An investigation of phosphate adsorbed on aluminium oxyhydroxide and oxide phases by nuclear magnetic resonance

The adsorption of phosphate by soil minerals controls availability of P to plants, but the chemical environments of adsorbed phosphate are poorly known. We used ³¹P MAS NMR to study the adsorption of phosphate on to boehmite (γ‐AlOOH) and γ‐Al₂O₃ with large surface areas. The solid phases were reacted in 0.1 m phosphate solutions at pH from 3 to 11 and in solutions with pH 5 at concentrations from 10^?1 m to 10^?4 m . The spectra suggested three different phosphate environments: (i) orthophosphate precipitated from the residual solution after vacuum filtering, (ii) surface‐adsorbed phosphate in inner‐sphere complexes, and (iii) Al‐phosphate precipitates on the surfaces of the minerals. The chemical shifts of both the inner‐sphere complexes and surface precipitates became progressively less shielded with increasing pH and decreasing concentration of phosphate solution. For the inner‐sphere complexes, we interpret these changes to be the result of decreasing phosphate protonation combined with rapid proton exchange among phosphate tetrahedra with different numbers of protons, which causes peak averaging. The chemical shifts of ³¹P of the Al‐phosphate precipitates were more negative than those of the surface phosphates at a given pH and solution concentration, probably because of a larger number of P–O–Al linkages per tetrahedron. The observed trend of decreasing shielding is probably due to the decreasing average number of P–O–Al linkages per tetrahedron combined with decreasing protonation and an increasing number of K⁺ next‐nearest neighbours. Even at small concentrations of phosphate solution, a significant amount of Al‐phosphate precipitate was present.     

Die Veränderung der Phosphatintensität der Bodenlösung eines Pelosol-Pseudogleys nach Grünlandumbruch

Long-term change in phosphate intensity in a clay soil after turning grassland into arable land In a 7years field experiment on a poorly drained clay (Pelosol-Pseudogley) after turning permanent grassland into arable land, the phosphate concentration in the soil solution (P_l) decreased and P retention (P_s) increased even if as much as 265 kg P ha^?1 was added in excess of plant uptake. Neither loss in organic matter nor disaggregation of the well aggregated grassland soil by soil management seemed to be responsible for this decrease in P_l. Instead, a slight increase in pH within the 7 years and, to a lesser extent, a concomitant increase in exchangeable Ca are assumed to have caused the decrease in P_l. Laboratory experiments in which pH, exchangeable Ca and ionic strength were varied support the field results. A multiple correlation between P_l and pH plus exchangeable Ca explained 89% of the variation of P_l whereas pH and Ca alone explained only 76 and 42%, respectively. Solubility data placed the solutions around the hydroxyapatite (HA) isotherm even when P-fertilizer were added in excess of plant uptake. It seems unlikely, therefore that HA governs P_l in this soil. Instead, the results are in favour of surface P adsorption to determine P_l. The effect of exchangeable Ca and ionic strength is then explained by their effects on the thickness of the electric double layer which in turn influences P adsorption.     

Effects of acid sulphate on DOC release in mineral soils: the influence of SO42− retention and Al release

Dissolved organic carbon (DOC) in acid‐sensitive upland waters is dominated by allochthonous inputs from organic‐rich soils, yet inter‐site variability in soil DOC release to changes in acidity has received scant attention in spite of the reported differences between locations in surface water DOC trends over the last few decades. In a previous paper, we demonstrated that pH‐related retention of DOC in O horizon soils was influenced by acid‐base status, particularly the exchangeable Al content. In the present paper, we investigate the effect of sulphate additions (0–437 µeq l^?1) on DOC release in the mineral B horizon soils from the same locations. Dissolved organic carbon release decreased with declining pH in all soils, although the shape of the pH‐DOC relationships differed between locations, reflecting the multiple factors controlling DOC mobility. The release of DOC decreased by 32–91% in the treatment with the largest acid input (437 µeq l^?1), with the greatest decreases occurring in soils with very small % base saturation (BS, < 3%) and/or large capacity for sulphate (SO₄^2?) retention (up to 35% of added SO₄^2?). The greatest DOC release occurred in the soil with the largest initial base status (12% BS). These results support our earlier conclusions that differences in acid‐base status between soils alter the sensitivity of DOC release to similar sulphur deposition declines. However, superimposed on this is the capacity of mineral soils to sorb DOC and SO₄^2?, and more work is needed to determine the fate of sorbed DOC under conditions of increasing pH and decreasing SO₄^2?.     

