Al Oxide Content Research Articles

Soil organic carbon regulation by pH in acidic red soil subjected to long-term liming and straw incorporation

The manipulation of soil pH through liming and straw incorporation plays a pivotal role in influencing soil organic carbon (SOC) dynamics in acidic red soil. This study aimed to assess the impact of these practices on SOC and elucidate the relationship between SOC and pH. Over a 31-year field experiment, seven different fertilization treatments were implemented: unfertilized (CK), nitrogen and potassium fertilizers (NK), NK with lime (NKCa), nitrogen, phosphorous, and potassium fertilizers (NPK), NPK with lime (NPKCa), NPK with straw (NPKS), and NPKS with lime (NPKSCa). Results revealed that liming and straw incorporation significantly elevated soil pH by 0.13–0.73 units. Lime application boosted SOC and mineral-associated organic carbon (MAOC) by 20.2% and 28.7%, respectively, in NK treatment, whereas its impact on SOC in NPK and NPKS treatments were negligible. SOC witnessed a 17.1% increase with NPKS and a 15.2% increase with NPKSCa compared to NPK alone. Notably, NPKS and NPKSCa led to a significant surge in particulate organic carbon (POC) by 19.7% and 37.7%, respectively, albeit NPKSCa reduced MAOC by 14.9% relative to NPK. Linear regression analysis unveiled a positive correlation between POC and soil pH, while SOC and MAOC exhibited an initial rise at lower pH levels followed by stabilization as pH continuously increasing. A partial least squares path model showed two pathways through which pH influenced SOC: firstly, by positively affecting SOC through increasing Fe and Al oxides contents and enhanced aggregate stability, and secondly, by negatively influencing SOC through altered ratios of fungi/bacteria and Gram-positive bacteria/Gram-negative bacteria. In conclusion, the long-term effects of lime and straw application on SOC and MAOC were contingent upon soil pH, with more pronounced positive effects observed at lower pH levels. These findings underscore the importance of considering soil pH when implementing lime and straw strategies to mitigate acidification and regulate SOC in acidic red soil.

Texture and clay mineralogy as main drivers of the priming effect in temperate forest soils

Abstract Background and aims This work aimed to determine how the soil parameters affect the magnitude and direction of priming effect (accelerated or decreased decomposition of native SOM under addition of new organic substrates, PE) in temperate acidic forest soils. Methods Thirteen topsoil samples were incubated for 163 days with the addition of maize residues. Soil respiration was measured and natural isotope labelling was used in order to separate the respiration sources (SOM, maize and PE). The effect of soil parameters on PE was studied using linear regression and structural equation modelling (SEM). Results Soils with high C/N ratio showed the lowest magnitude of cumulative PE (R2 = 0.321, p < 0.05) and the longest negative PE period. A positive relationship was found between PE and the pH (R2 = 0.511, p < 0.05). SEM analysis showed that pH and C/N ratio has direct (β = 0.50) and indirect (β = 0.20, via modifying soil texture and mineralogy) effect on PE. Soil texture and mineralogy had a significant effect on PE: texture affects the proportions of soil respiration sources and PE was reduced by the dithionite–citrate–bicarbonate–extractable Al (AlDCB, R2 = 0.454, p < 0.05), silt + clay (R2 = 0.421, p < 0.05), non-swelling clay mineral (R2 = 0.575, p < 0.05) and illite (R2 = 0.522, p < 0.05) contents. SEM analysis also highlighted that the AlDCB, illite and silt + clay contents has a great effect (β=−0.59) on the PE. Conclusion The silt + clay content and mineral composition of the soil, including the Al oxide and illite contents may thus significantly inhibit the magnitude of PE, and consequently the decomposition of SOM under acidic conditions.

