•Calcium and ferrous ions promoted inorganic P (Pi) accumulation on the periphyton.•P precipitation and coprecipitation with carbonates or Fe(OH)3 were main mechanisms.•Efficiency of Fe(II) was higher than Ca(II) in enhancing Pi abiotic accumulation.•Periphyton would help reduce P fixation in soil and P loss from the paddy fields.The effect of periphyton propagation in paddy fields on phosphorus biogeochemical cycling has received little attention. In this phytotron study, inorganic phosphorus (Pi) accumulation by periphyton was investigated for varying inputs of calcium [Ca(II)] or ferrous‑iron [Fe(II)], and lighting conditions. Results indicated that additions of Ca(II) or Fe(II) enhanced abiotic accumulation of Pi by up to 16 times, and decreased solution Pi concentration by up to 50%, especially under light condition. The enhanced Pi accumulation into periphyton intensified with increasing Pi concentration, and Pi accumulation showed a positive linear relationship with Ca or Fe accumulation. Abiotic accumulation of Pi induced by Ca(II) was mainly through Ca-phosphate precipitation, and co-precipitation of P with carbonates at pH > 8. Accumulation with added Fe(II) was mainly considered to be through Fe(III) phosphate precipitation coupled with adsorption of Pi by ferric hydroxides. Moreover, Fe(II) was more effective than Ca(II) in promoting abiotic accumulation of Pi by periphyton. Our results indicate the potential for controlling environmental factors to enhance the role of periphyton in biogeochemical cycling and P-use efficiency in paddy rice fields and to reduce P discharged to neighboring water bodies.Download high-res image (279KB)Download full-size image

The chemical characteristics, element contents, mineral compositions, and the ameliorative effects on acid soils of five biomass ashes from different materials were analyzed. The chemical properties of the ashes varied depending on the source biomass material. An increase in the concrete shuttering contents in the biomass materials led to higher alkalinity, and higher Ca and Mg levels in biomass ashes, which made them particularly good at ameliorating effects on soil acidity. However, heavy metal contents, such as Cr, Cu, and Zn in the ashes, were relatively high. The incorporation of all ashes increased soil pH, exchangeable base cations, and available phosphorus, but decreased soil exchangeable acidity. The application of the ashes from biomass materials with a high concrete shuttering content increased the soil available heavy metal contents. Therefore, the biomass ashes from wood and crop residues with low concrete contents were the better acid soil amendments.Download high-res image (148KB)Download full-size image

Sorption of organic phosphates–myo-inositol hexakisphosphate (IHP) and glycerol phosphate (GP) and its effects on the early stage of hematite aggregation kinetics were investigated at different pH and electrolyte composition. KH2PO4 (KP) was taken as an inorganic P source for comparison. Results indicated that for all types of P, the sorption amounts decreased with increasing solution pH. Sorption amount of IHP was almost two times that of KP, while those of GP and KP were close. Both organic P and inorganic P interacted with hematite via ligand exchange through their phosphate groups, which conveyed negative charges to mineral surface and significantly decreased the zeta potential of hematite. In Na+ solution, critical coagulation concentrations (CCCs) of hematite suspensions increased with increasing P concentration and followed the order of KP < GP < IHP at pH 5.5. Compared with KP, the organic P could more effectively stabilize the hematite suspension not only through increasing the negative charges and electrostatic repulsive force, but also through steric repulsion between P-sorbed hematite nanoparticles. When the pH was increased from 5.5 to 10.0, the CCCs of the hematite suspensions with GP and IHP decreased mainly because of the great reductions in organic P sorption amounts and consequent decreases in electrostatic and steric repulsive forces. However, enhanced aggregation was observed in the presence of IHP at pH 4.5 and above in low Ca2+ solutions. The precipitation of calcium phytate formed net-like structure, which served as bridges to bind hematite nanoparticles and resulted in enhanced aggregation. These results have important implications for assessing the fate and transport of organic P and hematite nanoparticles in soil and aquatic environments.

The purpose of this study was to elucidate the mechanisms for pectin-enhanced adsorption of heavy metal cations on variable charge minerals.Batch experiments were conducted to investigate the adsorption of pectin and copper(II) by amorphous Fe/Al hydroxides. The morphology, mineralogy, and functional groups of pectin–Fe/Al hydroxides were examined using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis.The amount of pectin adsorbed by amorphous Al(OH)3 was much greater than that by amorphous Fe(OH)3 at pH values between 3.5 and 6.5 due to the higher positive charge density on Al(OH)3 and greater electrostatic attraction between the hydroxide and pectin compared with Fe(OH)3. The addition of pectin decreased the positive surface charge on amorphous Fe and Al hydroxides. The presence of pectin enhanced the adsorption of Cu(II) by the Fe and Al hydroxides. The increase in Cu(II) adsorption on amorphous Fe hydroxide was more obvious at low pH values than at higher pH values, while an opposite changing trend was observed for amorphous Al hydroxide. At pH 3.9, 4.3, and 4.9, pectin increased Cu(II) adsorption by Fe hydroxide from 24.4, 76.6, and 177.0 mmol/kg to 61.6, 98.8, and 192.0 mmol/kg, i.e., Cu(II) adsorption was increased by 37.2, 22.2, and 15.0 mmol/kg, respectively. At pH 4.3 and 4.9, pectin increased Cu(II) adsorption by Al hydroxide from 3.7 and 27.0 mmol/kg to 17.3 and 69.4 mmol/kg, i.e., Cu(II) adsorption was increased by 13.6 and 42.4 mmol/kg, respectively. The greater adsorption of pectin by Al hydroxide was mainly responsible for the larger enhancement of pectin on Cu(II) adsorption on Al hydroxide at higher pH values compared with Fe hydroxide.The adsorption of pectin on Fe and Al hydroxides decreased the positive charge on the hydroxides and thus enhanced the adsorption of Cu(II) by the hydroxides.

The purpose of this study is to examine the effects of combined application of biomass ash (BA), bone meal (BM), and alkaline slag (AS) on soil acidity, nutrient contents, uptake of the nutrients by wheat, and wheat growth.A pot experiment with an Ultisol collected from Anhui province, China, was conducted to compare the effects of BA, BM, and AS applied alone and combined on soil acidity; soil nutrient contents; uptake of N, P, K, Ca, and Mg by wheat, and wheat growth.Application of BA, BM, and AS alone and combined increased soil pH and decreased soil exchangeable Al3+. BA + BM + AS showed the greatest ameliorating effect on soil acidity, and soil pH of the treatment increased by 1.24 units compared with control. Application of BA + BM + AS reduced soil exchangeable Al3+ and increased soil exchangeable calcium and magnesium to a greater extent than BA + BM and single application of the amendments. The BM-containing amendments substantially increased soil available phosphorous by 66–93% compared with control. Application of the amendments alone and combined enhanced the uptake of N, P, K, Ca, and Mg by wheat and thus promoted wheat growth and increased yield of wheat grains. Application of BA + BM + AS and BA + BM showed greater effects on nutrient uptake and wheat growth than single application of the amendments. Wheat straw weights of the two treatments were 11.1 and 10.1 times greater than that of control. The data were 2.7, 4.8, and 5.6 times for the treatments of BA, AS, and BM. The contents of Cd, Cr, Zn, and Cu in wheat grains were lower than standard limits, except for the single BA treatment.BA + BM + AS is the best choice for amelioration of acid soils and promotion of crop production.