Controls on carbon accumulation and storage in the mineral subsoil beneath peat in Lakkasuo mire, central Finland

What processes control the accumulation and storage of carbon (C) in the mineral subsoil beneath peat? To find out we investigated four podzolic mineral subsoil profiles from forest and beneath peat in Lakkasuo mire in central boreal Finland. The amount of C in the mineral subsoil ranged from 3.9 to 8.1 kg m^?2 over a thickness of 70 cm and that in the organic horizons ranged from 1.8 to 144 kg m^?2. Rates of increase of subsoil C were initially large (14 g m^?2 year^?1) as the upland forest soil was paludified, but decreased to < 2 g m^?2 year^?1 from 150 to 3000 years. The subsoils retained extractable aluminium (Al) but lost iron (Fe) as the surrounding forest podzols were paludified beneath the peat. A stepwise, ordinary least‐squares regression indicated a strong relation (R² = 0.91) between organic C concentration of 26 podzolic subsoil samples and dithionite–citrate–bicarbonate‐extractable Fe (negative), ammonium oxalate‐extractable Al (positive) and null‐point concentration of dissolved organic C (DOC_np) (positive). We examined the ability of the subsoil samples to sorb dissolved organic C from a solution derived from peat. Null‐point concentration of dissolved C (DOC_np) ranged from 35 to 83 mg l^?1, and generally decreased from the upper to the lower parts of the profiles (average E, B and C horizon DOC_np concentrations of 64, 47 and 42 mg l^?1). The DOC_np was positively correlated with percentage of soil C and silt and clay content. The concentration of dissolved organic C in pore water in the peat ranged from 12 to 60 mg l^?1 (average 33 mg l^?1), suggesting that the sorptive capacity of the subsoil horizons for C had been exhausted. We suggest that the increase of C contents in the subsoil beneath mires is related to adsorption of dissolved organic C and slow mineralization under anaerobic conditions.     

Fluoro‐mobilization of metals in a Slovak forest soil affected by the emissions of an aluminum smelter

Depositions originating from a central Slovak Al smelter may increase metal solubility in adjacent soils because they contain F (mainly HF). The reason for fluoro‐mobilization of metals may be the formation of soluble fluoro‐metal complexes or the mobilization of organic matter and subsequent formation of organo‐metal complexes. The objectives of our work were (1) to assess the extent of metal mobilization by fluoride in a Slovak Lithic Eutrochrept affected by the emissions of an Al smelter and (2) to model the dissolved metal species with the help of a chemical equilibrium model (MINEQL+). The O (Moder), A, and B horizons were equilibrated with solutions at F concentrations of 0, 0.9, 2.7, and 9.0 mmol l^—1. In the extracts, the concentrations of Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, Zn, dissolved organic carbon (DOC), free and complexed F, and the pH and electrical conductivity (EC) were determined. The heavy metal concentrations in the O horizon (Cd: 0.99, Cr: 18.0, Cu: 44, Ni: 26, Pb: 110, and Zn: 84 mg kg^—1) were 2.5 to 9 times larger than those in the A and B horizons. The concentrations of H₂O‐soluble F decreased from the O (261 mg kg^—1) to the A (103 mg kg^—1) and B horizon (92 mg kg^—1). In batch experiments increasing addition of F increased the equilibrium concentrations of Al, Cr, Cu, Fe, Ni, Pb, and DOC in all samples, of Cd in the A, and of K in the B horizon. At the same time the concentrations of complexed F and pH increased whereas EC decreased. Chemical equilibrium modelling indicated that the mobilizing effect of F resulted from the formation of fluoro‐Al complexes and organo‐complexes of all other metals.     

Vermicomposting leachate as liquid fertilizer for the cultivation of sugarcane (Saccharum sp.)

In vermicomposting, the main product is the worm casts, but a leachate is generated that contains large amounts of plant nutrients. This leachate is normally diluted to avoid plant damage. We investigated how dilution of vermicompost leachate combined with different concentrations of nitrogen (N) - phosphorus (P) - potassium (K) triple 17 fertilizer, and polyoxyethylene tridecyl alcohol as dispersant and polyethylene nonylphenol as adherent to increase efficiency of fertilizer uptake, affected sugarcane plant development. The vermicomposting leachate with pH 7.8 and electrolytic conductivity 2.6 dS m^?1, contained 834 mg potassium (K) l^?1, 247 mg nitrate (NO₃^?) l^?1 and 168 mg phosphate (PO₄^3?) l^?1, was free of pathogens and resulted in a 65% germination index. Vermicompost leachate did not inhibit sugarcane growth and mixed with 170 g l^?1 NPK triple 17 fertilizer resulted in the best plant development. No dispersant or adherent was required to improve plant height and stem development.     