Pore size distribution and Al oxide content significantly regulated the effects of humic acid on perfluorooctanoic acid transport in natural soils

The ubiquity of dissolved organic matter (DOM) makes it encounter the released perfluorooctanoic acid (PFOA) in subsurface environment. However, the effect of DOM (e.g., humic acid, HA) on PFOA transport in soils and the critical influencing factors and mechanisms remain obscure. Column experiments were conducted to explore PFOA transport with the presence of different concentrations of HA in three types of soils and two types of Al oxide coated sand. Results revealed soil properties significantly regulate the effects of HA on PFOA transport, for which pore size distribution, minerals content (e.g., Al oxide) and pH were critical influencing soil–properties. For soil with large mesopore volume, pore blockage caused by HA controlled the effect of HA on PFOA transport. Large mesopore volume significantly alleviated pore blockage of HA, and led to insignificant effects of HA on PFOA transport. For soil exhibited minimum mesopore volume, Al oxide content and pH dominated the effect of HA on PFOA transport. Results from Al oxide coated sand (low mesopore volume) columns further proved that higher Al oxide content and lower pH caused more significant facilitating effect of HA on PFOA transport via site competition. Results highlighted the importance of considering pore size distribution and Al oxide content when assessing PFOA mobility capacity with co–transport with DOM in soils.

Distribution of Organic Carbon Fractions in Soil Aggregates and Their Contribution to Soil Aggregate Formation of Paddy Soils

ABSTRACT Different soil organic matter fractions play different roles in governing soil aggregation. Therefore, this study determined the distribution of labile organic carbon (OC) and stable OC fractions in soil aggregates and their contribution to the soil aggregation of paddy soils in the Central Plain of Thailand. Quantitative analysis of the OC fractions in soil aggregates was determined using wet oxidation method and their changes in the chemical composition of organic compounds were investigated using FTIR spectroscopy. The results showed that the large macroaggregate (LMA: 2–8 mm) content of studied soils generally increased with increasing soil organic carbon, clay, polyvalent cations and aluminum (Al) oxides contents. Generally, both the labile OC and stable OC fractions increased with increasing aggregate size classes, suggesting a hierarchical order of soil aggregation and physical protection of organic matter against microbial decomposition. The labile OC fraction in all aggregate size classes was positively correlated with the content of LMA (r = 0.54*–0.77**), suggesting that the labile OC fraction represented a major organic cementing agent that contributed to LMA formation. FTIR analysis revealed that the labile OC fraction mainly consisted of plant-derived and microbial-derived polysaccharides. Therefore, paddy soils with higher organic cementing agent (labile OC fraction) and inorganic cementing/flocculating agents may provide a stronger intimate association of these transient binding agents, particularly the microbial-derived polysaccharides with clay mineral surfaces and metal cations/Al oxides. In turn, the greater amounts of stabilized, microbial-derived polysaccharides also efficiently promoted the formation of LMA from a combination of microaggregates.

Glyphosate and AMPA have low mobility through different soil profiles of the prosecco wine production area: A monitoring study in north-eastern Italy

Contamination of the environment by glyphosate (GLP) and its metabolite aminomethylphosphonic acid (AMPA) is still of major concern worldwide due to specific interactions among these molecules and soil and water. Two monitoring sites were established in the Prosecco wine production area (Conegliano and Valdobbiadene) in northeastern Italy, which has been included in UNESCO’s World Heritage List since 2019. The study aims to increase the knowledge about GLP dynamics in this area where it has been intensively used by farmers and the potential risk for groundwater pollution is still debated. Each site was equipped with two soil-water monitoring stations consisting of multisensor soil probes and suction cups at three soil depths (10, 30, and 70 cm). Soil and water were sampled for 10 and 6 months, respectively, and analyzed for GLP and AMPA concentrations, for a total of 242 samples to describe their vertical movement and dissipation dynamics. Soil properties, in particular, the different forms of Fe and Al oxide contents, and Freundlich adsorption coefficients were quantified along the soil profile. First attempts showed that glyphosate dissipation time was 36 ± 8 days in Conegliano and Valdobbiadene soils and fully completed in both after 6 months. In contrast, AMPA dissipation dynamic —first described by an original equation—was longer than that of GLP and fully dissipated after almost 300 days. GLP showed a strong binding affinity with clay and Fe and Al chelated to soil organic matter, which likely acted as cation bridges and in turn led to low GLP mobility. GLP and AMPA were mostly detected after heavy rainfall events at 70 cm depth, likely bypassing the porous matrix of the intermediate layers.