•Adhesions of Fe and Al hydroxides decreased negative charge on rice roots.•Adhesions of Fe and Al hydroxides decreased adsorption of K+ and Cd2+ by rice roots.•Physical mask and diffuse layer overlapping were responsible for charge variation.•Adhesions of Fe and Al hydroxides enhanced cation desorption from rice roots.•Cation desorption experiments confirmed diffuse layer overlapping on rice roots.Iron (Fe) and aluminum (Al) hydroxides in variable charge soils attached to rice roots may affect surface-charge properties and subsequently the adsorption and uptake of nutrients and toxic metals by the roots. Adhesion of amorphous Fe and Al hydroxides onto rice roots and their effects on zeta potential of roots and adsorption of potassium (K+) and cadmium (Cd2+) by roots were investigated. Rice roots adsorbed more Al hydroxide than Fe hydroxide because of the greater positive charge on Al hydroxide. Adhesion of Fe and Al hydroxides decreased the negative charge on rice roots, and a greater effect of the Al hydroxide. Consequently, adhesion of Fe and Al hydroxides reduced the K+ and Cd2+ adsorption by rice roots. The results of attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and desorption of K+ and Cd2+ from rice roots indicated that physical masking by Fe and Al hydroxides and diffuse-layer overlapping between the positively-charged hydroxides and negatively-charged roots were responsible for the reduction of negative charge on roots induced by adhesion of the hydroxides. Therefore, the interaction between Fe and Al hydroxides and rice roots reduced negative charge on roots and thus inhibited their adsorption of nutrient and toxic cations.Download high-res image (164KB)Download full-size image

The purpose of the present study is to examine the effect of phosphate on the aggregation kinetics of hematite and goethite nanoparticles.The dynamic light scattering method was used to study the aggregation kinetics of hematite and goethite nanoparticles.Specific adsorption of phosphates could promote aggregation through charge neutralization at low P concentrations, stabilize the nanoparticle suspensions at medium P concentrations, and induce aggregation through charge screening by accompanying cations at high P concentrations. Two critical coagulation concentration (CCC) values were obtained in each system. In NaH2PO4, the goethite CCC at low phosphate concentrations was smaller than hematite and vice versa at high phosphate concentrations. Stronger phosphate adsorption by goethite rapidly changed the zeta potential from positive to negative at low phosphate concentrations, and the zeta potential became more negative at high phosphate concentrations. The clusters of hematite nanoparticles induced by phosphate adsorption had an open and looser structure. Solution pH and the phosphate adsorption mechanisms in NaH2PO4, KH2PO4, and Na3PO4 solutions affected zeta potential values and controlled the stability of hematite suspensions during aggregation. High pH and preference for non-protonated inner-sphere complexes in Na3PO4 solution decreased the zeta potential of positively charged hematite and promoted aggregation. Activation energies followed the order NaH2PO4 > KH2PO4 > Na3PO4 at low P concentrations. K+ was more effective than Na+ in promoting hematite aggregation due to the non-classical polarization of cations.Phosphate can enhance or inhibit the aggregation of hematite and goethite nanoparticles in suspensions by changing surface charge due to specific adsorption onto the particles. The phosphate-induced aggregation of the nanoparticles mainly depended on the initial concentration of phosphate.

The rapid increase in agricultural pollution demands judicious use of inputs and outputs for sustainable crop production. Crop straws were pyrolyzed under oxygen-limited conditions at 400 °C for 2 h to prepare peanut straw biochar (PB), canola straw biochar (CB), and wheat straw biochar (WB). Then, 300-g soils were incubated each with urea nitrogen (UN) and UN + biochars with or without dicyandiamide (DCD) for 60 days. During the incubations, soil acidification induced by urea was somewhat inhibited by biochars, but nitrification of hydrolyzed NH4+ produced much more acidity than the neutralization potential of the biochars. In single UN (200 mg/kg) treatment, soil pH decreased drastically and the final pH after incubation was lower than the control. Antagonistic to UN, all three biochars neutralized the soil acidity, which was consistent to their inherent alkalinity. DCD inhibited nitrification which was obvious throughout the incubations, as 30 mg/kg DCD + 200 mg/kg UN combined with 1  % PB, CB, and WB retained 0.94, 0.79, and 1.19 units higher pH, respectively, and significantly reduced exchangeable acidity over the treatments without DCD (P < 0.05). The treatments of UN + biochars with and without DCD had highly significant effects on soil pH, exchangeable Al3+, NH4+-N, (NO3−+NO2−)-N, and available P (P < 0.05). Amplified NH4+-N retentions at higher rates of PB referred increased negatively charged sites for nutrient adsorptions. Applied UN transformations varied among different treatments, and the maximum amounts of total mineral N recovered were 218.3, 218.5, and 223.8 mg/kg in the presence of DCD by PB, CB, and WB, compared to 198.2, 201.6, and 205.2 mg/kg, respectively, in no DCD treatments. Urea induced severe soil acidification and even lowered the ameliorative effects of applied biochars. Thus, ammonium-based fertilizers must include nitrification inhibitor (DCD) and, if used in combination with biochars will offer a suitable choice to reduce the acidity, improve base saturation and fertility of soil for sustainable agriculture.

•Transformation of Oxisol to paddy soil decreased free Fe oxides and increased amorphous ones.•Ferrolysis decreased arsenate adsorption by paddy soil.•Increasing amorphous Fe oxides increased arsenate adsorption on paddy soil.•Increasing pH and incorporating phosphate were unfavorable for arsenate adsorption.Iron oxides are dominant effective adsorbents for arsenate in iron oxide-rich variable charge soils. Oxisol-derived paddy soils undergo intensive ferrolysis, which results in high leaching and transformation of iron oxides. However, little information is available concerning the effect of ferrolysis on arsenate adsorption by paddy soil and parent Oxisol. In the present study, we examined the arsenate affinity of soils using arsenate adsorption/desorption isotherms, zeta potential, adsorption kinetics, pH effect and phosphate competition experiments. Results showed that ferrolysis in an alternating flooding–drying Oxisol-derived paddy soil resulted in a significant decrease of free iron oxides and increase of amorphous iron oxides in the surface and subsurface layers. There were more reactive sites exposed on amorphous than on crystalline iron oxides. Therefore, disproportionate ratios of arsenate adsorption capacities and contents of free iron oxides were observed in the studied Oxisols compared with paddy soils. The Gibbs free energy values corroborated that both electrostatic and non-electrostatic adsorption mechanisms contributed to the arsenate adsorption by bulk soils, and the kinetic adsorption data further suggested that the rate-limiting step was chemisorption. The zeta potential of soil colloids decreased after arsenate was adsorbed on the surfaces, forming inner-sphere complexes and thus transferring their negative charges to the soil particle surfaces. The adsorption/desorption isotherms showed that non-electrostatic adsorption was the main mechanism responsible for arsenate binding to the Oxisol and derived paddy soils, representing 91.42–94.65% of the adsorption capacities. Further studies revealed that arsenate adsorption was greatly inhibited by increasing suspension pH and incorporation of phosphate.

