Silicon Mobilization in Soils: the Broader Impact of Land Use

Dissolved Si (DSi) provision from land systems triggers diatom growth and CO2 sequestration. Soils and ecosystems act as a Si “filter”, transforming DSi originated from mineral weathering into biogenic Si (BSi) after DSi uptake by plants, or into other pedogenic forms of Si (non-BSi). Land use changes the quantity of BSi and non-BSi pools along the soil profile. However, methods used to isolate Si pools include chemical extractions at high temperatures and alkaline environments and therefore are unable to provide information concerning the dissolution potential of BSi and non-BSi pools under normal conditions of temperature and pH. Here, we conducted a batch experiment where forest, pasture and cropland soil samples were mixed with water at 25 °C and pH 7. The soil samples were collected from a temperate land use gradient located in the Belgian Loess Belt. We measured dissolved Si and aluminium (Al) during 80 days. BSi and non-BSi pool contents along the soil profile were known, as they had been established previously through chemical extraction. Results show that BSi and non-BSi enriched samples present distinct Si and Al dissolution curves. While non-BSi pools contribute significantly with immediate availability of Si, BSi pools present an initial slow dissolution. Therefore, croplands that were depleted of phytoliths and had poorly organic horizons display higher concentrations of initial dissolved Si, while pastures and forests, where pedogenic pools dominate only at depths below 40 cm, have more limited initial Si release.info:eu-repo/semantics/publishedVersio

Ground Deformations Related to an Old Drainage Adit in The Abandoned Coal Concession Around Saint-Vaast (Wallonia, Belgium) Analyzed Using PS InSAR and Piezometric Wells Time Serie

peer reviewedmonitoring LAnd SUbsidence caused by Groundwater exploitation through gEOdetic measurements15. Life on lan

Opal-CT precipitation in a clayey soil explained by geochemical transport model of dissolved Si (Blégny, Belgium)

Opal-CT precipitation controlling dissolved Si export Dissolved Si (DSi) exported by rivers are controlled by geological, hydrological and biological cycle processes [1]. The DSi concentrations measured in a river of an upstream catchment in eastern Belgium (Blégny, Land of Herve) don’t vary seasonally (6.91±0.94mgL-1; n=363). Si concentrations in pore water are often higher and vary more (8.65±3.65mgL-1; n=128). The decrease of DSi along the flowpath of water is due to sink processes, i.e. precipitation, adsorption or uptake by vegetation. As the DSi in the river does not show any seasonal variation, uptake by vegetation can be ruled out [1] whereas precipitation or adsorption can control the DSi drained by the stream water. This hypothesis is confirmed by XRD and DeMaster analysis. At 0.1m depth the soil is constituted of 62% quartz, 7% K-feldspar, 6% plagioclase, 3.2% carbonates, 18.9% Al-clay, 1.47% Kaolinite, 0.63% Chlorite and 0.2% amorphous Si, probably of biogenic origin. At 1.5m depth, the amounts of several minerals (35.8% quartz, 0.6% K-feldspars, 0.9% plagioclase, Al-clay 14.7%) drop drastically. Carbonates, chlorite and kaolinite are absent whereas 40.4% opal-CT appears. The precipitation of opal-CT controls the DSi export of this catchment. Development of geochemical transport model To descripe DSi export from a catchment a geochemical transport model is developped in HP1 which couples the water flux model Hydrus with the geochemical model PHREEQC [2]. Our model is based on the conceptual model developped in [3]. First results show different DSi export dynamics in the unsaturated zone than in the aquifer due to different pCO2 values and varying soil moisture conditions. Further development of the model will help to find out the reason of opal-CT precipitation in this setting. [1]Fulweiler, Nixon (2005) Biogeochemistry 74:115–130. [2] Simunek, Jacques, van Genuchten, Mallants (2006) JAWRA 42:1537-1547. [3] Ronchi et al. (2013). Silicon, 5(1), 115–133

Controlling factors of dissolved Silicon in upstream catchments with different land uses

