Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

The quantification of silicon (Si) uptake by tree species is a mandatory step to study the role of forest vegetations in the global cycle of Si. Forest tree species can impact the hydrological output of dissolved Si (DSi) through root induced weathering of silicates but also through Si uptake and restitution via litterfall. Here, monospecific stands of Douglas fir, Norway spruce, Black pine, European beech and oak established in identical soil and climate conditions were used to quantify Si uptake, immobilization and restitution. We measured the Si contents in various compartments of the soil-tree system and we further studied the impact of the recycling of Si by forest trees on the DSi pool. Si is mainly accumulated in leaves and needles in comparison with other tree compartments (branches, stembark and stemwood). The immobilization of Si in tree biomass represents less than 15% of the total Si uptake. Annual Si uptake by oak and European beech stands is 18.5 and 23.3 kg ha -1 year -1 , respectively. Black pine has a very low annual Si uptake (2.3 kg ha -1 year -1 ) in comparison with Douglas fir (30.6 kg ha -1 year -1 ) and Norway spruce (43.5 kg ha -1 year -1 ). The recycling of Si by forest trees plays a major role in the continental Si cycle since tree species greatly influence the uptake and restitution of Si. Moreover, we remark that the annual tree uptake is negatively correlated with the annual DSi output at 60 cm depth. The land-ocean fluxes of DSi are certainly influenced by geochemical processes such as weathering of primary minerals and formation of secondary minerals but also by biological processes such as root uptake.

Abstract. Silicon (Si) released as H 4 SiO 4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0

The quantification of silicon isotopic fractionation by biotic and abiotic processes contributes to the understanding of the Si continental cycle. In soils, light Si isotopes are selectively taken up by plants, and concentrate in secondary clay-sized minerals. Si can readily be retrieved from soil solution through the specific adsorption of monosilicic acid (H 4 SiO 4 0 ) by iron oxides. Here, we report on the Si-isotopic fractionation during H 4 SiO 4 0 adsorption on synthesized ferrihydrite and goethite in batch experiment series designed as function of time (0-504 h) and initial concentration (ic) of Si in solution (0.21-1.80 mM), at 20°C, constant pH (5.5) and ionic strength (1 mM). At various contact times, the d 29 Si vs. NBS28 compositions were determined in selected solutions (ic = 0.64 and 1.06 mM Si) by MC-ICP-MS in dry plasma mode with external Mg doping with an average precision of ±0.08& (±2r SEM ). Per oxide mass, ferrihydrite (74-86% of initial Si loading) adsorbed more Si than goethite (37-69%) after 504 h of contact over the range of initial Si concentration 0.42-1.80 mM. Measured against its initial composition (d 29 Si = +0.01 ± 0.04& (±2r SD )), the remaining solution was systematically enriched in 29 Si, reaching maximum d 29 Si values of +0.70 ± 0.07& for ferrihydrite and +0.50 ± 0.08& for goethite for ic 1.06 mM. The progressive 29 Si enrichment of the solution fitted better a Rayleigh distillation path than a steady state model. The fractionation factor 29 e (±1r SD ) was estimated at À0.54 ± 0.03& for ferrihydrite and À0.81 ± 0.12& for goethite. Our data imply that the sorption of H 4 SiO 4 0 onto synthetic iron oxides produced a distinct Si-isotopic fractionation for the two types of oxide but in the same order than that generated by Si uptake by plants and diatoms. They further suggest that the concentration of light Si isotopes in the clay fraction of soils is partly due to H 4 SiO 4 0 sorption onto secondary clay-sized iron oxides.

