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Context: Carbon (C) and nitrogen (N) transformation processes in soils play an important role in the fluctuation of soil pH. Incorporation of chitosan and crop straws, byproducts from fishery and agriculture, into acidic soils can increase soil pH through decarboxylation, decomposition, N immobilisation and ammonification. Aims: The study was designed to evaluate the transformation of organic N and C from chitosan and/or crop straws and their effects on soil physicochemical properties. Methods: Chitosan, rice straw and maize straw were incubated with two acidic Ultisols from Langxi (Soil 1) and Yingtan (Soil 2) differing in initial pH. Six treatments were prepared in triplicate: control (no amendment), 4% chitosan, 4% rice straw, 4% maize straw, 2% chitosan + 2% rice straw, and 2% chitosan + 2% maize straw. Soil pH, N transformation and CO2 evolution were estimated at different time intervals. Key results: After 40 days of incubation, control soil pH decreased by 0.35 and 0.32 units for Soils 1 and 2, respectively. Rice straw, maize straw, chitosan, rice straw–chitosan and maize straw–chitosan significantly increased soil pH by 0.51, 0.17, 2.27, 1.78 and 2.02 units for Soil 1, and 0.71, 0.16, 0.67, 0.49 and 0.68 units for Soil 2 (P < 0.01). The respective treatments decreased exchangeable acidity by 62%, 51%, 95%, 95% and 95% for Soil 1 and 75%, 69%, 88%, 88% and 87% for Soil 2. In treatments containing chitosan, the pH increase resulted from ammonification of organic N and mineralisation of organic C, with the effect higher in Soil 1 than Soil 2. Conclusions: Amending acidic soils with chitosan and crop residues can effectively increase soil pH and slow soil acidification rate. Implications: This study provides useful information for amelioration of acidic soils. Chitosan and crop straws, byproducts from fishery and agriculture, can be used to increase soil pH in acidic soils through decarboxylation, decomposition, N immobilisation and ammonification. Combined application of chitosan and crop straws showed greater ameliorating effect on soil acidity than single applications of straw. This practice provides an effective strategy for improving soil acidity, inhibiting soil acidification, regulating N transformation, and reducing N loss in farmlands. [ABSTRACT FROM AUTHOR]

BACKGROUND: Manganese (Mn) is an essential micronutrient for plants, whereas excess Mn(II) in soils leads to its toxicity to crops. Mn(II) is adsorbed onto plant roots from soil solution and then absorbed by plants. Root charge characteristics should affect Mn(II) toxicity to crops and Mn(II) uptake by the roots of the crops. However, the differences in the effects of root surface charge on the uptake of Mn(II) among various crop species are not well understood. RESULTS: The roots of nine legumes and six non‐legume poaceae were obtained by hydroponics and the streaming potential method and spectroscopic analysis were used to measure the zeta potentials and functional groups on the roots, respectively. The results indicate that the exchangeable Mn(II) adsorbed by plant roots was significantly positively correlated with the Mn(II) accumulated in plant shoots. Legume roots carried more negative charges and functional groups than non‐legume poaceae roots, which was responsible for the larger amounts of exchangeable Mn(II) on legume roots in 2 h and the Mn(II) accumulated in their shoots in 48 h. Coexisting cations, such as Ca2+ and Mg2+, were most effective in decreasing Mn(II) taken up by roots and accumulated in shoots than K+ and Na+. This was because Ca2+ and Mg2+ could compete with Mn(II) for active sites on plant roots more strongly compared to K+ and Na+. CONCLUSION: The root surface charge and functional groups are two important factors influencing Mn(II) uptake by roots and accumulation in plant shoots. © 2023 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Extracellular polymeric substances (EPS) secreted by rhizobacteria are ubiquitous in the environment and can enhance the negative charge of variable‐charge soils. Soil negative charge is directly related to pH buffering capacity (pHBC); it is essential to examine the effect of EPS on soil acidity and pHBC. We investigated the effect of adsorbed EPS on the acidification of five variable‐charge soils in China, which were subjected to different land use: an Oxisol and four Ultisols. Spectroscopic analysis revealed that EPS was specifically adsorbed onto soil surfaces through iron–oxygen–phosphorus/aluminum–oxygen–phosphorus (Fe‐O‐P/Al‐O‐P) bonds formed between phosphoryl groups of EPS and soil Al/Fe oxides. During the formation of Fe‐O‐P/Al‐O‐P bonds, there was ligand exchange between the EPS phosphoryl groups and the surface hydroxyl groups of Fe/Al oxides of the soils. The released OH– played an important role in soil acidity amelioration by the adsorbed EPS through neutralizing H+ and increasing soil suspension pH. After acidification, the pH of EPS‐treated soils was higher than that of the controls. This was attributed to a combined effect of enhanced soil pHBC, and protonation of organic anions (e.g., COO– to COOH) of EPS to decrease the activity of protons. The adsorbed EPS ameliorated prior soil acidity, inhibited new soil acidification, and promoted hydrolysis and re‐adsorption of Al. This was evident as the proportion of organically bound Al and adsorbed hydroxyl Al increased, whereas that of exchangeable Al decreased after EPS adsorption. Therefore, this study can serve as a reference for the molecular design of cost‐effective and environmentally friendly amendments for acid soils. Core Ideas: Variable charge soils can adsorb extracellular polymeric substances (EPS).Binding of EPS phosphoryl groups with soil Fe/Al oxides in soil released OH−.The released OH− from EPS adsorption increased soil pH and ameliorated soil acidity.Adsorbed EPS increased soil pH buffering capacity and inhibited soil acidification.EPS enhanced transformation of soil exchangeable Al to inactive solid Al in acid soils. [ABSTRACT FROM AUTHOR]