Dissolved Si (DSi) is an important nutrient in aquatic systems. River discharge delivers DSi to the oceans. Riverine DSi fluxes are affected by land use changes as DSi export from forests are higher than from croplands. Hypothesis were suggested to explain the differences in DSi export but the causes were poorly analyzed. Previous studies mentioned the higher amount of biogenic Si (BSi) in forest soils as the main reason for higher DSi fluxes from forests, since BSi is an important source of DSi in soils. This hypothesis was confirmed with a leaching test. Our experiment proved BSi in soils reacted kinetically in contrast to other soil minerals. In intensive agriculture, no kinetic reaction occurred: the small amount of BSi (2-3 mg Si g-1 soil) became inert. We made a conceptual model based on a literature review which suggested that other parameters would also influence DSi export. The leaching test showed Si release was high at pH <4 and varied with water flux rates. Dissolution processes were enhanced in the acid forest soils compared to neutral cropland soils. An increase in water flux with a factor 2.5 resulted in an increase in Si flux with a factor close to 2.5 in cropland, where reactions were (near) equilibrium. The impact of water flux variation was less important in the forest (factor 1.36 - 2.10). Chemical analysis of soil water sampled in situ proved that the biogeochemical Si cycle is controlled by different processes along a land use gradient. In forests, dissolved Si can complex with Al and clays (illite, smectite) dissolution/precipitation likely affects with the Si cycle. In arable land, hydrological conditions clearly control Si concentrations as they are inversely correlated with soil water content. To quantify DSi fluxes through soils, we simulated the kinetic reaction of BSi in soils and evaluate the effect of recent deforestation on DSi fluxes. Therefore, we set up a numerical model which coupled transport and geochemical calculations in the software HP1. Due to the increase in water flux after deforestation, the amounts of reacted BSi decreased but the total Si flux exported from the soil was higher. Our model also proved that clay stability was influenced by the chemistry of the infiltrating rain: cation-rich throughfall solution enhanced clay stability compared to rain water. We also showed that Si reprecipitation could affect significantly Si export from a catchment. In one of the studied catchments, we noticed a clear decrease in DSi concentrations from groundwater to the river. Precipitation of opal-CT was also observed in a coring. Our calculations showed that currently in this catchment DSi flux was 25% lower than expected, due to the Si reprecipitation. Finally we compared DSi fluxes through the soil from all land uses. Deforestation could increase the DSi flux of 66% compared to forest. The development of agriculture decreased DSi export of 33% in the grassland and 44% in the cropland compared to the forest. Our study proved that the export of DSi is controlled by different processes according to the land use type and the regional geochemical conditions (precipitation of opal-CT). In forests, DSi export from soil is the high as important BSi amounts are present and the soils are acid. After deforestation, the increase in water flux increases the DSi flux exported from the soil. When intensive agriculture develops, soils become neutral, BSi amounts diminish and become less reactive, water infiltration diminishes. All these processes together cause a decrease in DSi export along the land use gradient.1 Problem statement and research questions 1.1 Background 1.2 Research questions 1.3 Thesis outline 2 Transport of dissolved Si from soil to river: a conceptual mechanistic model 2.1 Introduction 2.2 Sources and sinks of Si in soils 2.2.1 Typology of Si-particles 2.2.2 Si dissolution and weathering 2.2.3 Sinks of DSi 2.2.4 Towards an integrated conceptual model 2.3 Delivery of Si from the soil to the river 2.4 External forces altering internal dynamics 2.4.1 Effect of land use 2.4.2 Seasonal climatic variation 2.5 Discussion and conclusions 3 Controls on Si export from different land uses: a soil column experiment 3.1 Introduction 3.2 Material and methods 3.2.1 Study sites and soil sampling 3.2.2 Column experiment 3.2.3 Si concentration 3.2.4 Si flux calculations 3.2.5 Dissolution rates 3.3 Results 3.3.1 Si release from A and B horizons 3.3.2 Equilibrium dissolution flux 3.3.3 Relation between pH and Si 3.3.4 Si dissolution rates 3.4 Discussion 3.4.1 Time dependency of DSi concentrations 3.4.2 Differences in DSi dissolution rates 3.4.3 Equilibrium vs. rate control 3.4.4 Turnover rates 3.4.5 Synthesis 3.5 Conclusions 4 Biogeochemical cycling of Si in temperate climates: land use impact and implication of clay in the Si cycle 45 4.1 Introduction 4.2 Study areas 4.3 Methodology 4.4 Results 4.4.1 Soil 4.4.2 Soil water 4.4.3 Spearman’s rank-order correlation 4.4.4 Speciation of DSi 4.4.5 Saturation indexes 4.5 Discussion 4.5.1 Forest 4.5.2 Pasture 4.5.3 Arable 4.5.4 Comparison of the different land uses 4.5.5 Uncertainties 4.6 Conclusions and Perspectives 5 One-dimensional numerical model of Si transport in a forest soil and in a deforested soil 5.1 Introduction 5.2 Method 5.2.1 Kinetic BSi dissolution 5.2.2 Study area 5.2.3 Modelling of a soil profile 5.3 Results 5.3.1 Approximation of BSi kinetics 5.3.2 Si concentrations in the soil column models 5.3.3 Processes controlling Si concentrations in the unsaturated zone 5.3.4 BSi dissolution 3.5 Clay saturation indices 5.4 Discussion 5.4.1 Net Si flux 5.4.2 The effect of water flow (TF and R vs R-200) 5.4.3 Chemical composition of influent (TF vs R and R-200) . 5.4.4 Uncertainties on the models 5.5 Conclusion 6 Opal-CT precipitation controls DSi export towards river Lhonneux (Blégny, Belgium) 6.1 Introduction 6.2 Material and methods 6.2.1 Site description 6.2.2 Methods 6.2.3 Rates of Opal-CT precipitation 6.2.4 Simulation of opal-CT precipitation in PhreeqC 6.3 Results 6.3.1 Soil analysis 6.3.2 Chemical composition of water 6.3.3 Potential rates of Opal-CT precipitation 6.3.4 Opal-CT precipitation in PhreeqC 6.4 Discussion 6.4.1 Formation of Opal-CT 6.4.2 Source of high DSi concentrations 6.5 Conclusion and perspectives 7 General conclusions and recommendations for further research 7.1 Research Questions 7.2 General conclusions and implications 7.3 Suggestions for further researchstatus: publishe