The determination of the plant-induced Si-isotopic fractionation is a promising tool to better quantify their role in the continental Si cycle. Si-isotopic signatures of the different banana plant parts and Si source were measured, providing the isotopic fractionation factor between plant and source. Banana plantlets (Musa acuminata Colla, cv Grande Naine) were grown in hydroponics at variable Si supplies (0.08, 0.42, 0.83 and 1.66 mM Si). Si-isotopic compositions were determined on a multicollector plasma source mass spectrometer (MC-ICP-MS) operating in dry plasma mode. Results are expressed as d 29 Si relative to the NBS28 standard, with an average precision of ± 0.08& (±2r D ). The fractionation factor 29 e between bulk banana plantlets and source solution is -0.40 ± 0.11&. This confirms that plants fractionate Si isotopes by depleting the source solution in 28 Si. The intra-plant fractionation D 29 Si between roots and shoots amounts to -0.21 ± 0.08&. Si-isotopic compositions of the various plant parts indicate that heavy isotopes discrimination occurs at three levels in the plant (at the root epidermis, for xylem loading and for xylem unloading). At each step, preferential crossing of light isotopes leaves a heavier solution, and produces a lighter solution. Si-isotopic fractionation processes are further discussed in relation with Si uptake and transport in plants. These findings have important implications on the study of continental Si cycle.

Three contrasted genotypes of Musa spp. (M. acuminata cv Grande Naine, M. acuminata spp. Banksii and M. balbisiana spp. Tani) were grown for 6 weeks under optimal conditions in hydroponics and were submitted to a wide range of Si supply (0-1.66 mM Si) to quantify the Si uptake and distribution in banana, as well as the effect of Si on banana growth. The level of Si supply did not affect plant growth, nor the rate of water and nutrient uptake. The rate of Si uptake and the Si concentration in plant tissues increased markedly with the Si supply. At the highest Si concentrations (1.66 mM), silicon absorption was essentially driven by mass flow of water (passive transport). However, at lower Si concentrations (0.02-0.83 mM), it was higher than its uptake by mass flow and caused the depletion of silicon in the nutrient solution, suggesting the existence of active processes in silicon transport. The distribution of silicon among shoot organs (pseudostem < petiole and midrib < young lamina < old leaf) confirmed the major role of transpiration in silicon accumulation and was not dependent on silicon supply. However, other mechanisms of transport might be operating in the roots and in the petiole and midrib of young leaves, whose silicon concentration was unexpectedly high at low Si supply (0.02 mM) compared to higher levels of Si. The three genotypes did not exhibit consistent differences in their responses to silicon supply.

Summary 1.Soil is the primary source of plant silicon (Si) and therefore a key reservoir of the Si biological cycling. Soil processes control the stock of Si-bearing minerals and the release of dissolved Si (DSi), hence the Si fluxes at the Earth's surface. Here, we review the interdependent relationship between soil processes and the return of plant Si in soils, and their controls on the biological Si feedback loop. 2. Dissolution and precipitation of soil silicate minerals govern the bioavailability of Si. Plants affect Si biocycling through mineral weathering, root uptake, phytolith formation, return and dissolution in soil. Thus, soil processes and Si biocycling readily interact in soil-plant systems. 3. Rock mineral weathering and soil formation are driven by the five soil-forming factors: parent rock, climate, topography, age and biota. These factors govern Si fluxes in soil-plant systems since they impact both the mineral weathering rate and fate of DSi. The variability of soil-forming factors at a global scale explains both the soil diversity and high variability of the rates of Si cycling in terrestrial ecosystems. 4. Plants play a crucial role in soil evolution by promoting weathering and forming phytoliths (plant silica bodies). They thus act as Si sinks and sources. With increasing depletion of lithogenic (LSi) and pedogenic (PSi) silicates, the biological Si feedback loop progressively takes over the Si plant uptake from weatherable LSi and PSi minerals. With rising weathering, the soil becomes increasingly concentrated in phytoliths, phytogenic amorphous silicates (PhSi), which are constantly formed in plant and dissolved in soil. Paradoxically, the Si biocycling is thus more intense in soils depleted in primary LSi source. By converting soil LSi and PSi into PhSi, plants increase the mobility of Si in soil and alleviate desilication in the topsoil. Nonessential plant Si is therefore an essential link between mineral and living worlds. 5. The dynamics of Si in terrestrial ecosystems is thus largely governed by pedogenesis and its relationship with plant community and diversity. Consequently, the appraisal of soil constituents and processes is central to further understand their interaction with the biological Si feedback loop.

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