The increased use of antibiotics by humans for various purposes has left the environment polluted. Antibiotic pollution remediation is challenging because antibiotics exist in trace amounts and only highly sensitive detection techniques could be used to quantify them. Nevertheless, their trace quantity is not a hindrance to their transfer along the food chain, causing sensitization and the development of antibiotic resistance. Despite an increase in the literature on antibiotic pollution and the development and transfer of antibiotic-resistant genes (ARGs), little attention has been given to the behavior of antibiotics at the soil-solution interface and how this affects antibiotic adsorption-desorption interactions and subsequent uptake and transformation by plants. Thus, this review critically examines the interactions and possible degradation mechanisms of antibiotics in soil and the link between antibiotic soil-solution chemistry and uptake by plants. Also, different factors influencing antibiotic mobility in soil and the transfer of ARGs from one organism to another were considered. The mechanistic and critical analyses revealed that: (a) the charge characteristics of antibiotics at the soil-root interface determine whether they are adsorbed to soil or taken up by plants; (b) antibiotics that avoid soil colloids and reach soil pore water can be absorbed by plant roots, but their translocation to the stem and leaves depends on the ionic state of the molecule; (c) few studies have explored how plants adapt to antibiotic pollution and the transformation of antibiotics in plants; and (d) the persistence of antibiotics in cropland soils can be influenced by the content of soil organic matter, coexisting ions, and fertilization practices. Future research should focus on the soil/solution-antibiotic-plant interactions to reveal detailed mechanisms of antibiotic transformation by plants and whether plant-transformed antibiotics could be of environmental risk. [Display omitted] • Soil-solution chemistry greatly affects antibiotics sorption-desorption processes. • The charge state of antibiotics determines whether they are adsorbed or taken up by plants. • Ionic partitioning of antibiotics controls transport, uptake, and transformation by plants. • Soil fertilization practices have significant effects on antibiotics uptake by plants. • Organic matter and heavy metals alter antibiotic sorption, persistence, and bioavailability. [ABSTRACT FROM AUTHOR]