We evaluated the ameliorative effects of crop straw biochars either alone or in combination with nitrate fertilizer on soil acidity and maize growth.Low energy-consuming biochars were prepared from canola and peanut straws at 400 °C for 2 h. Incubation experiment was conducted to determine application rate of biochars. Afterward, maize crop was grown in pots for 85 days to investigate the effects of 1 % biochars combined with nitrate fertilizer on soil pH, exchangeable acidity, and maize growth in an Ultisol collected from Guangdong Province, China.Application of 0.5, 1.0, and 1.5 % either canola straw biochar (CSB) or peanut straw biochar (PSB) increased soil pH by 0.15, 0.27, 0.34, and 0.30, 0.58, 0.83 U, respectively, after 65-day incubation. Soil pH was increased by 0.49, 0.72, 0.78, and 0.88 U when 1 % CSB or PSB was applied in combination with 100 and 200 mg N/kg of nitrate, respectively, after maize harvest in greenhouse pot experiment. These low-cost biochars when applied alone or in combination with nitrate not only reduced soil exchangeable acidity, but also increased Ca2+, Mg2+, K+, Na+, and base saturation degree of the soil. A total of 49.91 and 80.58 % decreases in exchangeable acidity were observed when 1 % CSB and PSB were incubated with the soil for 65 days, compared to pot experiment where 71.35, 78.64, 80.2, and 81.77 % reductions of exchangeable acidity were observed when 1 % CSB and PSB were applied in combination with 100 and 200 mg N/kg of nitrate, respectively. The higher contents of base cations (Ca2+, Mg2+, K+, Na+) in biochars also influenced the plant growth. The higher biomass in CSB-treated pots was attributed to the higher K content compared to PSB. The higher percent reduction in exchangeable Al3+ by applying 1 % CSB combined with 200 mg N/kg of nitrate consistently produced maximum biomass (129.65 g/pot) compared to 100 mg N/kg of nitrate and 1 % PSB combined with 100 and 200 mg N/kg of nitrate. The exchangeable Al3+ mainly responsible for exchangeable acidity was decreased with the application of biochars and nitrate fertilizer. A highly significant negative relationship was observed between soil exchangeable Al3+ and plant biomass (r2 = 0.88, P < 0.05).The biochars in combination with nitrate fertilizer are cost-effective options to effectively reduce soil acidity and improve crop growth on sustainable basis.

•Both B. subtilis and P. fluorescens inhibited P sorption on gibbsite.•Bacteria mobilized covalently bound P while adhering onto gibbsite.•Bacteria greatly reduced the positive charge on gibbsite surface.•The inhibitory effect of bacteria on P adsorption was due to the competition for adsorption sites.•Decrease in electrostatic attraction also contributed to the inhibitory effect.Sorption and desorption of phosphate (P) on Fe and Al (hydr)oxides may be affected by bacteria in soils because their ubiquitous and strong interactions. The role of Bacillus subtilis and Pseudomonas fluorescens in adsorption of P on gibbsite (γ-AlOOH) was systematically investigated under a wide range of conditions by combining in-situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy with batch macroscopic experiments. In-situ ATR-FTIR observations of the ternary systems (bacteria, P, and gibbsite) showed simultaneous desorption of P from, and adhesion of the bacteria to, gibbsite, indicating a competition between the two for surface sites. Batch desorption experiments showed that bacteria could mobilize the P from gibbsite into solution, and macroscopic adsorption data showed that the amount of P adsorbed on the bacteria–gibbsite complex was less than that on gibbsite alone over durations from 0 h to 26 h, concentrations of P from 0.1 mM to 2.0 mM, pH from 5 to 8, and ionic strength from 0 M to 0.5 M, suggesting that bacteria inhibit the adsorption of P on gibbsite. The degree of inhibition increased with the number of bacteria in the system and was significantly but non-linearly correlated with the decline in the positive charge on gibbsite induced by the bacteria. Therefore, competition for suitable sites on the surface of gibbsite between P and the bacteria and reduction in the positive charge on the surface of gibbsite induced by bacteria are proposed as two important mechanisms that inhibit P adsorption. These findings highlight the role of bacteria in regulating the availability of P to plants and its mobility in natural environments.

Our objectives were to compare effects of root charge properties on Al adsorption by the roots of rice that differed in Al-tolerance, and to examine effects of different nitrogen forms on charge properties of rice roots and Al adsorption.Streaming potential and chemical methods were used to measure root zeta potential and investigate Al chemical forms adsorbed on the roots of rice obtained from solution culture experiments.Rice roots of the Al-sensitive variety Yangdao-6 carried greater negative charge than the Al-tolerant variety Wuyunjing-7, which meant the roots of Yangdao-6 adsorbed more exchangeable and complexed Al. When both rice varieties were grown in NH4+-containing nutrient solutions, there were less functional groups and lower negative surface charge on their roots, which reduced Al adsorption compared to the rice grown in NO3− containing nutrient solutions. The decline in nutrient solution pH due to NH4+ uptake by rice roots was responsible for the reduced numbers of functional groups and the lower negative surface charge on the roots compared to the rice grown in NO3− containing solutions.Integrated root surface charge, as expressed by zeta potential, played an important role in Al adsorption by the roots of rice with different Al-tolerance.

This study was conducted in order to examine the effect of colloidal particles on electrochemical properties of charged larger size materials.A self-made streaming potential apparatus was used to measure the zeta potentials of Fe/Al oxide-coated quartz. The effects of colloidal particles of kaolinite and montmorillonite on the electrochemical properties of Fe/Al oxide-coated quartz were investigated through comparing the difference in zeta potential of the coated quartz in electrolyte and clay suspension.The change of zeta potentials of the coated quartz, when clay suspensions flowed through, increased with the increasing concentrations of kaolinite and montmorillonite and degree of coating with Fe/Al oxides, and decreased with increased ionic strength of the suspensions. Electrostatic attraction between clay colloids and the coated quartz was the key factor influencing the interaction between the oppositely charged particles. The deposition of colloidal particles of kaolinite and montmorillonite on coated quartz and the overlapping of the diffuse layers of electrical double layers between the oppositely charged particles were responsible for the change in zeta potential of Fe/Al oxide-coated quartz. The relative contribution of the deposition of clay particles to the change in zeta potential was greater than that of the overlapping of diffuse layers.When clay suspensions flowed through the saturated sand of Fe/Al oxide-coated quartz, both overlapping of diffuse layers between charged sand and clay particles and deposition of clay particles contributed to change of zeta potential of the coated quartz.

The purpose of this study is to obtain insights into how bacterial adhesion to soils affect the surface charges of soil Fe/Al hydroxides.Adhesion of Escherichia coli and Bacillus subtilis by amorphous Fe and Al hydroxides was investigated using a batch method. Zeta potential measurements and potentiometric titrations were used to study the effects of bacteria adhesion on the surface charges of Fe and Al hydroxides.The amorphous Al(OH)3 adhesion capacity of the two bacteria species was greater than that of amorphous Fe(OH)3 due to the much larger positive charge on amorphous Al(OH)3 compared to amorphous Fe(OH)3. The maximum Al(OH)3 adhesion capacities, obtained by fitting a Langmuir equation, were 2,689.6 and 2,358.7 g/kg for E. coli and B. subtilis, respectively, and the maximum Fe(OH)3 adhesion capacities of the two bacteria were 750.5 and 893.6 g/kg, respectively. Adhesion of both E. coli and B. subtilis to amorphous Fe and Al hydroxides decreased with increasing pH at pH < their points of zero charge (PZC), which was consistent with the change in interaction energy, as calculated by the classical DLVO theory, between the bacteria and the Fe/Al hydroxides. The electrostatic force plays an important role in bacterial adhesion by the Fe and Al hydroxides. The FTIR spectra revealed that chemical bond formation also contributed to the interactions between the bacteria and amorphous Fe and Al hydroxides. E. coli and B. subtilis adhesion decreased the positive charges and zeta potentials of the Fe and Al hydroxides, and shifted the PZCs of the Fe and Al hydroxides to lower pH. The larger number of negative charge on B. subtilis meant that it had a greater effect on the surface charge properties of the Fe and Al hydroxides compared to E. coli.Electrostatic force plays an important role in E. coli and B. subtilis adhesion to amorphous Fe and Al hydroxides. Bacterial adhesion led to the decreases in positive charges and PZCs of the hydroxides.

Our objective was to determine the in situ zeta potential at the root surfaces of rice.A streaming potential apparatus made in our laboratory was used to measure zeta potential at the root surfaces.Zeta potential at the root surfaces could be calculated from streaming potential measured by the apparatus, and the measurements of streaming potential had good stability, reproducibility and accuracy. Ionic strength, electrolyte type, pH influenced the zeta potentials, with being consistent with theoretical calculations. The absolute value of the zeta potential at the root surfaces decreased with increasing ionic strength due to compression of diffuse layer of the electric double layer. Cations had much greater impact on zeta potential than anions due to the electrostatic attraction of cations by the negatively charged root surfaces and electrostatic repulsion of anions. The effects of different cations on the zeta potential followed the order: Al3+ > H+ > Cu2+ > Mg2+ ≈ Ca2+ > K+ ≈ Na+. The zeta potential of root surfaces became more negative with increasing pH due to deprotonation of functional groups on roots.Streaming potential measurement is a novel method that can be used to calculate zeta potential of root surfaces.