Relations between soil organic carbon content and the pore size distribution for an arable topsoil with large variations in soil properties

AbstractSoil organic carbon (SOC) in arable topsoil is known to have beneficial effects on soil physical properties that are important for soil fertility. The effects of SOC content on soil aggregate stability have been well documented; however, few studies have investigated its relationship with the soil pore structure, which has a strong influence on water dynamics and biogeochemical cycling. In the present study, we examined the relationships between SOC and clay contents and pore size distributions (PSDs) across an arable field with large spatial variations in topsoil SOC and clay contents by combining X‐ray tomography and measurements of soil water retention. Additionally, we investigated the relationships between fractionated SOC, reactive Fe and Al oxide contents and soil pore structure. We found that porosities in the 0.2–720 μm diameter class were positively correlated with SOC content. A unit increase of SOC content was associated with a relatively large increase in porosity in the 0.2–5 and 480–720 μm diameter classes, which indicates that enhanced SOC content would increase plant available water content and unsaturated hydraulic conductivity. On the other hand, macroporosities (1200–3120 μm diameter classes) and bioporosity were positively correlated with clay content but not with SOC content. Due to strong correlations between soil texture, carbon‐to‐nitrogen ratios and reactive iron contents, we could not separate the relative importance of these soil properties for PSDs. Reactive aluminium and particulate organic carbon contents were poorer predictors for PSDs compared with clay and SOC contents. This study provides new insights on the relations between SOC and soil pore structure in an arable soil and may lead to improved estimations of the effects of enhanced SOC sequestration on soil water dynamics and soil water supply to crops.Highlights Relations between soil organic carbon (SOC) and pore size distribution (PSD) in an arable soil were explored. We used X‐ray tomography and soil water retention to quantify a wide range of PSD. There were positive correlations between SOC and porosities in 0.2–720 μm diameter classes. Porosities in 0.2–5 and 480–720 μm diameter classes were more strongly correlated with SOC than clay. Our results have implications for improved estimates of effects of SOC sequestration on soil water dynamics.

Modifying Effect of Soil Properties on Bio-Accessibility of As and Pb from Human Ingestion of Contaminated Soil

Exposure to soils contaminated with heavy metals can pose human health risk to children through ingestion of contaminated soil. Soil properties such as soil pH, reactive Fe and Al oxide content, clay content, soil organic matter (SOM), and cation exchange capacity (CEC) can reduce contaminant bio-accessibility and exposure. In vitro bio-accessibility (%IVBA) of As and Pb in 19 soils was determined using U.S. EPA Method 1340. Soil properties reduced the bio-accessibility of As by 17–96.5% and 1.3–38.9% for Pb. For both As and Pb, bio-accessibility decreased with increasing Al and Fe oxide content. Al oxides were found to be the primary driver of As and Pb bio-accessibility. Multiple regressions with AlOx, soil pH, %clay and/or FeOx predicted %IVBA As (p < 0.001). The multiple regression including log (FeOx + AlOx) and %clay explained 63% of the variability in %IVBA Pb (p < 0.01). Fe and Al oxides were found to be important drivers of As and Pb bio-accessibility, regardless of in vitro method. These findings suggested soil pH should be used in addition to reactive oxides to predict bio-accessible As. Risk-based adjustments using soil properties for exposure via incidental ingestion should be considered for soils contaminated with As and/or Pb.

Origin and implications of early diagenetic quartz in the Mississippian Bowland Shale Formation, Craven Basin, UK

Silica cementation exerts a key control on the compaction and geotechnical properties of mudstones, and by extension, the style of hydrocarbon and/or mineral systems present in a given sedimentary basin. Integrated microscopic and bulk geochemical observations demonstrate that siliceous mudstones in the Bowland Shale Formation, a target for UK shale gas extraction, exhibit abundant dispersed, discrete, μm-scale quartz cements, and exhibit silica enrichment (‘excess’) above a local detrital Si/Al threshold of 2.5. Dissolution of siliceous radiolarian tests during early diagenesis is identified as the main source of silica (opal A) required for quartz precipitation, either via opal CT or directly to quartz, and potentially generated as a product of anoxic marine ‘weathering’ (dissolution) of reactive silicates during early diagenesis. Excess silica correlates with free hydrocarbons (S1) normalised to total organic carbon (oil saturation index; OSI); we propose early diagenetic quartz precipitation suppressed pore collapse (‘buttress effect’), retaining the pore space capacity to host oil. Quartz precipitation was likely catalysed, for example via low porewater pH, elevated Al and/or Fe oxide content, and/or abundant labile organic matter. Juxtaposition of siliceous mudstones and mudstones lacking quartz cement indicates silica was immobile beyond the bed scale. Thus metre-scale siliceous packages likely represent more prospective units within the Bowland Shale (in terms of unconventional hydrocarbons), on the basis of early diagenetic biogenic-derived quartz cementation leading to improved hydrocarbon storage capacity coupled to enhanced brittleness. These findings are relevant for shale oil and shale gas systems, specifically where oil retained in pores subsequently cracks to generate gas. These findings also suggest the Bowland Shale is a sub-class of black shale, defined by the potential to host a relatively large volume of early diagenetic fluids, derived from anoxic bottom waters, which were potentially S- and/or metal-bearing. This is potentially relevant for understanding the genesis of adjacent and related Pb-Zn mineral deposits.

A spacial analysis of organic matter, oxides and granulometric distribution on sediments as a support for geochemical analysis on Três Marias reservoir – Minas Gerais

This research evaluates the granulometric, organic matter and oxides distribution on sediments of the Tres Marias lake. Thus, a sampling network was established based on the main input of sediment in the lake and on the morphology of the lake, totaling 40 samples. After collection, the material was dried at 40 ° C for 36 hours and then treated by dry sieving. The Fe and Al oxides content was determined by ICP-OES. The measurements using loss-on-ignition methods gave the organic matter content. The organic concentration was compared to the environmental guidelines established by CONAMA No. 354/2012. The concentrations was exceeded by seven samples. The sediment supply comming from Sao Francisco, Sao Vicente, Paraopeba, Extrema, Ribeirao do Boi, Borrachudo and Indaia rivers pointed to an overall predominance of the sand fraction along the margin sediments and high disparities between their granulometry. The higher aluminum concentrations are disposed at central-east zone of the Lake, concordantly with the higher organic matter ratios. The higher iron concentrations are mainly on Indaia River.

Desorption of Arsenic from Calcareous Mine Affected Soils by Phosphate Fertilizers Application in Relation to Soil Properties and As Partitioning

The ability of fertilizer phosphates to desorb arsenates from soils is not yet adequately studied especially in cases of mining lands severely contaminated with arsenic (As). In this study, two soils with different physicochemical properties and heavily contaminated with As equilibrated with solutions containing various rates of phosphates either in the form of triple superphosphate fertilizer (TSP) or as NH4H2PO4 using NaNO3 as background electrolyte. A treatment with TSP in water was also applied to mimic agronomic practices. In general, increased P rates resulted in higher As release and to lower P sorption. Depending on the P rate, desorbed As ranged between 8 and 64.4 mg/kg for soil 1 and between 16.5 and 35.3 mg/kg for soil 2, corresponding to more than 50% of the potentially available As, as defined by the sum of the two first fractions of Wenzel sequential extraction scheme. Arsenic desorption patterns substantially differ between the two soils, mainly affected by active carbonates, organic matter and Fe and Al oxides contents. Though the differences between P treatments were not always significant, the presence of NaNO3 increased the desorbing strength of the solutions. Phosphorus sorption capacity was high for both soils, but excess P addition led to high P concentrations in the equilibrium solutions, implying leaching hazard.

