Interpretation of humic acid coagulation and soluble soil organic matter using a calculated electrostatic potential

The coagulation of humic substances and its role in controlling the solubility of organic matter in soils are not well understood. We therefore studied the physico‐chemical behaviour of purified humic acid from forest soil coagulated with Na, Ca, Cu, Al at pH 4 and 6, and then modelled the behaviour with the Non‐Ideal Consistent Competitive Adsorption Donnan (NICA‐Donnan) model. We found that the coagulation of humic acid occurs when the Donnan potential is less negative than ?0.08 V. Based on this result, an empirical relation between the Donnan potential of humic acid and its concentration in solution was derived. In addition, the Donnan potential of the dissolved organic matter in the soil solution of six soil profiles from forests was calculated using the NICA‐Donnan model under the assumption that all the dissolved organic matter behaves as humic acid. The measured concentration of dissolved organic matter also decreases in a soil profile, as the calculated potential becomes less negative. The results are in many cases in semi‐quantitative agreement with the predicted concentration based on the humic acid coagulation experiment. Acid soils contain more dissolved organic matter, which may result from the presence of a fairly large fraction of more soluble organic molecules, such as fulvic acid.     

Speciation of cadmium,copper, lead,and zinc in contaminated soils

Abstract

To investigate the activity of free cadmium (Cd²⁺), copper (Cu²⁺), lead (Pb²⁺), and zinc (Zn²⁺) ions and analyze their dependence on pH and other soil properties, ten contaminated soils were sampled and analyzed for total contents of Cd, Cu, Pb, and Zn (Cd_T, Cu_T, Pb_T, and Zn_T, respectively), 0.43 MHNO₃‐extractable Cd, Cu, Pb, and Zn (Cd_N, Cu_N, Pb_N, and Zn_N, respectively), pH, dissolved organic matter (DOC), cation exchange capacity (CEC), ammonium oxalate extractable aluminum (Al) and iron (Fe), and dissolved calcium [Ca²⁺]. The activity of free Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺ ions in soil solutions was determined using Donnan equilibrium/graphite furnace atomic absorption (DE/GFAA). The solubility of Cd in soils varied from 0.16 to 0.94 μg L^‐1, Cu from 3.43 to 7.42 μg L^‐1, Pb from 1.23 to 5.8 μg L^‐1, and Zn from 24.5 to 34.3 μg L^‐. In saturation soil extracts, the activity of free Cd²⁺ ions constituted 42 to 82% of the dissolved fraction, for Cu²⁺the range was 0.1 to 7.8%, for Pb²⁺ 0.1 to 5.1% and for Zn²⁺2 to 72%. The principal species of Cd, Cu, Pb, and Zn in the soil solution is free metal ions and hydrolyzed ions. Soil pH displayed a pronounced effect on the activity of free Cd²⁺, Cu^2t, Pb²⁺, and Zn²⁺ ions.     

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.     

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.     

中国东部太湖地区酸性水稻土团聚体上铜的专性和非专性吸附动力学研究

The Taihu Lake region in East China has become prone to soil acidification, which changes heavy metals such as copper(Cu) in soil into water-soluble species and increases the mobility and contamination risks of heavy metals in the biological environment. In this study, the kinetics of Cu2+sorption by the bulk soil and the aggregate size fractions of an acidic paddy soil collected from the Taihu Lake region, the effects of temperature on Cu2+sorption, and the p H changes of the solution were investigated by static sorption and magnetic stirring. The aggregate size fractions were prepared by low-energy ultrasonic dispersing and freeze-drying. The total sorption amounts of the bulk soil and the aggregate size fractions for Cu2+followed a descending order of clay > coarse sand > bulk soil > silt> sand, corresponding to those of organic matter content, free iron oxide content, free aluminum oxide content, and cation exchange capacity. The kinetic sorption curves of Cu2+by the bulk soil and the aggregates, which were divided into two stages(rapid and slow sequentially), were well fitted by the first-order equation, the diffusion equation, and the Elovich equation, showing significant correlations(P < 0.05). Specific and non-specific sorption dominated in the fast and slow stages, respectively, and the former was predominant throughout the sorption process. The specific sorption accelerated and the non-specific sorption decelerated with rising temperature. The p H of the solution decreased significantly during the specific sorption and remained unchanged or increased slightly during the non-specific sorption. When the specific sorption terminated, the p H of the solution was minimized nearly simultaneously.The sorption progress of Cu2+by the bulk soil significantly preceded that by the aggregates. Therefore, heavy metal contamination may be another factor reducing soil p H and metal sorption forms should be taken into consideration in studies of mitigating soil heavy metal pollution or determining environmental capacity of heavy metal in soil.     