Fate of TiO2 Nanoparticles in Carbonate and Silicate Aquifers

The increasing use of engirneered nanoparticles leads to their relaese in the environment, i.e. in aquifers. However, their transport through aquifers remains unclear up until now. Nanoparticle stability in solutions depends on its surface charge which varies with pH conditions but also on the ionic strength of the solution. Consequently, nano-sized TiO2 (nTiO2) are expected to behave differently in silicate and carbonate aquifers as groundwater chemistry is influenced by the host rock mineralogy. To assess the vulnerability of these types of aquifers to engineered nanoparticles, the stability of nTiO2 is evaluated in four different types of water pumped from carbonate and silicate aquifers and in Milli-Q water. A suspension of nTiO2 (30 g/l, pH 1.3 with HNO3) was diluted in each type of water (2 replicates) to reach a concentration of 30 ppm and sonicated at 330W during 3 minutes. These suspensions were then allowed to settle undisturbed for up to 5 days for a stabilisation experiment with daily measurements of Ti concentrations. Diameters of the particles were measured for samples of the first day of the stabilisation experiment but also on a second bench of samples. These suspensions were prepared by diluting the same nTiO2 suspension in filtered groundwater (0.45 μm). TiO2 concentrations in supernatant Milli-Q solution (pH= 3.9) were stable during the experiment. In contrast, nTiO2 concentrations measured in groundwater (pH=7.6±0.5) dropped from 30 ppm to ca 1 ppm in 1 day. Sedimentation rates were slightly higher for water from carbonate aquifers compared to silicate aquifers during the first day. After two days, this difference was already insignificant. Diameters of particles present in the samples were the largest in natural groundwater, smaller in filtered groundwater and the smallest in Milli-Q water. This experiment proved that groundwater composition favours rapid aggregation, the likely effect of pH-conditions and the presence of natural suspended particles. Further analyses are needed to determine how these aggregates behave in the aquifer matrix

Temporal dynamics of bio-available Si fluxes in a temperate forested catchment (Meerdaal forest, Belgium)