Phosphorus sorption characteristics and related properties in urban soils in southeast China

Phosphorus (P) is usually enriched in urban soils but the environmental consequences of this P enrichment are not well understood. The objective of this study was to investigate P sorption characteristics as a measure of potential P loss from urban soils due to P enrichment via runoff and leaching. We collected soil samples at 0–5 and 5–15 cm soil depths from urban park areas in Nanchang, China, a rapidly expanding city with low P contents in its natural soils. Soil P sorption indices were characterized by a multiple-point isotherm batch experiment. The 0–5 cm layer had lower Langmuir P sorption maximum (Smax) than the 5–15 cm layer, probably because of its lower amorphous Al oxide (Alox) content and higher soil organic carbon (SOC) content. Stepwise multiple regression showed that the combination of Alox and SOC were the two most important soil properties related to Smax of the studied soils. A two-piece segmented linear regression was fitted to the relationships between labile P fractions (i.e., water extractable-P and anion exchange resin-extractable P) and binding energies (k) and the change point values were identified for k below which labile P decreased linearly. Soils below the change point had greater contents of SOC and exchangeable Ca and Mg and higher pH than those above the change point. These results suggest that parent materials played a major role in regulating P sorption capacities of the urban soils, particularly for the soils in urban green spaces. However, the enrichment of Ca and Mg derived from dissolution of calcareous materials in urban areas could significantly influence P bonding strength and saturation ratio of urban soils.

Reduction in throughfall reduces soil aggregate stability in two subtropical plantations

SummaryClimate change has altered global precipitation regimes in terms of intensity and frequency of drought stress and, consequently, it is likely to affect soil moisture and soil aggregation. However, we know little about the effects of drought on soil aggregate size, distribution and stability, and how that affects carbon sequestration. A drought manipulation experiment was conducted by throughfall exclusion treatment (TET) of 50% in two planted forests (Pinus massoniana Lamb. and Castanopsis hystrix A.DC.) in subtropical China. The aim was to investigate the effects of a reduction in throughfall on aggregate size, distribution and stability. The results from the 4‐year experiment show that the TET affected soil moisture content significantly (with 14.5 and 20.4% decreases in the P. massoniana and C. hystrix plantations, respectively) compared with the control. Soil temperature at 0–5‐cm soil depth and soil texture were not affected significantly. Soil porosity in the TET plots was greater than that of the control, whereas soil bulk density and free Al oxide content were less. The mass fractions of macroaggregates (> 0.25 mm) and mean weight diameter (MWD) of aggregates, an indicator of aggregate stability, decreased in the TET compared with the control. Variation in aggregate size, distribution and stability can be caused by Fe and Al oxides, soil texture, bulk density and porosity. Our results indicated that TET reduced soil aggregate stability in subtropical plantations because of a decrease in free Al oxide and an increase in porosity. Slaking effects from variation in soil moisture were also possible for the decreased MWD. This study suggests disturbance of forest soils should be minimized in the context of a decline in precipitation.Highlights Size, distribution and stability of forest soil aggregates were evaluated under throughfall exclusion. TET reduced free Al oxide content but increased soil porosity, leading to breakdown of macroaggregates. Soil aggregate stability was reduced by TET. Fe and Al oxides and soil texture contributed more than SOC to aggregate size and stability.