•Streaming potential method can characterize oppositely charged particle interaction.•Electric double layer overlapping occurs between Fe/Al oxide suspensions and quartz.•Electric double layer overlapping decreased with increase in ionic strength.Interaction of the electrical double layers (EDLs) between negatively charged quartz grains and positively charged iron (Fe)/aluminum (Al) oxide colloids was investigated in this work by comparing the difference in zeta potentials between the single quartz in electrolyte and quartz with Fe/Al oxide suspensions. The Fe/Al oxides used were α-Fe2O3, amorphous Fe hydroxide, gibbsite and amorphous Al hydroxide. The zeta potential of quartz was measured using a self-made streaming potential apparatus. Our results indicated that the overlapping degree of diffuse layers of the EDLs between quartz and Fe/Al oxides increased with increased concentration of Fe/Al oxides and decreased with the decrease in negative surface charge on quartz due to ferric coating. The overlapping degree of diffuse layers of the EDLs between quartz and Fe/Al oxides decreased with increased ionic strength due to the decreased thickness of the diffuse layers. The results should provide a useful reference for using the streaming potential method to investigate the effect of the colloidal transport process on the properties of bulk soil in situ.

The Al forms on maize and soybean roots were investigated to determine the main factors affecting the distribution of Al forms and its relationship with Al plant toxicity.Solution culture experiments were conducted to obtain the fresh roots of maize and soybean. KNO3, citric acid, and HCl were used to extract the exchangeable, complexed, and precipitated forms of Al on the roots.The complexed Al was higher than the exchangeable and precipitated Al. Root CECs of soybean and maize were 77 and 55 cmol kg−1, and functional groups on the soybean roots (262.4 cmol kg−1) were greater than on maize roots (210.8 cmol kg−1), which resulted in more exchangeable and complexed Al on soybean roots than on maize roots, and was one of the reasons for the increased Al toxicity to soybean. The total and exchangeable Al were the highest on the plant root tips and decreased gradually with increasing distance from the tips. Ca2+, Mg2+, and NH4+ cations reduced the exchangeable Al on the roots. Oxalate and malate also reduced the adsorption and absorption of Al by roots, and the effect of oxalate was greater than malate.Higher exchangeable and complexed Al on plant roots led to increased Al plant toxicity. Ca2+, Mg2+, and NH4+ and oxalate and malate can effectively alleviate Al plant toxicity.

Industrial by-products are being explored as alternatives to lime and gypsum for correcting soil acidity in top- and sub-soil because they are cheap and easy to obtain. In this study, the ameliorating effect of alkaline slag (AS) on soil acidity in an Ultisol profile was compared with lime (CaCO3) and phosphogypsum (PG).A column leaching experiment was used, and the leaching columns received a total of 1,632 mm of simulated rainfall which was divided into ten events over 2 months.Results indicated that lime increased soil pH and exchangeable calcium (Ca2+) in top- and sub-soil of 0–20 cm but did not affect soil acidity of layers below 20 cm. AS had a similarly positive effect to lime in increasing soil pH and decreasing exchangeable acidity in top- and sub-soil but increased total amount of soil exchangeable Ca2+ and magnesium (Mg2+) throughout the 40-cm profile. Surface application of AS had a comparable effect to PG, lime + PG, and AS + PG in increasing soil exchangeable Ca2+ in the sub-soil. Moreover, AS was more effective than lime and PG in enriching Mg in the Ultisol. Lime + PG and AS + PG had similar effects in alleviating soil acidity throughout the soil profile. Sulfate (SO42−) in the PG and SO42− and chloride (Cl−) in AS, as accompanying anions, promoted the migration of Ca2+ or Mg2+ from the top- to sub-soil and induced their ameliorating effects on sub-soil acidity.AS can play the same role as lime + PG in correcting soil acidity in the top- and sub-soil in this Ultisol, since AS contains both calcium carbonate and calcium sulfate.

The biochars were prepared from straws of canola, corn, soybean, and peanut at different temperatures of 300, 500, and 700 °C by means of oxygen-limited pyrolysis. Amelioration effects of these biochars on an acidic Ultisol were investigated with incubation experiments, and application rate of biochars was 10 g/kg. The incorporation of these biochars induced the increase in soil pH, soil exchangeable base cations, base saturation, and cation exchange capacity and the decrease in soil exchangeable acidity and exchangeable Al. The ameliorating effects of biochars on acidic soil increased with increase in their pyrolysis temperature. The contribution of oxygen-containing functional groups on the biochars to their ameliorating effects on the acidic soil decreased with the rise in pyrolysis temperature, while the contribution from carbonates in the biochars changed oppositely. The incorporation of the biochars led to the decrease in soil reactive Al extracted by 0.5 mol/L CuCl2, and the content of reactive Al was decreased with the increase in pyrolysis temperature of incorporated biochars. The biochars generated at 300 °C increased soil organically complexed Al due to ample quantity of oxygen-containing functional groups such as carboxylic and phenolic groups on the biochars, while the biochars generated at 500 and 700 °C accelerated the transformation of soil exchangeable Al to hydroxyl-Al polymers due to hydrolysis of Al at higher pH. Therefore, the crop straw-derived biochars can be used as amendments for acidic soils and the biochars generated at relatively high temperature have great ameliorating effects on the soils.

Hydroxyl ion release by maize (Zea mays L.) roots under acidic conditions was investigated with a view to develop a bioremediation method for ameliorating acid soils in tropical and subtropical regions.Two hydroponic culture experiments and one pot experiment were conducted: pH, nitrogen state, and rhizobox condition, which investigated the effects of different nitrogen forms on hydroxyl release by maize roots under acidic conditions.The pH of the culture solution increased as culture time rose. The gradient of change increased with rising NO3−/NH4+ molar ratios. Maize roots released more hydroxyl ions at pH 4.0 than at pH 5.0. The amount of hydroxyl ions released by maize roots at a constant pH was greater than those at a nonconstant pH. Application of calcium nitrate reduced exchangeable acidity and increased the pH in an Ultisol rhizosphere, compared with bulk soil. The increasing magnitude of soil pH was greater at higher doses of N. The absorption of NO3−–N increased as the NO3−/NH4+ molar ratios rose, which was responsible for hydroxyl ion release and pH increases in culture solutions and rhizosphere.Root-induced alkalization in the rhizosphere resulting from nitrate absorption by maize plants can be used to ameliorate acidic Ultisols.

Biochars derived from the straws of rice, soybean, and peanut were prepared and modified with aluminum [Al(III)]. These modifications shifted zeta potential–pH curves of the biochars in a positive-value direction and changed surface charge of biochars from negative to positive under acidic conditions. The isoelectric points for 0.6 M Al(III)-modified rice, soybean, and peanut straw biochars were 8.0, 7.8, and 7.5, respectively. Electrostatic attraction of the positively charged surfaces on Al(III)-modified biochars to arsenate [As(V)] enhanced its sorption. The sorption of As(V) by these Al(III)-modified biochars was investigated in batch experiments. Al(III)-modified biochars had greater sorption capacity under acidic conditions compared with corresponding unmodified biochars. While unmodified biochars sorbed negligible amounts of As(V), their Al(III)-modified forms sorbed 445–667 mmol kg−1 at pH 5.0, which were predicted by the Langmuir equation. Modifications with 0.3 M Al3+ improved sorption capacity of As(V) on soybean straw biochar to 445 mmol kg−1, which was further increased by 50 % after modification with 0.6 M Al3+. These As(V) sorption capacities of biochars modified with 0.6 M Al3+ were larger than those of Fe/Al oxides determined at the same pH, which were < 500 mmol kg−1. Thus, biochars modified with 0.6 M Al3+ could substitute Fe/Al oxides used for water purification. However, the sorption of As(V) by the Al(III)-modified biochars increased with decreasing suspension pH. Thus, As(V) removal by Al(III)-modified biochars is suggested to be conducted under acidic conditions, but at pH > 4.0.