Silicon (Si) is a key element in global biogeochemical cycling and recent research has shown that changes in the biological component of the Si cycle may lead to more rapid variations in the land–ocean Si transfer than previously thought. The objective of this paper is to better understand the controls on temporal Si dynamics in terrestrial ecosystems, by studying Si fluxes from a small forested catchment in central Belgium. An intensive monitoring program (2008–2010) of dissolved and amorphous silica (DSi and ASi) concentrations and load patterns show that DSi concentrations are significantly lower during winter–spring periods than during summer–autumn periods. In contrast to what was found in other studies, seasonal dynamics in Meerdaal forest are not controlled by variations in biological uptake or temperature, but mainly by the more important supply of pore-water to the groundwater table in winter–spring periods. Analysis of seasonal and event fluctuations in stream water DSi concentrations showed that final stream water is a mixture of old, DSi rich water pushed out of the soil, and new, DSi poor water delivered by quick flow. The mixing of old and new water finally resulted in streamwater DSi concentrations responding only moderately to variations in discharge (near-chemostatic behaviour). We estimated the total DSi export from the system to be ca. 65.1 9 103 mol km-2 year-1. Because Si delivery is biologically regulated through an important Si cycle in the vegetation-soil continuum, an anthropogenic (e.g. agricultural expansion) or climatic disturbance of terrestrial ecosystems may alter both water residence times through shifts in hydrological regimes and the DSi chemical equilibrium concentration in soils. In turn, these perturbations will potentially alter long-term DSi and ASi inputs to aquatic systems

Halloysite occurrence at the karstified contact of Oligocene sands and Cretaceous calcarenites in Hinnisdael quarries, Vechmaal (NE of Belgium)

© 2017, Geologica Belgica. All rights reserved. In Belgium, numerous karstified sand-carbonate contacts are known for their association with halloysite precipitation. In the Hinnisdael caves of Vechmaal, eastern of Belgium, a similar geological setting is observed in dolines of karstified calcarenite of the Cretaceous Maastricht Formation filled with sand of the Oligocene St. Huibrechts-Hern Formation. At the sand-calcarenite interface, a discontinuous succession occurs of white clay and oxidized rust almost perfectly segregated. These lithologies are authigenic precipitations of high purity consisting of only halloysite-allophane and goethite-ferrihydrite respectively. The Al, Si and Fe necessary for this precipitation were derived from the short-range distance dissolution of glauconite and other silicate minerals present in the overlying sand unit. The mobilization of these ions was realized by the progressive oxidation of pyrite to jarosite leading to a significant acidification of the percolating water. It is furthermore observed that seemingly small variations in local conditions in the Curfs quarry of Valkenburg, the Netherlands, result in alunite, gibbsite, halloysite, allophane precipitation instead of jarosite, halloysite-allophane and goethite-ferrihydrite.status: publishe

Identifying the Transport Pathways of Dissolved Organic Carbon in Contrasting Catchments

Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end-member mixing analysis to determine the contributing end-members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end-members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone. © Soil Science Society of America.status: publishe

Silicon mobilization in soils : the broader impact of land use

Dissolved Si (DSi) provision from land systems triggers diatom growth and CO2 sequestration. Soils and ecosystems act as a Si “filter”, transforming DSi originated from mineral weathering into biogenic Si (BSi) after DSi uptake by plants, or into other pedogenic forms of Si (non-BSi). Land use changes the quantity of BSi and non-BSi pools along the soil profile. However, methods used to isolate Si pools include chemical extractions at high temperatures and alkaline environments and therefore are unable to provide information concerning the dissolution potential of BSi and non-BSi pools under normal conditions of temperature and pH. Here, we conducted a batch experiment where forest, pasture and cropland soil samples were mixed with water at 25 °C and pH 7. The soil samples were collected from a temperate land use gradient located in the Belgian Loess Belt. We measured dissolved Si and aluminium (Al) during 80 days. BSi and non-BSi pool contents along the soil profile were known, as they had been established previously through chemical extraction. Results show that BSi and non-BSi enriched samples present distinct Si and Al dissolution curves. While non-BSi pools contribute significantly with immediate availability of Si, BSi pools present an initial slow dissolution. Therefore, croplands that were depleted of phytoliths and had poorly organic horizons display higher concentrations of initial dissolved Si, while pastures and forests, where pedogenic pools dominate only at depths below 40 cm, have more limited initial Si release.info:eu-repo/semantics/publishedVersio

Identifying the Transport Pathways of Dissolved Organic Carbon in Contrasting Catchments

An extensive sampling study identified the transport pathways of dissolved organic C to the surface water in contrasting catchments. An end-member mixing analysis revealed that the transport pathways of dissolved organic C can differ seasonally and highly depend on the degree of saturation of the vadose zone.Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end-member mixing analysis to determine the contributing end-members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end-members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone