Al Pool Research Articles

Coupled Si–Al Biogeochemistry: Occurrence of Aluminum in Diatom‐Derived Biogenic Silica

AbstractDiatoms play an important role in the biogeochemical cycling of aluminum (Al) in oceans. This occurs via the uptake of biological Al (Albio), which is incorporated into the structure of diatom‐derived biogenic silica (DBSi) and the formation of adsorbed Al (Alads) on the DBSi surface of post‐mortem diatoms. Al occurrence influences DBSi dissolution and thus diatom‐driven carbon sequestration. However, the mechanism of Al occurrence in DBSi remains unclear. In this study, Albio and Alads of DBSi from various diatom fossils in marine diatomaceous sediments were identified and quantified by combined focused ion beam thinning, elemental analysis, and the Al K‐edge X‐ray absorption near edge structure. Results showed the coexistence of Albio and Alads in all diatomaceous sediments and Al‐bearing DBSi thus constitutes a biological Al pool. Albio and Alads were mainly fourfold‐ and sixfold‐coordinated, respectively and Alads was much more abundant than Albio. Moreover, even at low concentrations (with an Al/Si atomic ratio of 0.0031), Albio can inhibit DBSi dissolution, effectively decreasing the extent of DBSi dissolution by ∼14%. Albio also significantly increased the mechanical strength of DBSi. The average Young's modulus (a measure of the stiffness of a material) of cribrum layers in Al‐incorporated DBSi was ∼1.4 times higher than that of Al‐free DBSi. Our results further demonstrate that diatoms play a dominant role in the biogeochemical cycling of Al in oceans, and the Al of DBSi participates in diatom‐driven Si and C coupled cycles in oceans, influencing the effectiveness of diatom‐driven carbon export by regulating the dissolution and mechanical strength of DBSi.

Operando X-ray diffraction study of thermal and phase evolution during laser powder bed fusion of Al-Sc-Zr elemental powder blends

Elemental powder blends are an emerging alternative to prealloyed powders for high-throughput alloy design via additive manufacturing techniques. Elemental Al+Sc(+Zr) powder blends were processed by laser powder bed fusion into Al-Sc and Al-Sc-Zr alloys, with operando X-ray diffraction at the Swiss Light Source extracting the structural and thermal history of the process. The pure Sc and Zr particles were found to react with the molten Al pool at 550–650 °C, well below their respective melting temperatures. Various scan areas (1 × 1, 2 × 2, 4 × 4, and 8 × 2 mm 2 ) were studied to compare (i) the base plate “preheating” effect caused by prior laser scans, (ii) the return temperature reached after the melting scan and before the following scan, (iii) the initial cooling rate immediately after solidification, and (iv) the time spent in the “intrinsic heat treatment range”, defined as 300–650 °C, where secondary Al 3 (Sc,Zr) precipitation occurs. Microstructural analysis of the as-built samples show 110–140 nm L1 2 -Al 3 (Sc,Zr) primary precipitates at the bottom of the melt pool. The 1 × 1 mm 2 samples exhibit the most elongated grains (long axis of 10 ± 5 µm), which correlates with the highest build plate temperature and the slowest initial cooling rate (3–5 × 10 5 K/s). In comparison, the 4 × 4 mm 2 samples exhibit the smallest equiaxed grains (2 ± 0.6 µm), corresponding to the lowest build plate temperature and the fastest initial cooling rate (6–7 × 10 5 K/s). These results indicate the need for establishing a minimum feature size during part design, or for modifying the laser parameters during processing, to mitigate microstructure and performance differences across features of different sizes.

Soil organic carbon accrual due to more efficient microbial utilization of plant inputs at greater long-term soil moisture

High long-term soil moisture may either stimulate or inhibit soil organic carbon (SOC) losses through changes to mineral and chemical composition, and resultant organo-mineral interactions. Yet, the trade-off between mineralization and accrual of SOC under long-term variation in unsaturated soil moisture remains uncertain. We tested the underexplored relationships between long-term soil moisture and organo-mineral chemical composition and its implications for SOC persistence in an experimental field in New York, USA, with differences in long-term mean soil volumetric water content (0–0.15 m depth) ranging from 0.40 to 0.63 (v/v) during the growing season. Long-term soil moisture across 20 subplots on four fallow plots were positively correlated with SOC (R2 = 0.23; P = 0.019, n = 20), mineral-associated organic matter (MAOM) content (g fraction/g soil) (R2 = 0.44; P = 0.001; n = 20) and occluded particulate organic matter (oPOM) content (R2 = 0.18; P = 0.033; n = 20). Higher long-term soil moisture was associated with a decrease in the relative content of sodium pyrophosphate extractable Fe (R2 = 0.33; P < 0.005; n = 20), an increase in sodium dithionite extractable Fe (R2 = 0.44; P < 0.001; n = 20), and an increase in SOC retention by non-crystalline Al pools (R2 = 0.51; P = 0.0002 for sodium pyrophosphate extracts, R2 = 0.41; P = 0.0014 for hydroxylamine hydrochloride extracts; n = 20 for both). Increasing long-term soil moisture was associated with a four-fold increase in microbial biomass C (per unit SOC) and lower metabolic quotient (R2 = 0.56, P < 0.001). MAOM fractions of high-moisture soils had lower C:N (from C:N 9.5 to 9.0, R2 = 0.27, P = 0.011, n = 20). Consistent with decreasing C:N, increasing decomposition with increasing moisture was reflected by a 15% and 10% greater proportion of oxidized carboxylic-C to aromatic-C and O-alkyl C, respectively, as measured with 13C NMR, and a more pronounced FTIR signature of N-containing proteinaceous compounds in high-moisture MAOM fractions, indicative of microbial metabolites and transformation products. A partial least squares regression showed that SOC content increased with greater long-term moisture (P = 0.019), pyrophosphate-extractable Al (P = 0.0001), and exchangeable Ca (P = 0.013). Taken together, our results show that higher long-term soil moisture resulted in SOC accrual by enhancing conversion of plant inputs into microbial biomass that interacts with reactive minerals.

Spatio-temporal ablation dynamics and plasma chemistry of aluminum induced by temporally modulated ytterbium fiber laser

In this work, we studied single-pulse ablation dynamics of a temporally modulated continuous wave laser–material interaction with Al using in situ multimodal time-resolved diagnostics that describe in detail the associated physical and chemical processes. Time-resolved scattering, emission imaging, and optical emission spectroscopy unveiled a sequence of events spread out across three distinct phases: (i) early phase ablation process, associated with particle generation and liquid Al column formation (&amp;lt;20 μs), (ii) secondary detonation when sufficient ejected material is accumulated over the surface (20–50 μs), and (iii) molten liquid Al pool oscillation on the surface, followed by large droplet ejection from the liquid pool (100–500 μs). Atomic Al and AlO were observed with optical emission spectroscopy at different ratios during the entire lifetime of the event, verifying the formation of oxidized Al vapor upon its interaction with air. Morphological and compositional characterization confirmed surface oxidation and material re-solidification in the form of protrusions produced during the irradiation process. This work provides insights into the complex physical and chemical mechanisms of single-pulse ablation in the sub-millisecond laser pulse regime, which are critically important for parameter optimization in a variety of laser processing, microfabrication, and deposition applications.

Interaction of land use history, earthworms, soil chemistry and tree species on soil carbon distribution in managed forests in Vermont, USA

Forest soils store a globally important pool of carbon (C) and reforestation has the potential to increase this pool. Past land use, tree species composition, and current management are known to affect carbon storage rates and amounts. Another important factor, especially in the vertical distribution of soil C, is the presence or absence of earthworms. We investigated these interactions in eighteen managed forest sites in Vermont that have varied land use history (cultivation, pasture, woodlot, continuous woodland) and a range of time since reforestation from 60 to 100+ years. Soils were sampled to about 1 m, or shallower if bedrock was reached, and both C and exchangeable cation pools quantified. Earthworms were enumerated to a depth of 20 cm and identified by species. Basal area by tree species was also measured and aboveground C calculated. Detailed land use history was documented using archival sources. Forest type was primarily northern hardwood, with a few lowland spruce-fir sites included. Eleven different species of earthworms were found, representing all ecotypes, and numbers ranged from 0 (at 8 of the 18 sites) to 319 individuals/m2. Carbon pools in the forest floor (1.5–30.1 Mg/ha) were greater with higher site elevation (range of 154–651 masl) and also greater with lower numbers of earthworms. Besides elevation, the best predictor of earthworm presence or absence, and overall numbers, was the soil exchangeable calcium (Ca) pool in the Oa and A horizons (range of 9.4–1288 cmolc/m2). Because soil exchangeable Ca and aluminum (Al) pools were inversely related, higher exchangeable Al was associated with lower earthworm numbers. In the mineral soil, the C pool (82.1–210.8 Mg/ha) was best explained by the thickness of the B horizons with the exchangeable Al pool as a positive, secondary factor. The thickness of the A horizon (0–16.3 cm), and its contribution to the entire profile C pool (0–54.7 Mg/ha), decreased relative to past land use in the order: cultivated < pasture < woodlot. No earthworm metric was strongly related to the C pool in either the mineral soil or the full soil profile. Dramatic differences in the vertical distribution of carbon were related to past land use and earthworm presence, which in turn was negatively correlated with elevation and the presence of conifers. While it is clear that earthworms and prior land-use have a long-lasting effect on the forest floor C pool, it is less clear whether or not there is an effect on full profile C sequestration over time.

Aluminum geochemical characterization in agricultural soils: sequential chemical extraction combined with mineralogical analysis of the fine fraction

Understanding both the concentration and distribution of aluminum (Al) in agricultural soils is essential to provide a benchmark on the productivity of such soils. In this study, a combination of sequential chemical extraction and mineralogical investigation of the fine fraction was used in order to evaluate Al pools in agricultural soils in Galicia (NW Spain), an Atlantic European region where the wet climate favors leaching of soil basic cations. Determination of Al (soluble/exchangeable/specifically adsorbed, bound to manganese oxides, associated with amorphous compounds, bound to oxidizable organic matter, associated with crystalline iron oxides, and residual fraction) in the topsoil samples was performed to evaluate its potential environmental availability in the study area. Aluminum contents were determined by flame atomic absorption spectrophotometry and the mineralogy of the fine fraction of these soils was studied by X-ray diffraction (XRD). The results showed that Al is mainly in the residual phase, which represents Al incorporated in the lattice minerals, and consequently the threat of toxicity to plants and the environment is reduced. The XRD results showed the presence of hydroxy-Al interlayered vermiculites and kaolinite, accompanied by small amounts of goethite, mica, and quartz.

Lake sedimentary biogenic silica from diatoms constitutes a significant global sink for aluminium

Diatoms play an important role in marine biogeochemical cycle of aluminum (Al), as dissolved Al is taken up by diatoms to build their siliceous frustules and is involved in the sedimentation of diatomaceous biogenic silica (BSi). The Al incorporation in BSi facilitates decreasing the dissolution of marine BSi and thus substantially influences the biochemical processes driven by diatoms, such as CO2 sequestration. However, the role of lake BSi in the terrestrial biochemical Al cycle has not been explored, though lakes represent the second-largest sink for BSi. By identifying the previously unexplored high Al/Si atomic ratios (up to 0.052) in lake BSi, here we show lake BSi is a large terrestrial Al pool due to its high Al content, and lake sedimentary BSi constitutes a significant global sink for Al, which is on the same magnitude as the Al sink in global oceans.

Aluminium in glacial meltwater demonstrates an association with nutrient export (Werenskiöldbreen, Svalbard)

The aluminium (Al) cycle in glacierised basins has not received a great deal of attention in studies of biogeochemical cycles. As Al may be toxic for biota, it is important to investigate the processes leading to its release into the environment. It has not yet been ascertained whether filterable Al (passing through a pore size of 0.45 μm) is incorporated into biogeochemical cycles in glacierised basins. Our study aims to determine the relationship between the processes bringing filterable Al and glacier‐derived filterable nutrients (particularly Fe and Si) into glacierised basins. We investigated the Werenskiöldbreen basin (44.1 km2, 60% glacierised) situated in SW Spitsbergen, Svalbard. In 2011, we collected meltwater from a subglacial portal at the glacier front and at a downstream hydrometric station throughout the ablation season. The Al concentration, unchanged between the subglacial system and proglacial zone, reveals that aluminosilicate weathering is a dominant source of filterable Al under subglacial conditions. By examining the Al:Fe ratio compared with pH and the sulphate mass fraction index, we found that the proton source for subglacial aluminosilicate weathering is mainly associated with sulphide oxidation and, to a lesser degree, with hydrolysis and carbonation. In subglacial outflows and in the glacial river, Al and Fe are primarily in the forms of Al(OH)4‐ and Fe(OH)3. The annual filterable Al yield (2.7 mmol m‐2) was of a magnitude similar to that of nutrients such as filterable Fe (3.0 mmol m‐2) and lower than that of dissolved Si (18.5 mmol m‐2). Our results show that filterable Al concentrations in meltwater are significantly correlated to filterable and dissolved glacier‐derived nutrients (Fe and Si, respectively) concentrations in glaciers worldwide. We conclude that a potential bioavailable Al pool derived from glacierised basins may be incorporated in biogeochemical cycles, as it is strongly related to the concentrations and yields of glacier‐derived nutrients.

Aluminum and acidity suppress microbial activity and biomass in acidic forest soils

To evaluate the effects of aluminum (Al) toxicity on microbial processes in acidic forest soils, we determined microbial biomass, soil enzyme activities associated with carbon (C), nitrogen, and phosphorus cycling (β-d-glucosidase and polyphenol oxidase, l-asparaginase, and acid phosphatase, respectively), exchangeable base cations, soluble and exchangeable Al (CaCl2–Al and KCl–Al), and organically bound (Alp and Fep) and both organically bound and noncrystalline forms (Alo and Feo) of Al and iron (Fe) in Japanese forest soils (25 Inceptisols, seven allophanic Andisols, and eight nonallophanic Andisols). The exchangeable calcium (Ca) and magnesium (Mg) concentrations decreased significantly with decreasing soil pH. In contrast, the KCl–Al and CaCl2–Al concentrations exponentially increased with decreasing pH, and the increase was steeper in Andisols than in Inceptisols. The Alp concentration also increased exponentially with decreasing soil pH in Andisols, and the Fep concentration increased in Andisols and Inceptisols, whereas the concentration of the noncrystalline Al pool (Alo − Alp) decreased with decreasing pH in Andisols. We showed that the β-d-glucosidase, polyphenol oxidase, and acid phosphatase activities were mainly suppressed by soluble and exchangeable Al, whereas l-asparaginase activity was depressed by soil pH. In Andisols, a negative effect of organically bound Al and Fe on the biomass C/organic C ratio was observed. These results suggest that Al toxicity and acidity repressed soil enzyme activities, leading to suppressed microbially mediated nutrient cycling, and that Al toxicity and a reduced availability of organic matter, as a result of Al and Fe binding, may protect a substantial pool of organic C from microbial degradation in acidic forest soils.

The biogeochemical behavior of dissolved aluminum in the southern Yellow Sea: Influence of the spring phytoplankton bloom

Many recent studies have investigated the nutrient-type profiles of dissolved aluminum (Al) in the ocean. Significant scavenging of dissolved Al can occur during phytoplankton blooms, but the mechanism remains unclear. The distribution of dissolved Al in the southern Yellow Sea (SYS) was investigated in winter and spring 2009. Following measurements at grid stations during the spring sampling cruise, two drifting anchor surveys of more than 100 h were conducted to trace the variation of dissolved Al concentration during the spring phytoplankton bloom (SPB). The concentration of dissolved Al in the SYS decreased from 40 nmol/L in February to 30 nmol/L in March and 10–20 nmol/L in April, while the concentration of Chl a increased from 4 μg/L in April. The concentration of dissolved Al in the SYS decreased significantly with the development of the phytoplankton bloom, which indicated biological scavenging of dissolved Al from water column. The proportion of dissolved Al scavenged from water column by different phytoplankton species differed at the two drifting stations, with greater removal efficiency demonstrated by diatoms than dinoflagellates. Phytoplankton samples collected from the Chl a maximum layer were washed with trace metal clean reagent (oxalate-EDTA-citrate, abbreviate as oxalate solution, Tovar-Sanchez et al., 2003) to enable the surface-scavenged (extracellular) and intracellular Al pools associated with phytoplankton to be differentiated. Thirty-nine to ninety-six percent of the total Al was found to be existed in the interior pools, which indicated that biological absorption was the important way to scavenge dissolved Al during phytoplankton blooms in the SYS.

Concurrent evolution of organic and mineral components during initial soil development after retreat of the Damma glacier, Switzerland

The foreland of the Damma glacier (Switzerland) was investigated to assess the build-up of organic carbon (OC) stocks in mineral soils and organic surface layers with increasing ice-free period (ca. 15, 60–70, 71–80 and 110–140 years). Mature soils outside the proglacial area (soil ages > 700 years) served as reference for advanced soil development. All soils were sampled in triplicate per surface age to estimate the variability of soil formation. Different selective dissolution methods were applied to quantify Fe and Al pools with respect to their role for soil organic carbon (SOC) stabilization during initial pedogenesis. The chemical composition of organic matter was characterized by using solid-state CPMAS 13 C NMR spectroscopy. Leptosols and Regosols were found in the glacier foreland which showed a high variability of development ranging from morphologically undeveloped to soils forming Ah horizons within 70 years. These different stages of soil development were present at a small scale within the same surface age according to past glacier movements. Particle-size distribution varied between soils of similar age and without chronological trend. These results point to the strong impact of different glacial deposition and subsequent glaciofluvial erosion, which was indicated by buried organic surface layers, on soil formation. In general, we found a rapid accumulation of OC in the mineral soils (7.1 g m −2 year −1) and organic surface layers with increasing soil age. Similarly, the amount of poorly crystalline Fe oxides and Al phases increased reflecting the growing potential for SOC stabilization. This was indicated by the strong relationship between SOC stocks and stocks of oxalate soluble Fe and Al. In contrast to strongly increasing quantities, only small changes in the composition of organic matter as well as Fe and Al pools were detected during initial pedogenesis. Fe oxides and inorganic Al phases mainly remained poorly crystalline. Our results point to the concurrent evolution of SOC and poorly crystalline Fe oxides and Al phases with positive feedback mechanisms during initial soil formation. In the Swiss Alpine environment, soil development on silica rich parent material proceeds to Cambisols within at least 700 years as evidenced by the reference soils found outside the proglacial area. They showed indications of weak podzolization as some Fe and Al were translocated downwards. The comparison between the foreland soils and the Cambisols showed decreasing accumulation rates of SOC and pedogenic Fe and Al. This indicates that soil formation processes slow down already after some hundred years or accumulation of Fe and Al increasingly occurs at greater soil depth.

Decreased DOC concentrations in soil water in forested areas in southern Sweden during 1987–2008

During the last two decades, there is a common trend of increasing concentrations of dissolved organic carbon (DOC) in streams and lakes in Europe, Canada and the US. Different processes have been proposed to explain this trend and recently a unifying hypothesis was presented, concluding that declining sulphur deposition and recovery from acidification, is the single most important factor for the long-term DOC concentration trends in surface waters. If this recovery hypothesis is correct, the soil water DOC concentrations should increase as well. However, long-term soil water data from Sweden and Norway indicate that there are either decreasing or indifferent DOC concentrations, while positive DOC trends have been found in the Czech Republic. Based on the soil water data from two Swedish integrated monitoring sites and geochemical modelling, it has been shown that depending on changes in pH, ionic strength and soil Al pools, the DOC solubility might be positive, negative or indifferent. In this study, we test the acidification recovery hypothesis on long-term soil water data (25 and 50 cm soil depth) from 68 forest covered sites in southern Sweden, showing clear signs of recovery from acidification. The main aim was to identify potential drivers for the DOC solubility in soil solution by comparing trends in DOC concentrations with observed changes in pH, ionic strength and concentrations of Al n+. As in earlier Swedish and Norwegian studies, the DOC concentrations in soil water decreased or showed no trend. The generally small increases in pH (median < 0.3 pH units) during the investigation period seem to be counterbalanced by the reduced ionic strength and diminished Al concentrations, increasing the organic matter coagulation. Hence, opposite to the conclusion for surface waters, the solubility of organic matter seems to decrease in uphill soils, as a result of the acidification recovery.

Decreasing DOC trends in soil solution along the hillslopes at two IM sites in southern Sweden — Geochemical modeling of organic matter solubility during acidification recovery

Numerous studies report increased concentrations of dissolved organic carbon (DOC) during the last two decades in boreal lakes and streams in Europe and North America. Recently, a hypothesis was presented on how various spatial and temporal factors affect the DOC dynamics. It was concluded that declining sulphur deposition and thereby increased DOC solubility, is the most important driver for the long-term DOC concentration trends in surface waters. If this recovery hypothesis is correct, the DOC levels should increase both in the soil solution as well as in the surrounding surface waters as soil pH rises and the ionic strength declines due to the reduced input of SO 4 2− ions. In this project a geochemical model was set up to calculate the net humic charge and DOC solubility trends in soils during the period 1996–2007 at two integrated monitoring sites in southern Sweden, showing clear signs of acidification recovery. The Stockholm Humic Model was used to investigate whether the observed DOC solubility is related to the humic charge and to examine how pH and ionic strength influence it. Soil water data from recharge and discharge areas, covering both podzols and riparian soils, were used. The model exercise showed that the increased net charge following the pH increase was in many cases counteracted by a decreased ionic strength, which acted to decrease the net charge and hence the DOC solubility. Thus, the recovery from acidification does not necessarily have to generate increasing DOC trends in soil solution. Depending on changes in pH, ionic strength and soil Al pools, the trends might be positive, negative or indifferent. Due to the high hydraulic connectivity with the streams, the explanations to the DOC trends in surface waters should be searched for in discharge areas and peat lands.

Nanocrystallization of aluminized surface of carbon steel for enhanced resistances to corrosion and corrosive wear

Aluminizing is often used to improve steel's resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100 nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate.

Assessment of soil aluminium pools along three mountainous elevation gradients

Anthropogenic soil acidification in mountain forests and consequent Al release still present a significant problem in many regions. The effect of deposition may differ according to stand conditions, including altitude. This contribution is focused on three elevation transects, two in the Jizera Mountains strongly influenced by acid deposition, one in the less affected Novohradske Mountains. Quantification of pools of different Al forms and related soil characteristics (organic carbon, exchangeable hydrogen cations, sorption characteristics, etc.) is evaluated. In the Novohradske Mountains, the pool of both organically bound and water-soluble Al increases with increasing altitudes. In the Jizera Mountains, the distribution is more complicated; it is strongly affected by different forest type (beech vs. spruce), deforestation, and other local differences. Higher amounts of Al are bound in the mineral horizons compared to the surface organic horizons, even in the case of organically bound Al pools. Further differences between different altitudes and between soil horizons in Al distribution were revealed by detailed Al speciation using HPLC/IC method.

Surface nanocrystallization of Al-plated steel for application in the exhaust system of vehicles

Aluminum-plated or aluminized steel has been used in the exhaust system of vehicles, which exhibits a reasonably high corrosion resistance in the exhaust gas condensate environment. However, due to the inhomogeneous element distribution the outmost surface layer, which is directly exposed to the exhaust gas, is more like an aluminum layer (with little intermetallic compounds) that, though corrosion-resistant, may not be strong enough to effectively resist erosion. In this work, efforts were made to increase the service life of Al-plated or aluminized steel surface, particularly to enhance the outmost surface layer by nanocrystallization for improved resistances to corrosion, wear and corrosive wear. The entire process includes dipping steel specimens into a molten Al pool to make an Al coat, sandblasting the surface coat to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. Electrochemical properties, electron work function (EWF), wear and corrosive wear of the nanocrystalline surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface exhibited improved resistances to corrosion, wear and corrosive wear.

Evolution of soil solution aluminum during transport along a forested boreal hillslope

Aluminum (Al) transfer from soils to surface waters is a crucial determinant of aquatic biodiversity in acidified and/or naturally acid surface waters. Defining how landscapes and acid deposition influence Al export depends on our understanding of the mobilization of Al within catchments. This paper documents the evolution of Al chemistry in water during its transport through a hillslope by using soil water chemistry combined with hydrometric data. Al levels moving laterally from upslope mineral soils were low (&lt;0.1 mg·L−1). This is not consistent with a vertically infiltrating acidification front mobilizing Al from across the catchment. As water moves laterally downslope into the peaty soils of the riparian zone (RZ), Al increased along with organic acids and iron. The RZ in this study was thus a net source of Al to soil solution on all 9 sampling occasions where Al was measured, as well as a net source of dissolved organic carbon (DOC) on another dozen measurement occasions. The potential for Al concentrations being modified during transport through the RZ needs to be accounted for in efforts to relate acid deposition to toxic stream Al concentrations. Organic carrying capacity was in excess of Al for all samples. Only samples from the deeper mineral soils showed supersaturation; however, none of the samples showed degrees of supersaturation high enough for spontaneous mineral precipitation. A key area for further research is how the RZ can sustain a larger Al pool than upslope mineral soils when the supply of Al from these upslope soils to lateral flux is currently lower than that from the RZ. Two possibilities are either episodic downslope transport during periods of high lateral flow, or drainage‐induced changes in hydrology that transformed the RZ from an accumulator of Al to a source of Al.