Chitosan is a biodegradable polymer with a vast range of applications. Along with its metal composites, chitosan has been applied in the remediation of polluted soils as well as a biofertilizer. However, little attention has been given to the degradation of chitosan composites in soil and how they affect soil respiration rate and other physicochemical parameters. In this study, the degradation of chitosan and its composites with gibbsite and hematite in an acidic Ultisol and the effect on urea (200 mg N kg−1) transformation were investigated in a 70-d incubation experiment. The results showed that the change trends of soil pH, N forms, and CO 2 emissions were similar for chitosan and its composites when applied at rates <5 g C kg−1. At a rate of 5 g C kg−1, the C and N mineralization trends suggested that the chitosan-gibbsite composite was more stable in soil and this stability was owed to the formation of a new chemical bond (CH–N–Al-Gibb) as observed in the Fourier-transform infrared spectrum at 1644 cm−1. The mineralization of the added materials significantly increased soil pH and decreased soil exchangeable acidity (P < 0.01). This played an important role in decreasing the amount of H+ produced during urea transformation in the soil. The soil's initial pH was an important factor influencing C and N mineralization trends. For instance, increasing the initial soil pH significantly increased the nitrification rate and chitosan decomposition trend (P < 0.01) and thus, the contribution of chitosan and its composites to increase soil pH and inhibit soil acidification during urea transformation was significantly decreased (P < 0.01). These findings suggest that to achieve long-term effects of chitosan in soils, applying it as a chitosan-gibbsite complex is a better option. [Display omitted] • Chitosan increased soil pH and inhibited the activation of soil exchangeable aluminum. • Chitosan-gibbsite complex was more stable in soil than chitosan-hematite complex. • Chitosan-gibbsite stability is owed to the formation of CH–N –Al -Gibb bond. • Chitosan and its composites reduced soil nitrification during urea transformation in soil. [ABSTRACT FROM AUTHOR]

To evaluate the effects of biochar on pH buffering capacity (pHBC) of paddy soils, rice straw biochar (RC) was modified by hydrogen peroxide (H2O2) and nitric acid/sulphuric acid (HNO3/H2SO4), respectively, and then added to the paddy soils developed from sandy Ultisol, clay Ultisol and granite Ultisol at the same proportion. After submerging and draining incubation, soil pHBC, soil pH and the available cadmium (Cd) extracted with calcium chloride were determined. The results indicated that the two modified biochars effectively promoted pHBC of three paddy soils and HNO3/H2SO4 modified biochar (HNO3/H2SO4-RC) led to greater increase in soil pHBC. Compared with control, HNO3/H2SO4-RC apparently increased pHBC of the paddy soils from sandy Ultisol, clay Ultisol and granite Ultisol by 32.7%, 37.4% and 25.3%, respectively. Correspondingly, soil pH increased by 1.59, 1.15 and 0.88 units, and the available Cd decreased by 74.1%, 67.5% and 75.0% in the paddy soils due to the incorporation of HNO3/H2SO4-RC. HNO3/H2SO4 modification significantly increased the carboxyl groups on the surface of RC. The dissociation of these functional groups produced a large number of organic anions, which could consume exogenous protons, and thus retarding the decline of soil pH and decreased soil available Cd. [ABSTRACT FROM AUTHOR]

Phosphorus (P) availability in highly weathered soils is significantly influenced by the contents of iron (Fe)/aluminum (Al) oxides, clay minerals, and organic matter. With the increasing interest in biofertilizers (e.g. chitosan), it is important to understand how they affect P adsorption profiles on colloids of weathered soils rich in Fe/Al oxides. Thus, the effect of chitosan on the adsorption of P to colloids of hematite, gibbsite, Oxisol, and Ultisol was studied through electrokinetic measurements, spectroscopic analysis, and adsorption edge/isotherm profiles. The presence of chitosan significantly improved the surface positive charge and the decreasing trend of surface positive charge was slower for chitosan-treated colloids compared to the control with increasing pH. At pH 5.0, all the colloids were positively charged, with the oxides containing more positve charges than the soil colloids. At this pH value, the surface coverage capacity of P was 99.1, 61.6, 50.5, and 37.5 mmol kg⁻1 for Oxisol, Ultisol, hematite, and gibbsite, respectively. This suggests that clay minerals in soil colloids were vital in enhancing P adsorption. In the presence of chitosan, the surface coverage capacity of P was increased by 111%, 173%, 647%, and 488% for Oxisol, Ultisol, gibbsite, and hematite, respectively. Drawing inferences from spectroscopic analysis, citric acid desorption profile, and zeta potential analysis, we suggest that chitosan (CH) enhanced P adsorption by promoting the formation of (i) citric acid "undisplaceable" inner-sphere P complexes such as [Colloid-OP- O -CH] and [Colloid-OP- N -CH] , (ii) citric acid "displaceable" outer-sphere P complexes such as {[Colloid- O -CH]-OP} and {[Colloid- N -CH]-OP} , and (iii) water "leachable or soluble" P complexes such as {[Colloid-CH] + PO 4 ³⁻} and {[Colloid-OP]⁻CH + }. Thus, applying chitosan as a biofertilizer (source of N) along with P in highly weathered soils could improve P availability while reducing P leaching. [Display omitted] • Soil colloids adsorbed more phosphate than gibbsite/hematite due to clay minerals. • Chitosan enhanced positive charge of soil colloids more than for gibbsite/hematite. • Chitosan enhanced P adsorption through electrostatic and chemical complexation. • Effect of chitosan on P adsorption was largely dependent on chemical complexation. [ABSTRACT FROM AUTHOR]