The purpose of this study is to compare the relative contribution of different mechanisms to the enhanced adsorption of Cu(II), Pb(II) and Cd(II) by variable charge soils due to incorporation of biochars derived from crop straws. The biochars were prepared from the straws of canola and peanut using an oxygen-limited pyrolysis method at 350 °C. The effect of biochars on adsorption and desorption of Cu(II), Pb(II) and Cd(II) by and from three variable charge soils from southern China was investigated with batch experiments. Based on the desorption of pre-adsorbed heavy metals, the electrostatic and non-electrostatic adsorptions were separated. EDTA was used to replace the heavy metals complexed with biochars and to evaluate the complexing ability of the biochars with the metals. The incorporation of biochars increased the adsorption of Cu(II), Pb(II) and Cd(II) by the soil; peanut straw char induced a greater increase in the adsorption of the three metals. The increased percentage of Cd(II) adsorption induced by biochars was much greater than that for the adsorption of Cu(II) and Pb(II). Cu(II) adsorption on three variable charge soils was enhanced by the two biochars mainly through a non-electrostatic mechanism, while both electrostatic and non-electrostatic mechanisms contributed to the enhanced adsorption of Pb(II) and Cd(II) due to the biochars. Peanut straw char had a greater specific adsorption capacity than canola straw char and thus induced more non-electrostatic adsorption of Cu(II), Pb(II) and Cd(II) by the soils than did the canola straw char. The complexing ability of the biochars with Cu(II) and Pb(II) was much stronger than that with Cd(II) and thus induced more specific adsorption of Cu(II) and Pb(II) by the soils than that of Cd(II). Biochars increased heavy metal adsorption by the variable charge soils through electrostatic and non-electrostatic mechanisms, and the relative contribution of the two mechanisms varied with metals and biochars.

The effect of Fe oxides on the natural acidification of highly weathered soils was investigated to explore the natural acidification process in variable charge soilsA variety of highly weathered soils with different Fe oxide contents were collected from the tropical and subtropical regions of southern China to investigate the soil acidity status. Electrodialysis experiments were conducted to simulate natural acidification process and promote accelerated acidification in a variety of systems such as relatively less weathered soils, mixtures of goethite with montmorillonite or kaolinite, an Alfisol, a limed Ultisol, and Fe oxides coated montmorillonite. The objective was to gather evidence for the occurrence of Fe oxide inhibited natural acidification in highly weathered soils.Highly weathered soils with free Fe2O3 < 100 g/kg (17 soils) had an average pH = 4.64 ± 0.06, while the soils with free Fe2O3 > 100 g/kg (49 soils) had an average pH = 5.25 ± 0.04. A significant linear relationship was found between the soil pH and Fe oxide content of these soils. Similar results were obtained in electrodialysis experiments, i.e., in soils that underwent accelerated acidification. A negative correlation was found between the Fe oxide content and exchangeable acidity or effective cation exchange capacity, respectively. In another set of experiments, goethite slowed down acidification in experiments conducted with this Fe oxide and montmorillonite, or kaolinite, or an Alfisol, or a limed Ultisol. The overlapping of the electrical double layers on the positively charged Fe oxide particles and negatively charged minerals may have caused the release and subsequent leaching of the base cations, but inhibited the production of exchangeable acidity cations. In addition, when montmorillonite or Fe oxide-coated montmorillonite were electrodialyzed in another set of experiments, exchangeable acidity of the former was much greater than that of the latter, suggesting that the positively charged Fe oxide coatings on montmorillonite have partially neutralized the permanent negative charge on montmorillonite surfaces, decreasing exchangeable acidity.Fe oxides may function as natural “anti-acidification” agents through electric double-layer overlapping and coating of phylliosilicates in highly weathered soils.

The key factors influencing pH buffering capacity of acid soils from tropical and subtropical regions, and effects of soil evolution and incorporation of biochars on pH buffering capacity were investigated to develop suitable methods to increase pH buffering capacity of acid soils.A total of 24 acid soils collected from southern China were used. The pH buffering capacity was determined using acid–base titration. The values of pH buffering capacity were obtained from the slope of titration curves of acid or alkali additions plotted against pH in the pH range 4.0–7.0. Two biochars were prepared from straws of peanut and canola using a low temperature pyrolysis method. After incubation of three acid soils, pH buffering capacity was then determined.pH buffering capacity had a range of 9.1–32.1 mmol kg–1 pH–1 for 18 acid soils from tropical and subtropical regions of China. The pH buffering capacity was highly correlated (R2 = 0.707) with soil cation exchange capacity (CEC) measured with ammonium acetate method at pH 7.0 and decreased with soil evolution due to the decreased CEC. Incorporation of biochars at rates equivalent to 72 and 120 t ha−1 increased soil pH buffering capacity due to the CEC contained in the biochars. Incorporation of peanut straw char which itself contained more CEC and alkalinity induced more increase in soil CEC, and thus greater increase in pH buffering capacity compared with canola straw char. At 5% of peanut straw char added, soil CEC increased by 80.2%, 51.3%, and 82.8% for Ultisol from Liuzhou, Oxisol from Chengmai and Ultisol from Kunlun, respectively, and by 19.8%, 19.6%, and 32.8% with 5% of canola straw char added, respectively; and correspondingly for these soils, the pH buffering capacity increased by 73.6%, 92.0%, and 123.2% with peanut straw char added; and by 31.3%, 25.6%, and 52.3% with canola straw char added, respectively. Protonation/deprotonation of oxygen-containing functional groups of biochars was the main mechanism for the increase of pH buffering capacity of acid soils with the incorporation of biochars.CEC was a key factor determining pH buffering capacity of acid soils from tropical and subtropical regions of China. Decreased CEC and content of 2:1-type clay minerals during evolution of tropical soils led to decreased pH buffering capacity. Incorporation of biochars generated from crop straws did not only ameliorate soil acidity, but also increased soil pH buffering capacity.

The amelioration effects of crop straws and their biochars on an acidic ultisol were compared in incubation experiments to determine suitable organic amendments for acid soils.Four crop straws, including non-legumes (canola straw and rice straw) and legumes (soybean straw and pea straw) were used to prepare biochars using a low temperature (350°C) oxygen-limited pyrolysis method. Two application rates of 1% and 2% were used for both crop straws and their biochars in incubation experiments lasting 90 days. Soil pH (1:2.5 soil to water), soil exchangeable acidity, soil exchangeable base cations, and soil cation exchange capacity (CEC) were determined to evaluate the amelioration effects of these crop straws and their biochars on an acidic ultisol.The incorporation of crop straws increased or decreased the soil pH depending on the relative contribution of alkalinity of the straws, mineralization of organic N and nitrification of NH4+. The incorporation of biochars produced from crop straws increased the soil pH, and their ameliorating effects increased with the application rates of biochars. The biochars from legume straws induced more increase in soil pH than non-legume biochars. The addition of both crop straws and their biochars decreased soil exchangeable acidity and exchangeable Al3+, and increased soil exchangeable base cations and base saturation degree. The biochars (especially legumes) induced a greater decrease in soil exchangeable acidity and a greater increase in soil exchangeable base cations compared to their feedstock due to their much higher contents of base cations. The CEC of biochars were 10–20 times that of soil CEC and thus biochar incorporation increased the soil CEC significantly, as well as the retention of Ca2+, Mg2+, K+, and NH4+ by acid soils.The biochars produced from legume crop straws were better choices as amendments for acid soils than their feedstock. Organic anions and carbonates were the main forms of alkali in the biochar; both contributed to neutralizing soil acidity and increasing soil pH. The incorporation of biochar cannot only neutralize soil acidity, but can also improve soil fertility.