Quality reference values and background concentrations of potentially toxic elements in soils from the Eastern Amazon, Brazil

Concentration quality reference value (QRV) concentrations are calculated based on the background of potentially toxic elements (PTEs) in their natural state, absent from human influences. QRVs are essential for ensuring that environmental legislation is consistent with the local conditions and they are also important for monitoring potential changes in the chemical compositions soils that can affect soil quality. The objective of this study was to obtain background concentrations to set QRVs for the PTEs in the soils of the state of Pará. The background concentrations and QRVs of the PTEs were obtained from 132 surface soils (0–0.2 m) and 132 subsurface (0.8–1.0 m) soils in the state of Pará, Eastern Amazon, Brazil. Under natural conditions, the values found for the QRVs and the background values of the PTEs were low, indicating low risk of environmental contamination and hazard to human health. The exception was Hg, whose content was higher than the prevention level defined by Brazilian law. The concentrations of Al, Cu, Cd, Cr, Fe, Hg, and Mn were positively correlated with the clay, Fe, Si, Ti, Al, and Mn oxide contents, whereas As, Ba, Ni, Pb, and Zn were positively correlated with the organic matter, Mn oxide and silt concentrations.Analysis of the main soil components indicated that Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, and Zn showed similar geochemical behaviors and common lithogenic origins. In the surface layers, Hg did not behave similarly to the other PTEs indicating that it can originate from different lithogenic sources or become or have elevated levels from anthropogenic sources. The results of this study serve as a basis for the monitoring of environmental contamination, prevention, and control of PTE pollution by authorities of the state of Pará and may serve as a reference for all Amazonian soils with similar soil characteristics.

Arsenic Speciation and Availability in Orchard Soils Historically Contaminated with Lead Arsenate.

Lead arsenate is an environmentally hazardous contaminant that was applied as a pesticide in orchards during the early 1900s. Elevated arsenic (As) concentrations persist in soils where lead arsenate was applied. To assess the risk associated with historic lead arsenate contamination, the retention, bioavailability, and speciation of soil As were evaluated in three historically contaminated orchard soils. Stirred-flow desorption studies and in vitro physiologically based extractions were used to assess the mobility and relative bioavailability of soil As, respectively. Synchrotron-based X-ray absorption spectroscopy was used to determine soil As speciation. Arsenic concentrations in former orchard soils ranged from 11.8 to 59.0 mg kg. Less than 22% of total As was considered bioavailable according to in vitro extractions. Up to 15% of soil As was desorbed in 10 mM KCl, but desorption with phosphate solutions resulted in release of up to 70% of total As dependent on soil type. Desorption data suggest that arsenate is primarily sorbed via inner-sphere complexation, and elevated concentrations of competing ions in soil solution may increase mobility of soil As. Arsenic was primarily present in the As(V) oxidation state, the less mobile form of As. Combined results from X-ray absorption spectroscopy and desorption studies indicate that As primarily exists as sorbed species, likely to ubiquitously present Fe- and Al-oxides in soils. Results demonstrate that soil As from these historically contaminated orchards is mostly in stable, nonhazardous forms, but factors such as Fe- and Al-oxide content and land management practices have a significant effect on As transport and bioavailability.

The Properties of Clay Minerals in Soil Particles from Two Ultisols, China

AbstractSoil aggregates consist of sand, silt, and clay size particles. Many of the clay size particles in soils are clay minerals, which actively influence soil behavior. The properties of clay minerals may change significantly as soil particle size decreases to the nanoscale; however, little information is available about these properties for the Ultisols in China. In the present study, the clay mineral components and structural characteristics of four particle-size fractions (i.e., <2000, 450–2000, 100–450, and 25–100 nm) of two Ultisol samples (Ult-1 and Ult-2) were investigated using elemental analysis, X-ray diffraction, Fouriertransform infrared spectroscopy, and thermal analysis. The molar SiO2 to Al2O3 ratios were lower in the nanoscale particle-size fraction (25–100 nm) than in the 450–2000 and <2000 nm fractions. This indicates greater desilicification and allitization of the smaller Ultisol particles. Furthermore, the Fe oxide and Al oxide contents increased and reached a maximum level in the 25–100 nm fraction of the two Ultisols. Goethite was mainly found in the 100–450 nm and 25–100 nm fractions. The dominant clay minerals in the Ultisol 25–100 nm fraction were kaolinite and illite with a small amount of a hydroxy-interlayered mineral in Ult-1 and gibbsite in Ult-2. The kaolinite crystallinity decreased as particle size decreased. The low crystallinity of the kaolinite in the A horizon 25–100 nm fraction was attributed to a reduction in the thickness of coherent scattering domains, as well as to decreases in OH groups and the dimensions of octahedral AlO6 sheets. A determination of the chemical and mineralogic properties of the different size fractions of the Ultisols is important to understand the desilicification and Al and Fe oxide enrichment mechanisms during soil formation. The significance of these results can help to reveal the nanoscale transformations of clay minerals. Analysis of clay mineral compositions in nanoparticles can provide the additional data needed to understand the adsorption and mobility of nutrients and pollutants.