The adsorption of glycine, cysteine and glutamic acid by gibbsite and montmorillonite was studied as a function of amino acid concentration and solution pH. Zeta potential measurements, X-ray diffraction (XRD), and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopic analysis were performed to deduce the mechanism(s) of adsorption and to observe changes in the basic structure of the minerals after interaction with amino acids. The ATR-FTIR analysis revealed that both NH 3 + and COO− groups of amino acids were very vital for interactions with the surface of gibbsite and montmorillonite. XRD analysis revealed that the formation of mineral-amino acid composites decreased the d 001 value of montmorillonite from 17.95 to 14.2 (glycine), 15.49 (cysteine), and 15.22 Å (glutamic acid). This observation suggests that amino acid adsorption induced structural stress on montmorillonite and expanded its interlayer space. Also, the adsorption of glycine and cysteine to gibbsite and montmorillonite increased their zeta potentials at pH 5.0 while glutamic acid decreased the zeta potentials. Thus, amino acid-induced increase/decrease in the surface positive/negative charge of the minerals indicates that chemical bonds were formed between glycine, cysteine, glutamic acid and the minerals. The findings of this study can serve as a reference for studies related to understanding the bonding interactions and distribution of different amino acids in soils. • More amino acids were adsorbed by gibbsite than by montmorillonite. • Amino acids adsorbed mainly through their amino and carboxylate groups. • Adsorption of amino acids to montmorillonite induced expansive phase formation. [ABSTRACT FROM AUTHOR]

This study was designed to investigate the relationship between the caloric value and salt tolerance of two varieties of Miscanthus sacchariflorus (Amur silvergrass: M127 and M022). The salt tolerance capacity, photosynthetic characteristics, Na+ and K+ uptake by the roots and aboveground parts, and caloric value of different parts of the aboveground parts were obtained under hydroponic conditions. The results showed that M022 was more tolerant to salt stress than M127 and the former had a higher photosynthetic efficiency as well as a lower aboveground Na+ accumulation, K+ efflux, and larger K+/Na+ ratio. The calorific values of stems, spear leaves, aging leaves, and functional leaves of the two varieties showed a decreasing trend with increasing NaCl concentration. At 270 mM NaCl, the calorific values of the stems, aging leaves, functional leaves, and spear leaves was reduced by 18.10%, 46.73%, 26.11%, and 18.35% for M022 and 41.99%, 39.41%, 34.82%, and 45.09% for M127 compared to the controls, respectively. We observed that the aging leaves of M022 had a faster decline rate in calorific value than those of M127, indicating that the aging leaves of M022 preferentially isolated the harmful Na+ ion, reduced its accumulation in other parts, and increased the K+/Na+ ratio in the corresponding parts, thus inhibiting the decrease in calorific value. Following this result, it can be inferred that M022 inhibited the decline in calorific values during stress by efficiently compartmentalizing the distribution of Na+ and K+. Our results provide a theoretical basis and technical support for the efficient cultivation of salt-tolerant energy plants in saline–alkaline soil. [ABSTRACT FROM AUTHOR]