Elemental bioavailability in nutrient solutions in relation to precipitation reactions

In hydroponic plant nutritional research, nutrient solutions can be considered as aqueous solutions of inorganic ions. In this aqueous solution, the ions are submitted to the laws of aquatic inorganic chemistry. This means that the ions are involved in the dynamic equilibria between complexation, dissociation, and precipitation reactions. These chemical reactions seriously impact elemental speciation and bioavailability. As a result, plant roots experience a different nutritional composition. Ions withdrawn from the‐nutrient solution due to precipitation reactions, change the nutritional composition and are not available for uptake by plant roots. Like complexes, precipitates can buffer a nutrient solution, exchanging nutrients as these decrease by plant uptake. This research looks into the precipitation reactions that occur in hydroponic nutrient solutions. In the concentration range of nutrient solutions, no precipitates are formed involving potassium (K⁺), nitrate (NO₃ ^‐), ammonium (NH₄ ⁺), or sulphate (SO₄ ^2‐), while calcium (Ca²⁺) and magnesium (Mg²⁺) form mainly at a higher pH precipitates with hydrogen phosphate (HPO₄ ^2‐). Preparing nutrient solutions with tap water, calcium carbonate (CaCO₃) is likely to precipitate. A good knowledge of the chemical reactions occurring in nutrient solutions is the first prerequisite in hydroponic plant nutritional research.     

Free energy and kinetics of dissolution of Sokoto rock phosphate and the implications for replenishing phosphorus in the savanna soil of Nigeria

The direct application of Sokoto phosphate rock to restore phosphorus in the savanna soil of Nigeria has not been very successful. The dissolution of Sokoto phosphate rock was investigated in three electrolyte solutions – 0.01 m CaCl₂, NaCl and KCl – at pH range 3.5–7.0 under laboratory conditions to provide solubility and kinetic data that are required to develop guidelines for direct application in the field. The phosphate rock dissolved in the salt solutions in the order KCl > NaCl > CaCl₂. Particle size and ionic strength had no significant effect on the dissolution. The standard free energy of reaction ΔG°_R in an acidic solution with no basic cations was ?38 kJ mol^?1. If Ca²⁺ ions were in the acidic solution, then ΔG°_R increased to 210 kJ mol^?1, 170 kJ mol^?1 for Na⁺ ions, and 107 kJ mol^?1 for K⁺ ions in the solution. The theoretical solubility constant (K_s) calculated from the relation ΔG° = ?RT ln K_s gave K_s = 10^6.7 in an acidic solution without basic cations, but decreased to 10^?36.8 with Ca²⁺ ions in solution, 10^?29.8 with Na⁺ ions, and 10^?18.8 with K⁺ ions in solution. At pH ≥ 5.5, the dissolution was more constrained by Ca²⁺ ions or basic cations in solution than by availability of protons. The kinetics of the dissolution reaction was best described by a power function: C_t = at^b, where C_t is the amount of P released from the rock phosphate at time t, and a and b are fitting parameters. An Elovich and a parabolic diffusion expression equally gave satisfactory fits to the dissolution data, suggesting that the rate of dissolution was limited by a combination of film‐ and intra‐particle diffusion. To utilize this rock phosphate as an effective source of P, management practices that increase Ca sinks and the supply of protons to the soil are necessary. In the savanna, increasing the soil's organic matter greatly enhances cation exchange capacity and availability of protons. The practice should provide adequate sinks for Ca²⁺ and the acidic environment required for the release of P from rock phosphate.     