Retention and release of dissolved organic matter in Podzol B horizons

The main objectives were to study the effects of pH on the retention and release of organic matter in acid soil, and to determine the main differences in results obtained from batch experiments and experiments in columns. We took soil material from the B horizons of a Podzol at Skånes Värsjö (southern Sweden). In batch experiments, soil was equilibrated with solutions varying in pH and concentration of dissolved organic C. In Bh samples, the release of dissolved C gradually increased with increase in pH. In the Bs1 material there was a minimum at pH 4.1, and in the Bs2 soil the minimum occurred at pH 4.6. The ability to retain added dissolved C increased in the order Bh < Bs1 < Bs2. The column experiment was run for 160 days under unsaturated flow conditions. Columns were packed with Bh, Bh + Bs1 or Bh + Bs1 + Bs2 samples to calculate mass balances for each horizon. Solutions either without any dissolved organic C or ones containing 49 mg C dm^?3 with pH of 4.0 or 3.6 were used to leach columns. The pH of input solutions only little affected the concentration of dissolved C in the effluent. Relative proportions of hydrophobic substances decreased with increasing column length and decreasing pH. For input solutions containing dissolved C, near steady state was achieved for both the Bs1 and Bs2 horizons with approximately 25% dissolved organic matter retention. Thus, no maximum sorption capacity for dissolved C could be defined for these horizons. This behaviour could not have been predicted by batch data, showing that column experiments provide useful additional information on interactions between organic compounds and solid soil material.     

Copper Release, Speciation, and Toxicity Following Multiple Floodings of Copper Enriched Agriculture Soils: Implications in Everglades Restoration

This study characterizes the effects of water–soil flooding volume ratio and flooding time on copper (Cu) desorption and toxicity following multiple floodings of field-collected soils from agricultural sites acquired under the Comprehensive Everglades Restoration Plan (CERP) in south Florida. Soils from four field sites were flooded with three water–soil ratios (2, 4, and 6 [water] to 1 [soil]) and held for 14 days to characterize the effects of volume ratio and flooding duration on Cu desorption (volume ratio and flooding duration study). Desorption of Cu was also characterized by flooding soils four times from seven field sites with a volume ratio of 2 (water) to 1 (soil) (multiple flooding study). Acute toxicity tests were also conducted using overlying waters from the first flooding event to characterize the effects of Cu on the survival of fathead minnows (Pimephales promelas), cladocerans (Daphnia magna), amphipods (Hyalella azteca), midges (Chironomus tentans), duckweed (Lemna minor), and Florida apple snails (Pomacea paludosa). Acute tests were also conducted with D. magna exposed to overlying water from the second and third flooding events. Results indicate that dissolved Cu concentrations in overlying water increased with flooding duration and decreased with volume ratio. In the multiple flooding study, initial Cu concentrations in soils ranged from 5 to 223 mg/kg (dw) and were similar to Cu concentration after four flooding events, indicating retention of Cu in soils. Copper desorption was dependent on soil Cu content and soil characteristics. Total Cu concentration in overlying water (Cu_w) was a function of dissolved organic carbon (DOC), alkalinity, and soil Cu concentration (Cu_s): log(Cu_w)?=?1.2909?+?0.0279 (DOC)?+?0.0026 (Cu_s)???0.0038 (alkalinity). The model was validated and highly predictive. Most of the desorbed Cu in the water column complexed with organic matter in the soils and accounted for 99% of the total dissolved Cu. Although total dissolved Cu concentrations in overlying water did not significantly decrease with number of flooding events, concentrations of free Cu²⁺ increased with the number of flooding events, due to a decrease in DOC concentrations. The fraction of bioavailable Cu species (Cu²⁺, CuOH⁺, CuCO₃) was also less than 1% of the total Cu. Overlying water from the first flooding event was only acutely toxic to the Florida apple snail from one site. However, overlying water from the third flooding of six out of seven soils was acutely toxic to D. magna. The decrease in DOC concentrations and increase in bioavailable Cu²⁺ species may explain the changes in acute toxicity to D. magna. Results of this study reveal potential for high Cu bioavailability (Cu²⁺) and toxicity to aquatic biota overtime in inundated agricultural lands acquired under the CERP.     