Excessive application of ammonium‐based N fertilizers promote nitrification, which is mainly responsible for intensive acidification in cropland soils. Thus, there is an urgent need for the development of control measures to manage soil acidification induced by N fertilization. In this study, we investigated the potential of enriching organic C bioavailability to mitigate acidification induced by N fertilization through microbial N immobilization using an indoor incubation experiment. Results showed that the sole application of ammonium sulfate [(NH4)2SO4] or urea to a cropland soil promoted nitrification of fertilizer N, and subsequently soil pH decreased by 0.48–1.00 units after 45‐d incubation. The combined application of glucose with N fertilizers induced microbial immobilization of almost all fertilizer N within 1 d and gradually induced mineralization of soil organic N after 1 wk of incubation. More than 65% of immobilized total extractable N was returned to soil solution and mainly existed as NH4+–N and extractable organic N, with the pH decrement being <0.10 units after 45‐d incubation. These results indicated that (a) enriching organic C bioavailability in cropland soils intensified microbial N immobilization, (b) intensification of microbial N immobilization induced a buffering effect to regulate soil inorganic N supply, and (c) microbial N immobilization had great potential to inhibit soil acidification and retard aluminum mobilization. The results obtained in this study provide a theoretical strategy to control soil acidification and increase N use efficiency in croplands. Core Ideas: Glucose stimulated microbial N immobilization for temporary storing fertilizer N in croplands.Glucose inhibited soil nitrification and acidification through microbial N immobilization.Intensifying microbial N immobilization had a buffering effect to regulate soil inorganic N supply.Improving organic C availability has potential to mitigate soil acidification due to N fertilization. [ABSTRACT FROM AUTHOR]

Mangroves are of important economic and environmental value and research suggests that their carbon sequestration and climate change mitigation potential is significantly larger than other forests. However, increasing salinity and heavy metal pollution significantly affect mangrove ecosystem function and productivity. This study investigates the tolerance mechanisms of rhizobacteria in the rhizosphere of Avicennia marina under salinity and copper (Cu) stress during a 4-y stress period. The results exhibited significant differences in antioxidant levels, transcripts, and secondary metabolites. Under salt stress, the differentially expressed metabolites consisted of 30% organic acids, 26.78% nucleotides, 16.67% organic heterocyclic compounds, and 10% organic oxides as opposed to 27.27% organic acids, 24.24% nucleotides, 15.15% organic heterocyclic compounds, and 12.12% phenyl propane and polyketides under Cu stress. This resulted in differential regulation of metabolic pathways, with phenylpropanoid biosynthesis being unique to Cu stress and alanine/aspartate/glutamate metabolism and α-linolenic acid metabolism being unique to salt stress. The regulation of metabolic pathways enhanced antioxidant defenses, nutrient recycling, accumulation of osmoprotectants, stability of plasma membrane, and chelation of Cu, thereby improving the stress tolerance of rhizobacteria and A. marina. Even though the abundance and community structure of rhizobacteria were significantly changed, all the samples were dominated by Proteobacteria, Chloroflexi, Actinobacteriota, and Firmicutes. Since the response mechanisms were unbalanced between treatments, this led to differential growth trends for A. marina. Our study provides valuable inside on variations in diversity and composition of bacterial community structure from mangrove rhizosphere subjected to long-term salt and Cu stress. It also clarifies rhizobacterial adaptive mechanisms to these stresses and how they are important for mitigating abiotic stress and promoting plant growth. Therefore, this study can serve as a reference for future research aimed at developing long-term management practices for mangrove forests. [Display omitted] • Salt-copper (Cu) stress decreased bacterial abundance but increased diversity. • Low and high salt stress significantly increased bacterial abundance. • High salt stress significantly downregulated the content of free amino acids in the rhizosphere. • High Cu and salt-Cu stress promoted N reduction function but inhibited C metabolism. • Phenylpropanoid biosynthesis was unique to copper stress. [ABSTRACT FROM AUTHOR]

Copper-containing amine oxidases (CuAOs) are known to have significant involvement in the process of polyamine catabolism, as well as serving crucial functions in plant development and response to abiotic stress. A genome-wide investigation of the CuAO protein family was previously carried out in sweet orange (Citrus sinensis) and sweet cherry (Prunus avium L.). Six CuAO (KoCuAO1-KoCuAO6) genes were discovered for the first time in the Kandelia obovata (Ko) genome through a genome-wide analysis conducted to better understand the key roles of the CuAO gene family in Kandelia obovata. This study encompassed an investigation into various aspects of gene analysis, including gene characterization and identification, subcellular localization, chromosomal distributions, phylogenetic tree analysis, gene structure analysis, motif analysis, duplication analysis, cis-regulatory element identification, domain and 3D structural variation analysis, as well as expression profiling in leaves under five different treatments of copper (CuCl2). Phylogenetic analysis suggests that these KoCuAOs, like sweet cherry, may be subdivided into three subgroups. Examining the chromosomal location revealed an unequal distribution of the KoCuAO genes across four out of the 18 chromosomes in Kandelia obovata. Six KoCuAO genes have coding regions with 106 and 159 amino acids and exons with 4 and 12 amino acids. Additionally, we discovered that the 2.5 kb upstream promoter region of the KoCuAOs predicted many cis elements linked to phytohormones and stress responses. According to the expression investigations, CuCl2 treatments caused up- and downregulation of all six genes. In conclusion, our work provides a comprehensive overview of the expression pattern and functional variety of the Kandelia obovata CuAO gene family, which will facilitate future functional characterization of each KoCuAO gene. [ABSTRACT FROM AUTHOR]