Effects of dissolved organic matter on the mobility of copper in a contaminated sandy soil

Changes in land use can result in increased soil organic matter content, and decreases in Ca and pH which will affect the mobility of Cu in soil. We studied how the mobility and coagulation of dissolved organic matter and pH affected the mobility of Cu in contaminated sandy soil by batch and column experiments in the laboratory. The soil, with pH ranging from 3.8 to 5.7, had been polluted with Cu in the range 0.13–1.9 mmol kg^?1 more than a decade ago. Calcium and Cu bound by dissolved organic matter (purified humic acid) was determined in the pH range 4–8; Cu²⁺ concentration ranged from 10^?4 to 10^?12M and Ca²⁺ concentration from 10^?3 to 10^?6M. Binding of Cu by dissolved organic matter as affected by Ca and pH could be predicted well with the non-ideal competitive adsorption (NICA) model. Coagulation of dissolved organic matter was affected by the amount of trivalent (Al³⁺) and divalent (Ca²⁺ and Cu²⁺) cations in solution. There was little effect of pH on coagulation between pH 4 and 6. The concentration of the divalent cations, Ca²⁺ and Cu²⁺, at which coagulation of dissolved organic matter appeared could be explained by differences in the binding of Ca and Cu by dissolved organic matter. Binding of Cu by dissolved organic matter as well as by solid organic matter, both affected by Ca and pH, could be described well with the NICA model. We investigated the coagulation and mobility of dissolved organic matter in column experiments and found that they enhanced Cu mobility. Three processes, Cu desorption by soil, dissolved organic matter coagulation and Cu complexation by dissolved organic matter, act simultaneously in the soil columns. All three with counteracting effects on Cu mobility are influenced by Ca and pH and could be adequately represented by the multicomponent NICA model.     

A method for the analysis of the δ18O of inorganic phosphate extracted from soils with HCl

The oxygen isotope composition of phosphate (δ¹⁸O‐PO₄) has successfully been used to study the biological cycling of phosphorus (P) in seawater and marine sediments. However, only a few studies have used this approach in soils. In order to analyse δ¹⁸O‐PO₄, phosphate must be extracted from the soil, purified and converted to silver phosphate (Ag₃PO₄). The published extraction methods, successfully applied to marine waters and sediments, lead to the precipitation of impure Ag₃PO₄when used with soils or organic‐rich samples. Here we present an improved purification protocol, designed for soils and other organic‐rich samples. After extraction with HCl, phosphate is purified with multiple mineral precipitations that do not require extreme pH adjustments of the solutions. We show that contaminant‐free Ag₃PO₄ can be produced from fertilizers and various soils with different chemical and physical characteristics. Our first isotopic results confirm that differences in P status and availability in soils are expressed in the δ¹⁸O‐PO₄ signal, indicating the potential of this isotopic tracer to understand P dynamics in soil systems.     

Cation binding by acid-washed peat, interpreted with Humic Ion-Binding Model VI-FD

A sample of ombrotrophic peat from Moor House in northern England was extensively extracted with dilute nitric acid (pH 1) to free it of bound cations. Suspensions of the acid‐washed peat (5–30 g l^?1), prepared with different concentrations of background electrolyte (NaCl and KCl), were used to conduct batch acid–base titrations. A strong dependence of proton release on ionic strength (I) was observed, the apparent acid dissociation constant (pK_app) being found to decrease by approximately 1.0 for each tenfold increase in I. This behaviour could not be explained satisfactorily with Humic Ion‐Binding Model VI, a discrete‐site/electrostatic model of cation binding by humic substances, parameterized with data from laboratory studies on isolated samples. More success was obtained by abandoning the impermeable‐sphere electrostatic submodel used in Model VI, and instead assuming the peat to consist of aggregates with fixed internal volume, and with counterion accumulation described by the Donnan model, as proposed by Marinsky and colleagues. The fixed‐volume Donnan model (Model VI‐FD) could also approximately explain other reported results from acid–base titrations of peat, including the effects on the titrations of complexing cations (Al, Ca, Cu). Copper titrations of the Moor House sample were performed using an ion‐selective electrode, with peat suspensions in the acid pH range, at two ionic strengths, and in the presence of Al and Ca. The measured concentrations of Cu²⁺ were in the range 10^?13?10^?5 m . Model VI‐FD provided reasonable fits of the experimental data, after optimization of the intrinsic binding constant for Cu, the optimized value being close to the default value derived previously from data referring to isolated humic substances. The optimized constants for Al and Ca, derived from their competition effects, were also close to their default values. Additional experiments were performed in which the centrifugation‐depletion method was used to measure the binding of a cocktail of metals (Al, Ni, Cu, Zn, Cd, Eu, Pb) at a single pH. The model correctly predicted strong binding of Al, Cu, Eu and Pb, and weaker binding of Ni, Zn and Cd. For the strongly binding metals, the dissolved forms were calculated to be mainly due to complexes with dissolved humic matter, whereas the free ions (Ni²⁺, Zn²⁺, Cd²⁺) dominated for the weakly binding metals. Acid‐washed soil appears to provide a valuable intermediate between isolated humic substances and untreated soil for the investigation of cation binding by natural organic matter in the natural environment.     