Relationship between soil clay mineralogy and carbon protection capacity as influenced by temperature and moisture

Environmental conditions like temperature and moisture could affect the carbon protection capacity of various clay types in soils. Using dominantly kaolinitic-illitic, smectitic and allophanic soils, we conducted systematic incubation experiments over 42 days at different temperatures (4, 22 and 37 °C) and moisture contents (30, 60 and 90% of water holding capacity (available water)). The basal respiration was monitored to study the relative effect of moisture contents and temperature on the carbon protection capacities and mechanisms of the three clay types. The results indicated that carbon decomposition increased with increasing moisture and temperature. A two-component quadratic equation could explain the carbon mineralisation process. The highest C respiration was observed at 37 °C with a 60% moisture level in each of the soil types. Under these conditions, the smectitic soil recorded the highest carbon decomposition followed by the kaolinitic-illitic and allophanic soils. The study of the priming effect using 14C labelled malic acid confirmed the trend of the bulk respiration results. The allophanic soil showed the lowest amount of carbon mineralisation under all experimental conditions. A strong inverse correlation (R2 = 0.90 at p < 0.05) was observed between CO2 emission rate and total sesquioxides (Fe and Al oxides) content. As evidenced by the pore size distribution, micromorphologies and thermogravimetric analyses, the microporous structure and microaggregate formation in the allophanic soil enhanced carbon sequestration. This study indicated that soil carbon stabilisation was related more to the sesquioxides content than to the clay types or their relative specific surface areas.

Iron [Fe(0)]-rich substrate based on iron–carbon micro–electrolysis for phosphorus adsorption in aqueous solutions

The phosphorus (P) adsorption properties of an iron [Fe(0)]-rich substrate (IRS) composed of iron scraps and activated carbon were investigated based on iron–carbon micro-electrolysis (IC–ME) and compared to the substrates commonly used in constructed wetlands (CWs) to provide an initial characterization of the [Fe(0)]-rich substrate. The results showed that P was precipitated by Fe(III) dissolved from the galvanic cell reactions in the IRS and the reaction was suppressed by the pH and stopped when the pH exceeded 8.90 ± 0.09. The adsorption capacity of the IRS decreased by only 4.6% in the second round of adsorption due to Fe(0) consumption in the first round. Substrates with high Ca– and Mg–oxide contents and high Fe– and Al–oxide contents had higher P adsorption capacities at high and low pH values, respectively. Substrates containing high Fe and Al concentrations and low Ca concentrations were more resistant to decreases in the P adsorption capacity resulting from organic matter (OM) accumulation. The IRS with an iron scrap to activated carbon volume ratio of 3:2 resulted in the highest P adsorption capacity (9.34 ± 0.14 g P kg−1), with minimal pH change and strong adaptability to OM accumulation. The Fe(0)–rich substrate has the considerable potential for being used as a CW substrate.