The copper transporter (COPT/Ctr) gene family plays a critical part in maintaining the balance of the metal, and many diverse species depend on COPT to move copper (Cu) across the cell membrane. In Arabidopsis thaliana, Oryza sativa, Medicago sativa, Zea mays, Populus trichocarpa, Vitis vinifera, and Solanum lycopersicum, a genome-wide study of the COPT protein family was performed. To understand the major roles of the COPT gene family in Kandelia obovata (Ko), a genome-wide study identified four COPT genes in the Kandelia obovata genome for the first time. The domain and 3D structural variation, phylogenetic tree, chromosomal distributions, gene structure, motif analysis, subcellular localization, cis-regulatory elements, synteny and duplication analysis, and expression profiles in leaves and Cu were all investigated in this research. Structural and sequence investigations show that most KoCOPTs have three transmembrane domains (TMDs). According to phylogenetic research, these KoCOPTs might be divided into two subgroups, just like Populus trichocarpa. KoCOPT gene segmental duplications and positive selection pressure were discovered by universal analysis. According to gene structure and motif analysis, most KoCOPT genes showed consistent exon–intron and motif organization within the same group. In addition, we found five hormones and four stress- and seven light-responsive cis-elements in the KoCOPTs promoters. The expression studies revealed that all four genes changed their expression levels in response to copper (CuCl2) treatments. In summary, our study offers a thorough overview of the Kandelia obovata COPT gene family's expression pattern and functional diversity, making it easier to characterize each KoCOPT gene's function in the future. [ABSTRACT FROM AUTHOR]

The dual role of biochar for inhibiting soil acidification induced by nitrification was determined through two-step incubation experiments in this study. Ca(OH) 2 or biochar was added respectively to adjust soil pH to the same values (pH 5.15 and 5.85), and then the amended soils were incubated in the presence of urea for 70 days. The results showed that compared with Ca(OH) 2 treatment, both rice straw biochar and peanut straw biochar inhibited the decrease in soil pH and the increase in exchangeable acidity during the incubation. The application of biochars suppressed soil nitrification during the incubation, and thus reduced 7.5 mmol kg−1 and 1.4 mmol kg−1 protons released from nitrification compared to Ca(OH) 2 treatments. Compared with Ca(OH) 2 treatment, the ammonia-oxidizing bacteria population size was decreased by 8% and 12% in rice straw biochar and peanut straw biochar treatments respectively, which was the main responsibility for the inhibited nitrification after biochar application. In addition, the application of rice straw biochar and peanut straw biochar increased soil pH buffering capacity (pHBC) respectively by 22% and 32%. The increased pHBC played the main role (75%) in inhibiting the acidification of the soil amended with peanut straw biochar, while the rice straw biochar inhibited soil acidification mainly through suppressing nitrification during the incubation. Overall, compared with lime application, biochars can inhibit soil acidification caused by urea application through suppressing the nitrification process and improving the resistance of soils to acidification. The crop residue biochars presented a longer-lasting effect on ameliorating acidic soils than mineral lime. Image 1 • Biochar played a dual role in inhibiting soil acidification from nitrification. • Biochar decreased proton release through suppressing soil nitrification. • Biochar increased soil pH buffering capacity and the resistance to acidification. • Biochar would be a better option to ameliorate acidic soils than liming. [ABSTRACT FROM AUTHOR]