The effect of a tea plantation on soil basic properties, chemical and mineralogical compositions, and magnetic properties of Alfisols from eastern China was studied. Under the tea plantation, acidification took place within a soil depth of 70 cm, with the maximum difference in pH in the upper 17 cm (ΔpH = 2.80). Both the tea plantation and unused soil profiles were predominated by free Fe and Al oxides, i.e. citrate/bicarbonate/dithionite extractable Fe (Fed) and Al (Ald). Tea plantation soil was characterized by higher Ald and Fed and lower Fe oxalate, Fe2O3 and Al2O3; CaO was depleted, whereas SiO2 accumulated. Acidification induced by the tea plantation led to destruction of vermiculite followed by dissolution of the hydroxy-Al interlayers within its structure. The data clearly demonstrated that significant soil weathering occurred with acidification caused by tea cultivation. This acidification also resulted in decreased content of ferrimagnetic minerals due to the dissolution of minerals and movement of Fe in the profile.

In the present investigation, the enhancement of Cu(II) adsorption on goethite and γ-Al2O3 by increasing ionic strength under acidic conditions was observed. The decrease of the electrostatic potential at the adsorption plane on the Fe/Al oxides with increasing ionic strength was responsible for the enhanced adsorption of Cu(II) by the oxides. The ζ-potential of goethite and γ-Al2O3 obtained under different ionic strengths gave evidence to support the interpretation. The value of the ζ-potential of goethite and γ-Al2O3 indicated that the potential at the adsorption plane decreased with the increase in ionic strength when the pH was less than the PZSE (point of zero salt effect) of the Fe/Al oxides, which was exactly opposite to the changing trends of surface charge of these oxides with ionic strength. Therefore, the decrease of the potential at the adsorption plane on these Fe/Al oxides with increasing ionic strength favored the Cu(II) adsorption.

Phyllosilicates with net negative surface charge and Fe/Al oxides with net positive surface charge co-exist in variable charge soils and the interaction between these oppositely charged particles affects the stability of mixed colloids, aggregation, and even the surface chemical properties of variable charge soils. The overlapping of the diffuse layers of electrical double layers between kaolinite and goethite, hematite, gibbsite and γ-Al2O3 was investigated in this article based on the change of zeta potential and electrokinetic charge density induced by the addition of Fe/Al oxides. The results indicated that in the range of pH from 3 to 7, the kaolinite is negatively charged and the Fe/Al oxides are positively charged, and the overlapping of the diffuse layers led to the increase in zeta potential and the decrease in surface charge density of binary-system containing kaolinite and Fe/Al oxides compared with the single kaolinite system. The presence of gibbsite resulted in the strongest interaction of electrical double layers between this and kaolinite, followed by goethite and hematite. The interaction extent of electrical double layers between kaolinite and Fe/Al oxides increased with the amount of Fe/Al oxides added. The interaction also resulted in the increase in IEP of the binary-system containing kaolinite and Fe/Al oxides. The increase in ionic strength led to the decrease in the thickness of colloid diffuse layer and thus resulted in the reduction of overlapping of the diffuse layers between kaolinite and Fe/Al oxides.

Low-molecular-weight (LMW) organic acids may be adsorbed by soils and the adsorption could affect their biodegradation and efficiency in many soil processes. In the present study, the adsorption of phthalic acid and salicylic acid and their effect on the exchangeable Al capacity of variable-charge soils were investigated. The results indicated that phthalic acid and salicylic acid were adsorbed by four variable-charge soils to some extent, oxisols showed a greater adsorption capacity for organic acids than ultisols, and the ability of the four variable-charge soils to adsorb the organic acids at different pH generally followed the order Kunming oxisol > Xuwen oxisol > Jinxian ultisol > Lechang ultisol, which was closely related to their content of free iron oxides and amorphous iron and aluminum oxides. The adsorption of organic acids induced a decrease in the zeta potentials of soils and oxides. Goethite has greater adsorption capacity for organic acid than Xuwen oxisol and the adsorption of organic acids resulted in a bigger decrease in the zeta potential of goethite suspensions. After free iron oxides were removed, less organic acid was adsorbed by Xuwen oxisol and no change was observed in zeta potential for the soil suspension after organic acid was added. The presence of phthalic acid increased the capacity of exchangeable Al and the increment in the four variable-charge soils also followed the order Kunming oxisol > Xuwen oxisol > Lechang ultisol and Jinxian ultisol. The presence of salicylic acid increased the capacity of exchangeable Al in Kunming oxisol, Xuwen oxisol, and Jinxian ultisol, but decreased it in Lechang ultisol due to less adsorption of the acid and formation of soluble Al–salicylate complexes in solution. After free iron oxides were removed, less effect of organic acid on exchangeable Al was observed for Xuwen oxisol, which further confirmed that the iron oxides played a significant role in organic acid adsorption and had a consequent effect on the capacity of exchangeable Al in variable-charge soils. Therefore, the higher the content of iron oxides, the greater the adsorption of organic acids by soils and the greater the increase in soil exchangeable Al induced by the organic acids.

Interaction of the diffuse layers of electric double layer between negatively charged kaolinite particle and positively charged particles of goethite, gibbsite, hematite or γ-Al2O3 is investigated in this article through the comparison of zeta potential between single kaolinite system and the binary-system containing kaolinite and Fe/Al oxides. The overlapping of the diffuse layers on kaolinite and Fe/Al oxides decreases the effective negative charge density on kaolinite and thus increases the zeta potential of the binary-system containing kaolinite and Fe/Al oxides. The interaction of the diffuse layers increases with the increase in the ratio of kaolinite to Fe/Al oxides and decreases with the rise of system pH. The interaction also relates to the charge characteristics on the Fe/Al oxides. Gibbsite which posses comparative maximum positive charge density leads to the strongest interaction of the diffuse layers between the oxide and kaolinite as followed by goethite, and hematite, and the γ-Al2O3 with the least positive charge density results in the weakest interaction of the diffuse layers. The outcome of the present investigation is likely to be useful reference for the interpretation of the interaction of the diffuse layers in authentic variable charge soils.

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes, such as nutrient availability, translocation of metals, fate of heavy metals, and mineral weathering. In this paper, the effect of the LMW organic anions on the exchangeable aluminum of two variable-charge soils was examined. The results showed that the organic anions induced an increase or a decrease in the exchangeable Al, and the extent and direction of the effect depended on the nature of organic anions, surface chemical properties of soils, and pH. For example, at pH 4.5, the quantity of exchangeable Al of Oxisol in the control system was 2.65 mmol kg−1, whereas the values in the citrate, oxalate, malonate, malate, tartarate, salicylate, and lactate systems increased by 3.25, 1.93, 1.95, 1.82, 1.28, 0.88, and 0.45 times, respectively. In contrast, the quantity of the exchangeable Al of Ultisol at pH 4.5 in the oxalate and the citrate systems decreased by 8.8 and 19.6%, respectively. The increase in the exchangeable Al was caused mainly by the increase in negative surface charge of the soils due to the specific adsorption of organic anions. The ability of organic anions at low concentrations to increase exchangeable Al for Oxisol followed the order citrate > oxalate and malonate > malate > tartarate > salicylate > maleate > lactate. This order is consistent with that of the effect of the adsorption of anions on the increase in the negative surface charge and/or the decrease in the positive surface charge of the soil. On the other hand, the organic anions could depress the exchangeable Al through the formation of soluble Al–organic anion complexes under certain conditions. The anions with small stability constants of Al–organic anion complexes, such as lactate, caused an increase in exchangeable Al with the change in surface charge of the soils, while those with large stability constants, such as citrate and oxalate, caused an increase in the exchangeable Al at low concentration and a decrease at high concentration.

