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The growing interest in amorphous silica (ASi) within the fields of soil science and ecology underscores the necessity for a reliable protocol to estimate ASi contents in soil. Alkaline wet chemical extraction methods are commonly employed for silicon (Si) extraction fromoperationally defined (x-ray) amorphous Si phases or short-range ordered mineral phases in soils and marine sediments. In our study we conducted a comparative analysis of four alkaline extraction methods (1% sodium carbonate, 0.5M sodium carbonate, 0.2M sodium hydroxide, and 0.1M Tiron), assessing their extraction selectivity as well as effectiveness using soils artificially enriched with varying, defined amounts of ASi. While extraction effectiveness was evaluated by determining the recovery rate of initially added ASi, extraction selectivity was determined by measuring aluminum (Al) and iron (Fe) concentrations as indicators of the dissolution of non-target mineral phases. Microwave plasma atom emission spectrometry was used to analyze Al, Fe, and Si concentrations in the extracts. Our results indicate that extraction with 0.2M sodium hydroxide yields the best outcomes in terms of both extraction effectiveness and selectivity. Thismore recent extraction technique is conducted at the most alkaline pH (13.3) of all four methods tested, but at ambient temperature (21°C) decreasing the dissolution of non-target mineral phases. Though, no wet-chemical extraction used on heterogeneous samples like soil is precisely selective, and thus able to quantify the target analyte only. Hence, data obtained by such procedures still need to be interpreted with caution considering all their limitations. [ABSTRACT FROM AUTHOR]

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding antiherbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiplechoice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species. [ABSTRACT FROM AUTHOR]

There is increased interest in the terrestrial silicon cycle in the last decades as its different compounds and species have large implications for ecosystem performance in terms of soil nutrient and water availability, ecosystem productivity as well as ecological aspects such as plant–microbe and plant-animal feedbacks. The currently existing analytical methods are limited. Fourier-transform infrared spectroscopy (FTIR) analysis is suggested being a promising tool to differentiate between the different Si species. We report here on the differentiation of varying Si-species/Si-binding (in synthetic material) using FTIR-analyses. Therefore, we collected FTIR-spectra of five different amorphous silica, Ca-silicate, sodium silicate (all particulate), a water-soluble fraction of amorphous silica and soil affected by volcanic activity and compared their spectra with existing data. A decrease of the internal order of the materials analyzed was indicated by peak broadening of the Si–O–Si absorption band. Peak shifts at this absorption band were induced by larger ions incorporated in the Si–O–Si network. Additionally, short-range ordered aluminosilicates (SROAS) have specific IR absorption bands such as the Si–O–Al band. Hence, SROAS and Si phases containing other ions can be distinguished from pure amorphous Si species using FTIR-analyses. [ABSTRACT FROM AUTHOR]

Arctic soils store large amounts of organic carbon and other elements such as amorphous silica, silicon, calcium, iron, aluminium, and phosphorous. Global warming is projected to be most pronounced in the Arctic leading to thawing permafrost, which in turn is changing the soil element availability. To project how biogeochemical cycling in Arctic ecosystems will be affected by climate change, there is a need for data on element availability. Here, we analysed amorphous silica (ASi), silicon (Si), calcium (Ca), iron (Fe), phosphorus (P), and aluminium (Al) availability from 574 soil samples from the circumpolar Arctic region. We show large differences in ASi, Si, Ca, Fe, P, and Al availability among different lithologies and Arctic regions. We summarized these data in pan-Arctic maps of ASi, Si, Ca, Fe, P, and Al concentrations focussing on the top 100 cm of Arctic soil. Furthermore, we provide values for element availability for the organic and the mineral layer of the seasonally thawing active layer as well as for the uppermost permafrost layer. Our spatially explicit data on differences in the availability of elements between the different lithological classes and regions now and in the future will improve Arctic Earth system models for estimating current and future carbon and nutrient feedbacks under climate change. [ABSTRACT FROM AUTHOR]

Global changes in temperature and nitrogen supply can influence key aspects of the global nutrient cycles, such as leaf litter decomposition and associated nutrient release. The effect of temperature and nitrogen supply on nutrient release during decomposition may be influenced by initial nutrient concentrations of the litter, which for grasses depends partly on silicon availability during plant growth. We conducted a five-month field experiment to test whether Phragmites australis litter silicon concentration alters the effect of climate warming and excess nitrogen supply on nutrient concentrations and litter mass loss during decomposition. Warming enhanced concentration of Ca, Mn and Fe in decomposed leaf litter, whereas excess N-supply increased the concentration of Fe and Ca in leaves. Initial Si concentrations of leaf litter influenced the stoichiometric ratios of the elements (in relation to P), and element:P ratios mostly increased for litter with intermediate and high Si concentrations, but patterns were less clear when initial litter Si concentrations were low. We show that grass litter decomposition was most strongly associated with litter Si:P, whereas C:N and N:P ratios were less important. Our results provide first evidence that global warming, enhanced N supply and changes in Si availability affect element cycling during litter decomposition. [ABSTRACT FROM AUTHOR]