Swelling Behavior of Montmorillonites Noncolloidally Crosslinked with ε‐Al13

Core Ideas The expansion properties of montmorillonites treated with PHA steeply decreased. A mixture of montmorillonites and PHA solution is not a colloidal system. Al13 groups are the main intercalator and factor influencing swelling behavior. The treatment may be applied in expansive soil research. Studies concerning noncolloidally crosslinked montmorillonites (NCMs) are significant for the chemical treatment of expansive soil. In this study, NCMs with OH/Al molar ratios of 1.0, 1.5, 2.0, and 2.5 and Al/montmorillonite ratios of 0.1, 0.3, 0.5, 0.8, and 1.0 mmol g−1 were prepared via AlCl3–NaOH reactions and subsequent Al13–montmorillonite interactions. The resulting NCMs exhibited the characteristics of silt as well as significant expansion and mechanical property changes. The expansion properties of the NCM samples, including the free swelling ratio, swelling force, free expansion ratio, and expansion ratio under 50 kPa all decreased significantly. According to the X‐ray diffraction data, the basal spacing of the NCM samples became relatively stable and slightly expanded after the drying and rewetting processes. Additionally, the cohesive force and shear strength of the NCM samples increased significantly after the crosslinking process. Furthermore, the X‐ray fluorescence spectrometry results confirmed that the Al oxide content of the NCMs increased as the swelling properties decreased, indicating that primarily the Al13 groups influenced the swelling behavior of the NCMs.

Soil Aggregate Stability and Iron and Aluminium Oxide Contents Under Different Fertiliser Treatments in a Long-Term Solar Greenhouse Experiment

Soil in greenhouses is likely to suffer a gradual decline in aggregate stability. Determination of the effects of different fertiliser practices on soil aggregate stability is important for taking advantage of solar greenhouses. Soil aggregate stability and iron (Fe) and aluminium (Al) oxide contents were investigated in a 26-year long-term fertilisation experiment in greenhouse in Shenyang, China, under eight fertiliser treatments: manure (M), fertiliser N (FN), fertiliser N with manure (MN), fertiliser P (FP), fertiliser P with manure (MP), fertiliser NP (FNP), fertiliser NP with manure (MNP), and control without any fertiliser (CK). A wet sieving method was used to determine aggregate size distribution and water-stable aggregates (WSA), mean weight diameter and geometric mean diameter as the indices of soil aggregate stability. Different fertiliser treatments had a statistically significant influence on aggregate stability and Fe and Al oxide contents. Long-term application of inorganic fertilisers had no obvious effects on the mass proportion of aggregates. By contrast, manure application significantly increased the mass proportion of macroaggregates at the expense of microaggregates. All treatments, with the exception of FNP, significantly increased the stability of macroaggregates but decreased that of microaggregates when compared with CK. Aggregation under MP and MN was better than that under M and MNP; however, no significant differences were found among inorganic fertiliser treatments (i.e., FN, FP, and FNP). A positive relation was found between pyrophosphate-extractable Fe and WSA (r = 0.269), but no significant relations were observed between other Fe and Al oxides and aggregate stability.

Phosphorus desorption affected by drying and wetting cycles in Ferralsols and Luvisols of Brazilian Northeast

ABSTRACTSemiarid soils are subjected to wetting and drying cycles which influence sorption and desorption of applied phosphorus fertilizer. Phosphorus desorption was determined in soils from toposequences of two soil groups (Ferralsol and Luvisol) from a semiarid area, subjected to wetting and drying cycles. Samples from surface and subsurface horizons of upslope, midslope, and downslope positions were incubated for 4 months with phosphorus doses corresponding to 0, 5, 10, 25, 50, 75, and 100% of the maximum adsorption capacity, under constant moisture (80% water retention capacity) or 12 cycles of wetting and drying. Phosphorus desorption was lower in the Ferralsol than in the Luvisol, and lower in the subsurface than in the surface horizons, probably due to greater clay, Fe, and Al oxides contents, but they were similar among slope positions, of same mineralogy. Desorption tended to be greater in samples submitted to wetting and drying cycles but differences were small. P recovery reached 40–50% in the Luvisol, and 30–40% in the Ferralsol. The relatively low P retention capacity suggests a high residual effect of the P applied. Therefore, in relation to P losses, water retention techniques are less important than those that prevent soil erosion.