•Biomass ash, bone meal and alkaline slag can be used as amendments of acid Ultisols.•Combined application of three amendments was more effective to correct soil acidity.•Combined application of three amendments increased contents of soil Ca, Mg, K and P.•Application of amendments did not increase soil available contents of heavy metals.Incubation experiments with biomass ash (BA), bone meal (BM) and alkaline slag (AS) applied alone and together were conducted to ameliorate acidity and increase nutrient contents of five Ultisols collected from four provinces of southern China. The results showed that the ameliorating effect of the combined application of three amendments on soil acidity was greater than that of BA + BM or their single applications. The combined application of BA, BM and AS increased the pH by 0.63–1.37 for five Ultisols. The most significant decrease of soil exchangeable acidity was also observed in the treatments of BA + BM + AS by 80.1–96.9% for five Ultisols. Meanwhile, the combined application of the amendments greatly increased the exchangeable potassium, calcium and magnesium of the five soils, respectively, by 0.4–3.8, 1.9–10 and 1.7–9.7 times. The contents of available phosphorus of the five soils were also increased significantly by 0.6–184 times due to the application of BA + HBM + AS. In general, the application of the amendments did not increase available heavy metals in the soils in this short-term incubation experiment, but the potential risk of the amendments applied should be further assessed under field conditions through long-term experiments.

The incorporation of two legume materials increased soil pH, while the nitrification of the ammonium–nitrogen (N) from the ammonification of organic N weakened the liming potential of these materials at the later stages of incubation experiments. The addition of dicyandiamide (DCD) enhanced the liming potential of the two legume materials through its inhibition of nitrification of ammonium-N. At the end of the 60-d incubation, soil pH of the treatment using Chinese milk vetch shoots with DCD was 1.48 units higher than that of control and 1.16 units higher than of the treatment using only Chinese milk vetch. The corresponding data for the pea straw were 1.24 and 1.01 units higher, respectively.

In this study, biochars from rice straw (Oryza sativa L.) were prepared at 200–600 °C by oxygen-limited pyrolysis to investigate the changes in properties of rice straw biochars produced at different temperatures, and to examine the adsorption capacities of the biochars for a heavy metal, copper(II) (Cu(II)), and an organic insecticide of cyromazine, as well as to further reveal the adsorption mechanisms. The results obtained with batch experiments showed that the amount of Cu(II) adsorbed varied with the pyrolysis temperatures of rice straw biochar. The biochar produced at 400 °C had the largest adsorption capacity for Cu(II) (0.37 mol kg−1) among the biochars, with the non-electrostatic adsorption as the main adsorption mechanism. The highest adsorption capacity for cyromazine (156.42 g kg−1) was found in the rice straw biochar produced at 600 ° C, and cyromazine adsorption was exclusively predominated by surface adsorption. An obvious competitive adsorption was found between 5 mmol L−1 Cu(II) and 2 g L−1 cyromazine when they were in the binary solute system. Biochar may be used to remediate heavy metal- and organic insecticide-contaminated water, while the pyrolysis temperature of feedstocks for producing biochar should be considered for the restoration of multi-contamination.

It is imperative to choose some low cost, available and effective ameliorants to correct soil acidity in southern China for sustainable agriculture. The present investigation dealt with the possible role of industrial byproducts, i.e., coal fly ash (CFA), alkaline slag (AS), red mud (RM) and phosphogypsum (PG) in correcting acidity and aluminum (Al) toxicity of soils under tea plantation using an indoor incubation experiment. Results indicated that CFA, AS and RM increased soil pH, while PG decreased the pHs of an Ultisol and an Alfisol. The increment of soil pH followed the order of RM > AS > CFA. All the industrial byproducts invariably decreased exchangeable Al and hence increased exchangeable Ca, Mg, K and Na and effective cation exchange capacity. RM, AS and lime decreased total soluble Al, exchangeable Al and organically bound Al. Formation and retention of hydroxyl-Al polymers were the principal mechanism through which Al phytotoxicity was alleviated by application of these amendments. In addition, the heavy metal contents in the four industrial byproducts constituted a limited environmental hazard in a short time at the rates normally used in agriculture. Therefore, the short-term use of the byproducts, especially AS and RM, as amendments for soil acidity and Al toxicity in acid soils may be a potential alternative to the traditional use of mined gypsum and lime.

The liming potential of some crop residues and their biochars on an acid Ultisol was investigated using incubation experiments. Rice hulls showed greater liming potential than rice hull biochar, while soybean and pea straws had less liming potential than their biochars. Due to their higher alkalinity, biochars from legume materials increased soil pH much compared to biochars from non-legume materials. The alkalinity of biochars was a key factor affecting their liming potential, and the greater alkalinity of biochars led to greater reductions in soil acidity. The incorporation of biochars decreased soil exchangeable acidity and increased soil exchangeable base cations and base saturation, thus improving soil fertility.

Potassium (K) and nitrogen (N) are essential nutrients for plants. Adsorption and desorption in soils affect K+ and NH+4 availabilities to plants and can be affected by the interaction between the electrical double layers on oppositely charged particles because the interaction can decrease the surface charge density of the particles by neutralization of positive and negative charges. We studied the effect of iron (Fe)/aluminum (Al) hydroxides on desorption of K+ and NH+4 from soils and kaolinite and proposed desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and the positively charged Fe/Al hydroxide particles. Our results indicated that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al hydroxides, as amorphous Al(OH)3 or Fe(OH)3, and negatively charged surfaces from an Ultisol, an Alfisol, and a kaolinite standard caused the effective negative surface charge density on the soils and kaolinite to become less negative. Thus the adsorption affinity of these negatively charged surfaces for K+ and NH+4 declined as a result of the incorporation of the Fe/Al hydroxides. Consequently, the release of exchangeable K+ and NH+4 from the surfaces of the soils and kaolinite increased with the amount of the Fe/Al hydroxides added. The greater the positive charge on the surfaces of Fe/Al hydroxides, the stronger was the interactive effect between the hydroxides and soils or kaolinite, and thus the more release of K+ and NH+4. A decrease in pH led to increased positive surface charge on the Fe/Al hydroxides and enhanced interactive effects between the hydroxides and soils/kaolinite. As a result, more K+ and NH+4 were desorbed from the soils and kaolinite. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of K+ and NH+4 in the soils and thus increase their leaching lsos.

To evaluate the role of kaolinite and variable charge soils on the hydrolytic reaction of Al, the hydrolysis of Al ions in suspensions of a kaolinite and an Oxisol influenced by organic anions was investigated using changes of pH, Al adsorption, and desorption of pre-adsorbed Al. Kaolinite and the Oxisol promoted the hydrolytic reaction of Al above a certain initial Al concentration (0.1 mmol L−1 for kaolinite and 0.3 mmol L−1 for the Oxisol). The Al hydrolysis accelerated by kaolinite and the Oxisol increased with an increase in initial concentration of Al and was observed in the range of pH from 3.7 to 4.7 for kaolinite and 3.9 to 4.9 for the Oxisol. The acceleration of Al hydrolysis also increased with the increase of solution pH, reached a maximum value at pH 4.5, and then decreased sharply. Al hydrolysis was promoted mainly through selective adsorption for hydroxy-Al. Soil free iron oxides compensated a portion of the soil negative charge or masked some soil surface negative sites leading to a decrease in Al adsorption, which retarded acceleration to some extent. For the Oxisol organic anions increased the proportion of adsorbed Al3+ in total adsorbed Al with the increase in soil negative surface charge and eliminated or reduced the acceleration of Al hydrolysis. Different organic anions inhibited the hydrolysis of Al in the order: citrate > oxalate > acetate (under initial pH of 4.5). The formation of Al-organic complexes in solution also inhibited the hydrolysis of Al.