Copper distribution and acid-base mobilization in vineyard soils and sediments from Galicia (NW Spain)

In northern Spain and elsewhere in the world, many vineyards are located on steep slopes and are susceptible to accelerated soil erosion. Contaminants, notably Cu, originating from repeated application of copper‐based fungicides to the vines to prevent mildew, are transported and stored in the sediments deposited close to valley bottoms. In this study, the contents and distribution of Cu in 17 soil samples and 21 sediment samples collected from vineyard stands were determined. In addition, the effect of pH on Cu release from vineyard soils and sediments was quantified. The total Cu content (Cu_T) in the soils varied between 96 and 583 mg kg^?1, and was between 1.2 and 5.6 times greater in sediment samples. The mean concentration of potentially bioavailable Cu (Cu_EDTA) in the sediments was 199 mg kg^?1 (46% of Cu_T), and was 80 mg kg^?1 (36% of Cu_T) in the soils. Copper bound to soil organic matter (Cu_OM) was the dominant fraction in the soils (on average, 53% of the Cu_T), while in sediment samples Cu_OM values varied between 37 and 712 mg kg^?1 and were significantly greater (P < 0.01) than in the soils. Copper associated with non‐crystalline inorganic components (Cu_IA) was the second most important fraction in the sediments, in which it was 3.4 times greater than in the soils. Release of Cu due to changes in the pH followed a U‐shaped pattern in soils and sediments. The release of Cu increased when the pH decreased below 5.5 due to the increased solubility of the metal at this pH. When the pH increased above 7.5, Cu and organic matter were released simultaneously.     

Analysing the preferential transport of lead in a vegetated roadside soil using lysimeter experiments and a dual-porosity model

Lead (Pb) from the traffic accumulates in roadside soils, which are usually vegetated to control erosion. Plants release soluble organic substances that bind Pb. Root macropores also create preferential pathways through which water can flow. Both these processes may enhance Pb mobility. We used large lysimeters to investigate the transport of Pb in a contaminated (445 mg Pb kg^?1) soil under vegetation (Phacelia tanacetifolia). Despite the high soil pH (7.2), Pb leached into the drainage water during the 5‐month experiment. The fast response of the system to intense rainfall events indicated the presence of preferential flow. By comparing Pb concentrations in filtered and unfiltered leachates, we found that Pb was leaching primarily on suspended material. An increase in Pb concentration in the leachate at the end of the experiment indicated the remobilization of Pb, possibly by decaying vegetation. We parameterized the dual‐porosity MACRO model using the experimental results. The simple parameterization of MACRO used to simulate the Pb concentrations in the drainage water produced an overall model efficiency of 0.81: MACRO simulated the Pb concentrations well, but it failed to predict the observed increase of Pb in the leachate at the end of the experiment. The model gave the best prediction of Pb concentrations with a small partition coefficient (k_d= 150 cm³ g^?1). Long‐term simulations of Pb mobility showed that for our specific conditions preferential flow was the main process determining the fate of Pb.     

Interactions of aluminium and fulvic acid in moderately acid solutions: stoichiometry of the H+/Al3+ exchange

Complexation with organic matter controls the activity of dissolved Al³⁺ in many soils. The buffering intensity of these soils is largely dependent on the H⁺/Al³⁺ exchange ratio, i.e. the number of protons consumed by the solid phase when one Al³⁺ is released. Here, the H⁺/Al³⁺ exchange ratio was determined from batch titrations using solutions of fulvic acid (FA) as a model for soil organic matter. Aluminium was added, from 1.04 to 6.29 mmol Al per g FA, which is within the range of humus‐bound Al found in the upper B horizon of podzolized soils. Furthermore, pH was varied with NaOH to give values between 3.5 and 5.0. The H⁺/Al³⁺ exchange ratio ranged between 1.49 and 2.23 with a mean of 1.94. It correlated positively with pH and the total concentration of Al present. Theoretically, this can be explained with a partial hydrolysis of bound Al. The slope of logAl (log₁₀ of Al³⁺ activity) against pH generally underestimated the actual exchange ratio, which can partly be attributed to the systems being diluted (100 mg FA l^?1). However, where 4 mmol Al or more had been added per g FA, the logAl slope gradually approached ?3 between pH 4.5 and 5.0. This might be the result of a shift from Al³⁺ activity control by humus complexation to control by Al(OH)₃(s).     