Modification of Soil Solid Aluminium Phases During an Extreme Experimental Acidification of A Horizons of Forest Soils from Southwest Europe

Four umbric A horizons from acid forest soils were acidified in a batch type experiment and its effect in the Al pools of the solid phase analysed by means of selective dissolution methods. The results showed that Al release accounted for the consumption of 85–99% of the added protons, and causes a decrease of 2–33% of the ‘reactive’ Al pool of the soil solid phase. In these A horizons, inorganic non-crystalline Al and high stability Al-humus complexes are the main sources of the dissolved Al. The contribution of the complexes with intermediate stability only was relevant in the more acid horizon developed from phyllites (P18-A). The increase of equilibration time from 96 to 720 h did not caused significant differences in the decrease of the ‘reactive’ Al pool suggesting the acid neutralising reactions occurred in less than 96 h. In most cases the quantity of released Al is in agreement with the decrease of the different reactive Al pools of the solid phase.

Evaluating main factors controlling aluminum solubility in acid forest soils, southern and southwestern China

The mechanism controlling Al solubility in Chinese acidic forest soils is not clearly understood. This is the main limitation to the ability to generate adequate dose-response prediction models of the ecological effect of acid rain. To evaluate the relative significance of possible processes, soils and soil solutions from five forest catchments, located in southern and southwestern China, were collected and analyzed for chemical parameters. Monitoring showed that inorganic Al (Al i) was the dominant fraction in most soil solutions; organic Al (Al o) was usually less than 10% of total monomeric Al (Al a). Aluminum fractions varied significantly between and within the different sites, though appearing to follow a similar pattern. Over the entire pH range of 3.6–5.6, the pAl (i.e. −log of the Al 3+ activity) closely correlated with solution pH, following regression slopes of 1.28 and 2.00 for upper and lower soil horizons, respectively. The variations in Al 3+ activity could not be explained satisfactorily using mineral dissolution equilibria. Partial least square (PLS) regression showed that soil acidity (quality) and ionic strength (intensity) of the solution were the main explanatory variables for the variation in the concentration of Al fractions. Aluminum in upper horizons originated from both organic and inorganic solid Al pools, while aqueous Al in lower horizons was dominantly of inorganic origin. Aluminum solubility was strongly influenced by cation exchange, especially in the upper horizon. In the upper horizon, ionic strength ( I) had a greater influence on Al solubility due to cation exchange reaction. In the lower horizon, dissolution of inorganic Al pools by the elevated H + concentrations was the main Al release mechanism. So Al activity was more dependent on H + (or pH) in the lower horizon.

Aluminium Mobilization from Acidic Forest Soils in Leigongshan Area, Southwestern China: Laboratory and Field Study

Aluminium mobilization from acidic forest soils in the remote Leigongshan area in southwestern China was investigated in the field and by a laboratory batch experiment using sequential extractions with HCl solutions with pH of approximately 3.4. Experimental data showed that strongly organically bound aluminium (Alp-Alcu) decreased significantly after sequential extraction, whereas exchangeable aluminium (Alex) increased in all three horizons. In the A-horizon with higher base saturation (BS), dissolved aluminium exchanged with base cations (BCs) on soil complex, causing lower aluminium release but significant leaching of BCs. In AB- and B-horizon with lower BS, more dissolved aluminium remained in solution. Al3+ activities in both experimental soil extracts and field soil waters strongly deviated from those predicted by the gibbsite model (i.e., pAl = pKsp + 3.0 pH) despite a significant increase in Al3+ with a decrease in pH. The soil organic-bound aluminium model, using Alcu and Alp to account for differences in the size of available Al pools in the soil, gave considerably better fits to both experimental and field data. SOM-Al model, approved by using Alex instead of organically bound aluminium (i.e., Alp and Alcu), could depict the changes in Al activities most successfully. We have concluded that Alp-Alcu is the main source of aqueous aluminium, whereas Alex plays an important role in regulating aluminium solubility during soil acidification.