This investigation was conducted by using alkaline slag and crop straw biochars to reduce acidity of an acidic Ultisol through incubation and pot experiments with lime as a comparison. The soil was amended with different liming materials: lime (1 g kg−1), alkaline slag (2 and 4 g kg−1), peanut straw biochar (10 and 20 g kg−1), canola straw biochar (10 and 20 g kg−1) and combinations of alkaline slag (2 g kg−1) and biochars (10 g kg−1) in the incubation study. A pot experiment was also conducted to observe the soybean growth responses to the above treatments. The results showed that all the liming materials increased soil pH and decreased soil exchangeable acidity. The higher the rates of alkaline slag, biochars, and alkaline slag combined with biochars, the greater the increase in soil pH and the reduction in soil exchangeable acidity. All the amendments increased the levels of one or more soil exchangeable base cations. The lime treatment increased soil exchangeable Ca2+, the alkaline slag treatment increased exchangeable Ca2+ and Mg2+ levels, and the biochars and combined applications of alkaline slag with biochars increased soil exchangeable Ca2+, Mg2+ and K+ and soil available P. The amendments enhanced the uptake of one or more nutrients of N, P, K, Ca and Mg by soybean in the pot experiment. Of the different amendments, the combined application of alkaline slag with crop straw biochars was the best choice for increasing base saturation and reducing soil acidity of the acidic Ultisol. The combined application of alkaline slag with biochars led to the greatest reduction in soil acidity, increased soil Ca, Mg, K and P levels, and enhanced the uptake of Ca, Mg, K and P by soybean plants.

Limited information is available on the changes of surface chemical properties of tropical soils with time during the pedogenesis. Soil samples of three profiles derived from basalts of 10, 1330 and 2290 kilo annum (ka) in age were collected from adjacent locations in a tropical region of Hainan Province, China. The changes in soil surface chemical properties and the mineralogy of the soil clay fraction with time were investigated using ion adsorption, micro-electrophoresis, and X-ray diffraction analysis. The content of 2:1-type clay minerals decreased, while those of kaolinite and gibbsite increased with increasing basalt age and degree of soil development. The content of pedogenic free iron (Fe) oxides and the ratio of free Fe oxides/total Fe oxides increased with soil development stage, while soil poorly crystalline Fe and aluminum (Al) oxides had an opposite trend. The positive surface charge of the soils increased with increasing basalt age and degree of soil development; this was consistent with the change in their contents of free Fe/Al oxides. However, the value of negative surface charge had an opposite behavior. The soil derived from 10-ka-basalt had much more negative charge than soils derived from 1330- and 2290-ka-basalt. Soil net surface charge and zeta potential of the soil clay-fraction decreased with the increase in basalt age. Both net charge–pH curves and zeta potential–pH curves shifted to positive values with increased basalt age and degree of soil development. Increasing age also elevated the point of zero net charge of the soil and the isoelectric point of soil colloids.Highlights► Content of kaolinite and gibbsite increased with increasing basalt age and degree of soil development ► Increase of free Fe2O3 led to the increase in positive charge of the tropical soils ► Increase in soil development elevated PZC and IEP and made negative surface charge less negative

Soil acidification is an important process in land degradation around the world as well as in China. Acidification of Alfisols was investigated in the tea gardens with various years of tea cultivation in the eastern China. Cultivation of tea plants caused soil acidification and soil acidity increased with the increase of tea cultivation period. Soil pH of composite samples from cultivated layers decreased by 1.37, 1.62 and 1.85, respectively, after 13, 34 and 54 years of tea plantation, as compared to the surface soil obtained from the unused land. Soil acidification rates at early stages of tea cultivation were found to be higher than those at the later stages. The acidification rate for the period of 0–13 years was as high as 4.40 kmol H+ ha−1 year−1 for the cultivated layer samples. Soil acidification induced the decrease of soil exchangeable base cations and base cation saturation and thus increased the soil exchangeable acidity. Soil acidification also caused the decrease of soil cation exchange capacity, especially for the 54-year-old tea garden. Soil acidification induced by tea plantation also led to the increase of soil exchangeable Al and soluble Al, which was responsible for the Al toxicity to plants.

Organic ligands in the environment hinder the formation of crystalline Al precipitation products by perturbing the hydrolytic and polymeric reactions of Al resulting in the formation of short-range ordered (SRO) mineral colloids with varying degrees of crystallinity. However, the effect of these ligands on the mechanisms of their formation and nature of the transformation products of Al (oxy)hydroxides at the atomic and molecular levels is not well understood. In this study, the coordination structure of Al in Al (oxy)hydroxides formed under the influence of varying concentrations of low molecular weight (LMW) organic acids such as citric, malic, salicylic and acetic acids and a humic acid (HA) was investigated with X-ray absorption near edge structure (XANES) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction analysis. The Al K- and L-edge XANES spectra showed that with increasing LMW organic acid concentration the coordination number of Al changed from 6-fold to a mixture of 4- and 6-fold, except for acetate as acetate was unable to perturb the formation of Al (oxy)hydroxides at the acetate/Al molar ratio (MR) = 0.1. The proportion of 4-fold to 6-fold coordinated Al in the Al precipitation products depended on the structure and functionality of the LMW organic acids. The incorporation of the LMW organic acid into the network structure of Al (oxy)hydroxides prevented the formation of sheets/inter-layer H-bonding that was required for the formation of crystalline Al (oxy)hydroxides. The HA used in this study only slightly perturbed the crystallization of the Al (oxy)hydroxides at the concentrations used. The Al K-edge data showed that Al coordination number had not been altered in the presence of HA. The findings obtained in the present study are of fundamental significance in understanding the physicochemical behavior of soils and sediments, and their relation to the accumulation and transport of nutrients and pollutants in the environment.

The surface chemical properties of soil samples i.e., surface charge and zeta potential, and the mineralogy of soil clay fraction were investigated with reference to soil weathering extent for four different soils derived from Quaternary red earth using the ion adsorption method, a micro-electrophoresis method and the X-ray diffraction analysis. Results indicated that all these soil samples contained kaolinite and gibbsite. The Ultisols from Guizhou, Hunan and Jiangxi possessed the 2:1 type clay minerals of mica and vermiculite. Hematite and magnetite were found in the Ultisols from Guangxi, Hunan and Jiangxi. Goethite was found in the Ultisols from Jiangxi, Hunan and Guizhou. The positive surface charge for these soils decreased with the order: the Ultisol from Guangxi ≅ the Ultisol from Guizhou > the Ultisols from Hunan and Jiangxi from south to north when pH < 5.0. This is consistent with the content of free Fe/Al oxides present in these soils. On the other hand, the value of negative surface charge on the Ultisol from Guangxi was found much lower than the other soils perhaps because of the intensive weathering of the soil. Both permanent and variable negative charges for the former were also lower than the latter, whereas the point of zero salt effect (PZSE) for the former was greater than that of the latter. The variability of soil negative surface charge followed the order: the Ultisol from Guangxi > the Ultisol from Guizhou > the Ultisol from Jiangxi ≅ the Ultisol from Hunan. The zeta potential and isoelectric point (IEP) of soil colloids and soil net surface charge followed the same order: the Ultisol from Guangxi > the Ultisol from Guizhou > the Ultisols from Hunan and Jiangxi. A good correlation between zeta potential and net surface charge of these soils was observed. Therefore, the magnitudes of the PZSE, IEP and zeta potential of these soils were in agreement with the weathering extent of the soils and can be employed as reference criteria for classification and evolution of soils.