf Si concentration. Two trials with potted cane plants were established with the following Si sources as treatments: calcium silicate slag, fused magnesium (thermo) phosphate, volcanic rock dust, magnesium silicate, and granular potassium silicate. Silicon sources were applied at rates intended to achieve equivalent elemental soil Si concentrations; controls were untreated or lime-treated. Analyses were conducted to determine soil and leaf elemental concentrations. Among the sources, calcium silicate produced the highest leaf Si concentrations, yet lower plant-available soil Si concentrations than the thermophosphate. The latter, with slightly higher total Si than the slag, produced substantially greater increases in soil Si than all other products, yet did not significantly raise leaf Si above the controls. All other sources did not significantly increase soil or leaf Si concentrations, despite their high Si content. Hence, the total Si content of sources does not necessarily concur with a product's provision of soluble soil Si and subsequent plant uptake. Furthermore, even where soil pH was raised, plant uptake from thermophosphate was well below expectation, possibly due to its limited liming capacity. The ability of the calcium silicate to provide Si while simultaneously and significantly increasing soil pH, and thereby reducing reaction of Si with exchangeable Al3+, is proposed as a potential explanation for the greater Si uptake into the shoot from this source.

Uptake of Silicon by Sugarcane from Applied Sources May Not Reflect Plant-Available Soil Silicon and Total Silicon Content of Sources

Soils of the tropics and sub-tropics are typically acid and depleted of soluble sources of silicon (Si) due to weathering and leaching associated with high rainfall and temperatures. Together with intensive cropping, this leads to marginal or deficient plant Si levels in Si-accumulating crops such as rice and sugarcane. Although such deficiencies can be corrected with exogenous application of Si sources, there is controversy over the effectiveness of sources in relation to their total Si content, and their capacity to raise soil and plant Si concentrations. This study tested the hypothesis that the total Si content and provision of plant-available Si from six sources directly affects subsequent plant Si uptake as reflected in leaf Si concentration. Two trials with potted cane plants were established with the following Si sources as treatments: calcium silicate slag, fused magnesium (thermo) phosphate, volcanic rock dust, magnesium silicate, and granular potassium silicate. Silicon sources were applied at rates intended to achieve equivalent elemental soil Si concentrations; controls were untreated or lime-treated. Analyses were conducted to determine soil and leaf elemental concentrations. Among the sources, calcium silicate produced the highest leaf Si concentrations, yet lower plant-available soil Si concentrations than the thermophosphate. The latter, with slightly higher total Si than the slag, produced substantially greater increases in soil Si than all other products, yet did not significantly raise leaf Si above the controls. All other sources did not significantly increase soil or leaf Si concentrations, despite their high Si content. Hence, the total Si content of sources does not necessarily concur with a product's provision of soluble soil Si and subsequent plant uptake. Furthermore, even where soil pH was raised, plant uptake from thermophosphate was well below expectation, possibly due to its limited liming capacity. The ability of the calcium silicate to provide Si while simultaneously and significantly increasing soil pH, and thereby reducing reaction of Si with exchangeable Al³⁺, is proposed as a potential explanation for the greater Si uptake into the shoot from this source.