Phyllosilicate minerals are the important components in soils and an important source of activated aluminum (Al) during soil acidification. However, the mechanisms for Al activation in phyllosilicate minerals were not understood well. In this paper, the effect of phyllosilicate surface hydroxyl groups on Al activation during acidification was studied after the minerals were modified with inorganic and organic materials. After modification of kaolinite, montmorillonite, and illite with fulvic acid (FA-), iron oxide (Fe-), Fe combined with FA (Fe-FA-), and siloxane (Si–O-), the interlayer spaces were altered. For instance, when modified with Fe, Fe entered the interlayer spaces of kaolinite and montmorillonite and changed the interlayer spaces of both minerals but did not affect that of illite. Also, the other modification methods had significant effects on the interlayer space of montmorillonite but not on kaolinite and illite. It was observed that all the modification strategies inhibited Al activation during acidification by reducing the number of hydroxyl groups on the mineral surfaces and inhibiting protonation reactions between H+ and hydroxyl groups. Nevertheless, the inhibition effect varies with the type of phyllosilicate mineral. For kaolinite (Kao), the inhibition effect of the different modification methods on Al activation during acidification followed: Fe-FA-Kao > Fe-Kao > Si–O-Kao > FA-Kao. Additionally, for montmorillonite (Mon), the inhibition effect was in the order: Si–O-Mon > Fe-Mon > Fe-FA-Mon > FA-Mon, while for illite, it was: Fe-illite > Si–O-illite ≈ Fe-FA-illite > FA-illite. Thus, the hydroxyl groups on the surfaces and edges of phyllosilicate minerals play an important role in the activation of Al from the mineral structure. Also, the protonation of hydroxyl groups may be the first step during Al activation in these minerals. The results of this study can serve as a reference for the development of new technologies to inhibit soil acidification and Al activation. [Display omitted] • Surface –OH of phyllosilicate minerals plays an important role in Al mobilization. • Surface modification reduced active hydroxyl groups on phyllosilicate minerals. • Surface modification inhibited Al activation in minerals induced by acidification. [ABSTRACT FROM AUTHOR]

To explore the effects of the increases in pH and pH buffering capacity (pHBC) induced by crop residue biochars on the changes in soil available Cd content, six acidic paddy soils developed from different parents were amended with seeded sunflower plate biochar (SSPBC), peanut straw biochar (PSBC) and corn straw biochar (CSBC). The pH, pHBC, and available Cd of the soils were measured after laboratory incubation. The results showed that the incorporation of crop residue biochars led to the increases in soil pH and pHBC, but a decrease in soil available Cd content. The decreasing order of available Cd content was SSPBC > PSBC > CSBC and was consistent with the changes in soil pH induced by the biochars. During submerging and draining, soil pH increased first and then declined, however the content of available Cd decreased first and then increased significantly. Soil pH in the treatments with biochars showed little change during draining, which was different from the control without the biochars added. This was attributed to the enhancing effect of the biochars on soil pHBC. Also, there was a significant negative correlation between the change in available Cd content and soil pHBC during submerging/draining alternation and suggested that higher pHBC corresponded to smaller soil available Cd content. Consequently, the amount of Cd absorbed by rice was reduced, thereby reducing the potential risk of soil Cd to humans. These results can provide useful references for the remediation of Cd-contaminated paddy soils. [Display omitted] • Biochars increased pH, and pHBC of acidic paddy soils and reduced soil available Cd. • Biochars inhibited pH decline of acidic paddy soils during drainage. • Biochars inhibited uptake of Cd from soil suspensions in stems and roots of rice. • Seeded sunflower plate biochar and peanut straw biochar are better soil amendments. [ABSTRACT FROM AUTHOR]