Total and soluble fluorine concentrations in relation to properties of soils in New Zealand

Soil fluorine (F) concentrations continue to increase in agricultural soils receiving regular applications of phosphatic fertilizer. Continued accumulation of soil F poses a risk to grazing ruminants and may pose a future risk to groundwater quality. This paper examines the range of total F (F_t) concentrations and forms of soluble F species and their relationship to selected soil properties in New Zealand agricultural soils. The F_t and soluble F (soil F extracted with water (F_water) and 0.01 m KCl (F_KCl)) concentrations in 27 soil samples (0–75 mm depth) taken from predominantly pasture sites in the North and South Islands of New Zealand were much less than those reported in the literature for sites contaminated with F from industry. The F_t concentrations ranged from 212 to 617 µg F g^?1 soil. The F‐toxicity risk to grazing animals in farms at these sites through soil ingestion is small at present, but farms with very large F_t concentrations (i.e. > 500 µg F g^?1) need to adopt suitable grazing and fertilizer management practices to avoid future F‐toxicity risk. The F_t concentration had very strong positive correlations with both total soil P and total soil Cd concentrations, reflecting the link between P fertilizer use and F accumulation in the soils. It also had significant positive correlations with organic matter and amorphous Al oxides contents, indicating that F is strongly bound to Al polymers adsorbed to organic matter and amorphous Al oxides. The F_water and F_KCl concentrations and free F^– ion concentrations in water (F^–_water) and 0.01 m KCl (F^–_KCl) extracts were generally two and three orders of magnitude, respectively, less than the F_t concentrations and were much less than the concentrations considered phytotoxic. The F_water and F_KCl concentrations were positively related to soil organic matter content and negatively related to soil pH. Regression models relating F_water and F_KCl concentrations to soil organic matter content and soil pH suggest that F can be very soluble in extremely acidic soils (pH(water) < 4.9) with large organic matter contents and therefore F potentially may contaminate groundwater if these soils are also coarse‐textured and the water table is shallow.     

Sorption of copper (II) and sulphate to different biochars before and after composting with farmyard manure

Biochar application has been suggested for reducing toxic levels of metals in contaminated soils and enhancing nutrient retention in agro‐ecosystems. We studied sorption of copper (Cu(II)) and sulphate‐sulphur (SO₄‐S) to charcoal, gasification coke and flash‐pyrolysis biochar in order to relate sorption to char properties. Furthermore, we investigated the effect of composting of charcoal and gasification coke on sorptive properties. Langmuir sorption affinity coefficients for Cu(II) for non‐composted biochars increased in the order flash‐pyrolysis char < charcoal < gasification coke. The sorption capacity for Cu(II) of the chars decreased in the order gasification coke (629 mg kg^?1) > flash‐pyrolysis char (196 mg kg^?1) > charcoal (56 mg kg^?1). Composting significantly increased the sorption affinity coefficient approximately by a factor of 5 for charcoal (up to 1.1 l mg^?1) and by a factor of 3–4 for gasification coke (up to 3.2 l mg^?1). Whereas Cu(II) sorption to gasification coke (composted or not) was largely irreversible, sorption to flash‐pyrolysis char and charcoal showed higher reversibility. Relationships between Cu(II) sorption and biochar properties such as cation exchange capacity, specific surface area or aromaticity suggest that sorption was largely determined by complexation with organic matter. Sorption of SO₄‐S was negligible by non‐composted and composted biochars. Composted gasification coke might be suited to reducing toxic Cu(II) concentrations in contaminated soils. Composted charcoal can potentially improve Cu(II) retention in a plant available form in acidic, sandy soils with small organic matter contents. Transient effects of biochars on soil pH can over‐ride the influence of sorption to biochars on concentrations of trace elements in soil solution and their availability to plants.     