Abstract: Organic matter is known to accumulate high amounts of metals/metalloids, enhanced during the process of decomposition by heterotrophic biofilms (with high fixation capacity for metals/metalloids). The colonization by microbes and the decay rate of the organic matter depends on different litter properties. Main litter properties affecting the decomposition of organic matter such as the nutrient ratios and the content of cellulose, lignin and phenols are currently described to be changed by silicon availability. But less is known about the impact of silicon availability during plant growth on elemental fixation during decay. Hence, this research focuses on the impact of silicon availability during plant growth on fixation of 42 elements during litter decay, by controlling the litter properties. The results of this experiment are a significantly higher metal/metalloid accumulation during decomposition of plant litter grown under low silicon availability. This may be explained by the altered litter properties (mainly nutrient content) affecting the microbial decomposition of the litter, the microbial growth on the litter and possibly by the silicon double layer, which is evident in leaf litter with high silicon content and reduces the binding sites for metals/metalloids. Furthermore, this silicon double layer may also reduce the growing biofilm by reducing the availability of carbon compounds at the litter surface and has to be elucidated in further research. Hence, low silicon availability during plant growth enhances the metal/metalloid accumulation into plant litter during aquatic decomposition. [Copyright &y& Elsevier]

Abstract: Silicon is described as beneficial for grasses by enhancing yield and fitness via a considerable contribution to pathogen, drought, and pest resistance. Silicic acid is the predominant form for uptake and transport within the plant and will precipitate in leaves. But it is unknown whether polymeric nanosilicon compounds in its synthetic form, with an increasing concentration in aquatic environments, can be suitable for plant nutrition. Therefore, we investigated the uptake, transport, and deposition of silicic acid/silica within plants using synthetic nanosilica. Our results show a significant difference in silicon (Si) content within the different tissues of Phragmites australis. The nanosilica had been dissolved prior to the uptake by plants. The chemical form of Si during uptake was not traceable. A significant enhancement in the condensation state of the silica was found from root to leaves especially from culm to leaf tips visible by the increasing content of Q4-groups in the NMR spectra. We conclude that synthetic nanosilica has the same quality as source for the beneficial element Si like natural silica. Since the condensation state is described to control silica solubility, we suggest that different condensation states within the plant may result in different remobilization of silicon during decomposition of the plant material. [Copyright &y& Elsevier]

Abstract: Silicon is not an essential element in sensu stricto but affects the productivity of Poaceae. Recent field studies has shown the effect of silicon on cellulose and lignin content in bulk analyses of green compartments of different species, sampled before matter translocation in the course of senescence. Nevertheless, there is a gap in information about silicon availability and its effect on cellulose, lignin and phenol content in different plant tissues after nutrient resorption. In order to prove the effect of controlled silica supply on functional relevant carbon compounds Phragmites australis Trin. was grown in pot trials under three different levels of silicon surplus. After resorption of nutrients into rhizomes the content of silicon, lignin, cellulose and phenol were measured in different plant tissues. The results show that different levels of silicon surplus changed the plant cellulose, lignin and phenol content depending on plant tissue function. Cellulose content in tissues with stabilization function is reduced contrasting enhanced cellulose content in tissues without stabilization function. Furthermore, higher silicon surplus decreased the phenol content in photosynthetic active tissues and increased the phenol content in culm. Only weak silicon to lignin interaction was found. It is concluded that silicon affects the cellulose and phenol metabolism and the tradeoff between productivity and stabilization/defense. Considering these compounds as crucial factors in decay processes, silicon may be very important for the terrestrial and semi-terrestrial carbon turnover [Copyright &y& Elsevier]

• Heat effects form fire potentially plays an important role in global Si cycling. • Heat increased Si availability up to ~700% in different mineral soils. • Heat increased amorphous Si up to 1.9 × 104% in these soils. Altered fire regimes as a response to climate change are significantly affecting terrestrial ecosystems and biogeochemical cycles. In this context, not only natural fires but also anthropogenic fires like rice straw burning have to be considered. While heat effects of fire on some nutrient cycles (e.g., carbon, nitrogen, phosphorus) have been well-studied, little is known about consequences of heat for the silicon (Si) cycle. There is some information that heat increases the release of Si from soil organic matter. However, nothing is known about the effect of heat on Si availability in mineral soils to the best of our knowledge. We analyzed the effect of heat on Si availability in six different mineral soils. Our data show that heat is increasing Si availability (Mehlich-3 extractable) of all tested soils. The same pattern was found for amorphous Si (alkaline extraction) for most of these soils. Consequently, heated soils might represent a strong and immediate source for plant available Si. Ecosystem characteristics (e.g., vegetation, soil type) that affect uptake-leaching balances determine the further fate of the released Si. In summary, heat might affect Si cycling not only by strongly increasing Si availability in soil organic matter but also in mineral soils. [ABSTRACT FROM AUTHOR]