[Display omitted] • More exchangeable Al was observed on montmorillonite than on kaolinite at pH < 5.1. • Exchangeable Al in Alfisol was larger than that in Ultisols under acidic conditions. • Soil CEC and organic matter play an important role in acid buffering performance. • Soil organic matter inhibited the production of exchangeable and soluble Al. Soil acidification is an increasing challenge to food security due to the production of free aluminum ion (Al3+) that is toxic to crops. The production of exchangeable Al3+ in soils during acidification is a complex process and its specific mechanism is not clear. In this study, kaolinite, montmorillonite, and three acidic soils (two Ultisols and one Alfisol) were used to investigate the changes in exchangeable and soluble Al with pH using a constant pH automatic potentiometric titrator. The effect of soil organic matter (SOM) on soil pH and Al change was also investigated. The results showed that montmorillonite consumed more acid than kaolinite under the same conditions, which were consistent with the larger cation exchange capacity (CEC) of montmorillonite than kaolinite. When the pH was adjusted from 4.8 to 4.3, the exchangeable Al of montmorillonite was significantly higher than that of kaolinite. This was consistent with their CEC and indicated that montmorillonite adsorbed more H+ than kaolinite at the same pH, resulting in more exchangeable Al on its surface. Besides CEC, SOM shows a significant effect on acid buffering capacity and Al activation of acid soils. When acidified to the same pH, the content of exchangeable Al in Alfisol was larger than that in Ultisols. This observation was consistent with the larger CEC and smaller organic matter content of Alfisol. The presence of more organic matter in the Ultisols not only improved their acid buffering performance, but also inhibited the production of exchangeable and soluble Al. In conclusion, CEC of soils and minerals play an important role in soil acid buffering performance. While, SOM not only improves soil acid buffering performance, but also inhibits the mobilization of soil Al. [ABSTRACT FROM AUTHOR]

Biochar can effectively alleviate the Al phytotoxicity in acidic soils due to its alkaline nature. However, the longevity of this alleviation effect of biochar under re-acidification conditions is still unclear. In the present study, the maize root growth responding to the simulated re-acidification of two acidic soils amended by peanut straw biochar or Ca(OH) 2 was investigated to evaluate the long-term effect of biochar on alleviating Al toxicity in acidic soils. Compared with Ca(OH) 2 amendment, the application of biochar significantly retarded Al toxicity to plant during soil re-acidification. When 4.0 mM HNO 3 was added, the maize seedling root elongation in an Oxisol with biochar was 99% higher than that in the Oxisol with Ca(OH) 2. Also, the Evans blue uptake and Al content in the root tip in the biochar treatment were 60% and 51% lower than those in the Ca(OH) 2 treatment. The retarding effect was mainly attributed to the slow decrease in soil pH during acidification and the release of dissolved organic carbon (DOC) in the soils amended by biochar. The slower decrease in soil pH resulting from the increased pH buffering capacity after biochar application inhibited the increase of soluble and exchangeable Al during re-acidification. The increased DOC after biochar application decreased the toxic soluble Al speciation at the same pH value and total Al concentration in soil solution. Therefore, given the re-acidification of soils, biochar presented a longer-term effect on alleviating Al toxicity of acidic soil than liming. Unlabelled Image • Biochar retarded the Al toxicity to plant during soil re-acidification. • Biochar decreased the concentration of soil active Al during soil re-acidification. • Biochar altered the distribution of Al speciation in soil solution. • Biochar presented a long lasting effect on alleviating Al toxicity in acidic soils. [ABSTRACT FROM AUTHOR]

Forest soil acidification caused by acid deposition is a serious threat to the forest ecosystem. To investigate the liming effects of biomass ash (BA) and alkaline slag (AS) on the acidic topsoil and subsoil, a three-year field experiment under artificial Masson pine was conducted at Langxi, Anhui province in Southern China. The surface application of BA and AS significantly increased the soil pH, and thus decreased exchangeable acidity and active Al in the topsoil. Soil exchangeable Ca2+ and Mg2+ in topsoil were significantly increased by the surface application of BA and AS, while an increase in soil exchangeable K+ was only observed in BA treatments. The soil acidity and active Al in subsoil were decreased by the surface application of AS. Compared with the control, soluble monomeric and exchangeable Al in the subsoil was decreased by 38.0% and 29.4% after 3 years of AS surface application. There was a minimal effect on soluble monomeric and exchangeable Al after the application of BA. The soil exchangeable Ca2+ and Mg2+ in the subsoil increased respectively by 54% and 141% after surface application of 10 t ha−1 AS. The decrease of soil active Al and increase of base cations in subsoil were mainly attributed to the high migration capacity of base cations in AS. In conclusion, the effect of surface application of AS was superior to BA in ameliorating soil acidity and alleviating soil Al toxicity in the subsoil of this Ultisol. [Display omitted] • Biomass ash (BA) and alkaline slag (AS) effectively amended the topsoil acidity. • BA surface application increased soil exchangeable K+ in both topsoil and subsoil. • The acidity and active Al in subsoil were reduced by surface application of AS. • High mobility of Ca and Mg in AS contributed to the reduced active Al in subsoil. [ABSTRACT FROM AUTHOR]