Nitrogen deposition impacts on the amount and stability of soil organic matter in an alpine meadow ecosystem depend on the form and rate of applied nitrogen

The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon (SOC) in terrestrial ecosystems. In this study, a multi‐form, small‐input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha^?1 year^?1. One hundred and eight soil samples were collected at 10‐cm intervals to a depth of 30 cm in 2011. Contents and δ¹³C values of bulk SOC were measured, as well as three particle‐size fractions: macroparticulate organic C (MacroPOC, > 250 µm), microparticulate organic C (MicroPOC, 53–250 µm) and mineral‐associated organic C (MAOC, < 53 µm). The results show that 5 years of N addition changed SOC contents, δ¹³C values of the bulk soils and various particle‐size fractions in the surface 10‐cm layer, and that they were dependent on the amounts and forms of N application. Ammonium‐N addition had more significant effects on SOC content than nitrate‐N addition. For the entire soil profile, small additions of N increased SOC stock by 4.5% (0.43 kg C m^?2), while medium and large inputs of N decreased SOC stock by 5.4% (0.52 kg C m^?2) and 8.8% (0.85 kg C m^?2), respectively. The critical load of N deposition appears to be about 20 kg N ha^?1 year^?1. The newly formed C in the small‐input N treatment remained mostly in the > 250 µm soil MacroPOC, and the C lost in the medium or large N treatments was from the > 53 µm POC fraction. Five years of ammonium‐N addition increased significantly the surface soil POC:MAOC ratio and increased the instability of soil organic matter (SOM). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai–Tibetan Plateau over the short term.     

Proton interactions with soil organic matter: the importance of aggregation and the weak acids of humin

Samples of three organic‐rich soils (ombrotrophic peat, podzol H‐horizon, humic ranker) were extensively washed with dilute nitric acid, dialysed against deionised water, and then subjected to acid‐base titrations over the pH range 3–10, in 0.3–300 mm NaNO₃, and with soil concentrations in the range 2–150 g l^?1. The results for the three soils were quantitatively similar. Comparison of the titration data with previously published results for humic acids isolated from the same soils showed the soil organic matter to have a greater ionic strength dependency of proton binding and to possess relatively greater buffering capacity at high pH, attributable to weak acid groups (c. 2–5 mmol g^–1) in the humin fraction of the soils. To describe the soil titration data quantitatively, we modified Humic Ion‐Binding Model VI‐FD, which utilizes a fixed Donnan volume to describe counterion accumulation, by increasing the content of weak acid groups. When artefacts in pH measurement caused by the suspension effect were taken into account, the resulting Model VI‐FD2 provided good or fair simulations of all the titration data. The results suggest that soil structure, specifically aggregation, plays a significant role in cation binding by organic soils in situ. The lack of dependence of the titration results on soil suspension concentration suggests that the findings can be applied to soils in situ.     

Factors affecting the degree of phosphate-removal in the system FeCl3-orthophosphate and nature of the precipitates

In this batch type parametric study the influence of phosphate concentrations on the extent of its removal by constant amount of FeCl₃ at different pH, temperature and aging time is evaluated. The efficiency of FeCl₃ in removing the phosphate is strongly dependent on the pH of the precipitation. At P/Fe molar ratios of higher than 0.5 the pH of maximum phosphate removal is found to be at pH 2.5 and is followed by pH 4, 6 and 9. The optimum pH for maximal phosphate removal greatly varies with the amount of phosphate present relative to that of iron present. It is also observed that maximal phosphate removal by FeCl₃-solution is achieved at P/Fe ratios of more than 1, 0 suggesting that in removal process not only Fe³⁺ but also colloidal hydroxide particles could play an important role. The samples of non-aged synthesized precipitates obtained at P/Fe ratios of 8, 4 and 2 at pH 2.5 are found to have characteristic lines of strengite. The formation of strengite at room temperature from fresh solutions has not previously been reported. Thermally treated precipitates with the characteristic lines of strengite transform into a quartz-like phase which suggests that the loss of the coordinated water leads to a compound like ABO₄ where A and B both are in tetrahedral coordination. Upon heat treatment, the amorphous precipitates, having P/Fe ratios > 0.5, obtained between pH 2.5 and 6 result in a quartzlike phase. However, the characteristic lines of the quartz phase are not of the same intensity. In contrast, the precipitates having P/Fe ratios < 0.5 transform into hematite after heat treatment. The thermally treated products of precipitates obtained at pH 9 are found to have η-Fe₂O₃ phase which indicates that basic phosphates formed at this pH are structurally similar to that of basic sulfates.