Peatlands are globally important sinks and sources for greenhouse gasses. Their carbon (C) balances are dependent on input of biomass, its quality and decomposition. Silicon (Si) is known to influence the C quality, especially of graminoids, and it influences decomposition under oxic conditions. Drying – rewetting events (D/W), which may influence C mineralization and change nutrient availability, have been predicted to increase in frequency. This raises the question if effects of Si on decomposition are modified by such D/W events. To elucidate this question, we conducted a study with mesocosms extracted from a fen peatland. We hypothesized, that (I, II) there would be higher gas and solute concentrations after Si addition compared to the control after D/W. Further we expected (III) that higher respiratory fluxes of CO 2 and CH 4 would occur after Si addition and D/W and that (IV) there would be a higher gross primary production (GPP) under Si addition due to more viable biomass that is less prone to drought effects. Finally we hypothesized that (V) the chemical composition of Carex rostrata Stokes leaves grown under Si fertilization after a D/W cycle would be less recalcitrant compared to of C. rostrata leaves from the controls. To test these hypotheses, a control and a Si treatment group of mesocosms were subjected to a controlled D/W cycle. We measured soil gas (DIC and CH 4), and solute concentrations (P, Si, Fe and TOC) and gas fluxes of CO 2 and CH 4. Furthermore, we characterized the graminoid biomass for major element contents and by infrared spectroscopy (FTIR) for organic matter quality. Higher CH 4 fluxes were observed in the Si fertilized mesocosms. There was no difference between the treatments in the ER CO 2 fluxes, whereas the NEE and GPP fluxes indicated higher CO 2 uptake in the Si treated mesocosms. Solute concentrations showed no differences, neither for dissolved gasses , nor for solutes. C. rostrata leaves had lower shares of lignin, waxes, fats, and phenols in plants from the Si fertilized mesocosms. We concluded, that during D/W, plants from Si fertilized mesocosms accumulated more Si than plants from the control mesocosms. These plants were more viable, produced potentially more exudates and had more readily decomposable litter than controls, due to their lower share in recalcitrant moieties. This may have stimulated CH 4 production and emission of CH 4 from the soil into the atmosphere through abundant aerenchyma roots, bypassing oxic, methanotrophic peat layers. [ABSTRACT FROM AUTHOR]

• Si has divergent effects on greenhouse gas production depending on redox conditions. • With oxidized OM and ferric Fe phases, Si addition led to higher P concentrations. • Oxidized OM showed higher CO 2 and CH 4 production rates under Si addition. • No effect of Si with reduced organic matter (OM) and without ferric Fe phases. Peatlands store about 30% of the global soil carbon (C) stock. The decomposition of peat C in these systems depends on environmental parameters – such as water table levels and corresponding availability of electron acceptors for microbial respiration. Due to the latter, potential peat decomposition depends also on whether the material is initially oxidized or reduced prior to decomposition experiments. Recent studies revealed the importance of silicon (Si) for peat decomposition. High amounts of biogenic Si were found in peatlands, in particular in minerotrophic fens, and the importance of Si for graminoids and decomposability of respective litter has been widely discussed. Furthermore, the availability of Si was reported to influence the binding of phosphorus (P) to iron (Fe) and thereby the conditions under which decomposition proceeds. Yet the influence of Si on greenhouse gas production in peat under different initial redox conditions is largely unknown. Therefore, we intended to test the effect of Si on greenhouse gas production under different initial electron acceptor availabilities for microbial respiration, such as the availability of ferric Fe. We conducted two incubation experiments with initially oxidized and reduced peat organic matter (OM). We hypothesized that Si can mobilize P from Fe minerals, which increases microbial activity, and leads to higher production rates of carbon dioxide (CO 2) and methane (CH 4). Using the two different materials, we studied how initial redox conditions would modify effects of Si. As the predominant form of Fe as either ferric Fe-(oxy)hydroxides or as ferrous Fe minerals (sulfides, carbonates) is important for interaction with Si, we further hypothesized that Si effects should be stronger in initially oxidized peat in presence of ferric Fe, compared to initially reduced peat with ferrous Fe only. For incubation experiments using formerly oxidized material the Si addition increased P concentrations in the pore water, and more CO 2 was produced. The onset of methanogenesis was much stronger with than without addition of Si, indicating a more rapid depletion of electron acceptors by faster rates of respiration. We explain this by more P being available stimulating microbial activity, and also by a direct effect of Si on microbial activity and methanogenesis. The incubation of formerly reduced OM did not show any effects of Si on respiration processes, presumably due to the absence of ferric Fe phases. In conclusion, there was a clear difference in the effect of Si addition on decomposition of formerly oxidized compared to long term reduced OM, with only oxidized peat OM or peat with ferric Fe phases present showing clear Si effects. Consequently, redox conditions and availability of ferric Fe are a main control for Si effects on OM decomposition and nutrient availability. Little effects of Si can be expected under permanently reducing conditions and in absence of ferric Fe phases. [ABSTRACT FROM AUTHOR]