Effects of Different Rates of Ca2+ Addition on Respiration and Sorption of Water-Extractable Organic C to a Vertisol Subsoil

It is well known that calcium (Ca²⁺) plays an important role in binding organic matter to clay. However, most previous studies were conducted with either topsoil or pure aluminosilicates. Less is known about the effect of Ca²⁺ on binding of organic matter to clay-rich subsoils, which have lower organic-matter contents than topsoils, and their clays are more strongly weathered than pure aluminosilicates. Two experiments were conducted with a Vertisol subsoil (69% clay): a laboratory incubation and a batch sorption. The mineral substrate in the incubation experiment was pure sand alone or sand amended with 300 g clay kg^?1. Powdered calcium sulfate (CaSO₄) at rates of 0, 5, 10, and 15 g Ca kg^?1 and mature wheat residue at a rate of 20 g kg^?1 were added to this mineral substrate and the water content was adjusted to 70% of water-holding capacity. Carbon dioxide release was measured for 28 days. Cumulative respiration per g soil organic carbon (C) (SOC from clay and residues) was increased by clay addition. Increasing Ca²⁺ addition rate decreased cumulative respiration in the sand with clay but had no effect on respiration in the pure sand. Clay and Ca²⁺ addition had no significant effect on microbial biomass carbon (MBC) per g SOC but clay addition reduced the concentration of potassium sulfate (K₂SO₄)–extractable C per g SOC. For the batch sorption experiment, the subsoil was mixed with 0 to 15 g Ca kg^?1 and water-extractable organic C (WEOC) derived from mature wheat straw was added at 0, 1485, 3267, and 5099 mg WEOC kg^?1. Increasing Ca²⁺ addition rate increased sorption of WEOC, particularly at the greatest concentration of WEOC added, and decreased desorption. This study confirmed the importance of Ca²⁺ in binding organic matter to clay and suggests that Ca²⁺ addition to clay-rich subsoils could be used to increase their organic C sequestration.     

Solubility control of Cu, Zn, Cd and Pb in contaminated soils

We developed a semiempirical equation from metal complextion theory which relates the metal activity of soil solutions to the soil's pH, organic matter content (OM) and total metal content (MT). The equation has the general form: where pM is the negative logarithm (to base 10) of the metal activity, and a, b and c are constants. The equation successfully predicted free Cu²⁺ activity in soils with a wide range of properties, including soils previously treated with sewage sludge. The significant correlation of pCu to these measured soil properties in long-contaminated soils suggests that copper activity is controlled by adsorption on organic matter under steady state conditions. An attempt was made from separate published data to correlate total soluble Cu, Zn, Cd and Pb in soils to soil pH, organic matter content and total metal content. For Cu, the total Cu content of the soil was most highly correlated with total soluble Cu. Similarly, total soluble Zn and Cd were correlated with total metal content, but were more strongly related to soil pH than was soluble Cu. Smaller metal solubility in response to higher soil pH was most marked for Zn and Cd, metals that tend not to complex strongly with soluble organics. The organic matter content was often, but not always, a statistically significant variable in predicting metal solubility from soil properties. The solubility of Pb was less satisfactorily predicted from measured soil properties than solubility of the other metals. It seems that for Cu at least, solid organic matter limits free metal activity, whilst dissolved organic matter promotes metal solubility, in soils well-aged with respect to the metal pollutant. Although total metal content alone is not generally a good predictor of metal solubility or activity, it assumes great importance when comparing metal solubility in soils having similar pH and organic matter content.     

EFFECT OF SOIL pH AND ORGANIC MATTER ON LABILE ALUMINIUM IN SOILS UNDER PERMANENT GRASS

The rates of extraction of Na, K, Mg, Ca, and Al with 1M NH₄ NO₃ from the mineral-and organic-rich layers of some Park Grass (Rothamsted) soils were measured at the pH of the soil. Below pH 3.7 exchangeable Al, derived from the kinetics curve, increases with decreasing soil pH and is less in the organic-rich layer. The sum of the basic exchangeable cations, ∑(Na + K + Mg + Ca), increases with increasing soil pH and is more in the organic-rich layer. The extraction of exchangeable Al obeys first order kinetics, the rate constant being similar for all the soils (mean value 36 ± 7 × 10^?6|s^?1), which implies that exchangeable Al is released from surfaces with similar properties for the adsorption of Al, and that the rate is not affected by soil pH and organic matter. The rate of extraction of non-exchangeable Al is the same in the mineral-and organic-rich layers of each soil, and is maximal at about pH 3.7, decreasing sharply at more and less acid pH values.     

The behavior of selected chlorinated organic micropollutants in the activated sludge process: A pilot plant study

The effects of Cu(II) sulfate on sediment respiration were investigated in a 3-phase aquatic microcosm, containing a calcareous, southern Ontario stream sediment. In Cu²⁺ treated flask-microcosms, with the pH restored to 7.1, both aerobic and anaerobic CO₂ evolution were unaffected by 5000 jig Cu g^?1 sediment over a 40-day period at 15 °C. Oxygen consumption in sediment was initially unaffected by 5000 μg Cu g^?1. However, after 35 to 40 days, a significant reduction of 28% was observed. The added Cu^?2+ was removed from the water column and the sediment solution. In microcosms containing 5000 μg g^?1 of total Cu, only 1.00 ± 0.76 μg g^?1 was water soluble Cu, and the free cupric cation (Cu²⁺) concentration was below the detection limit of the specific ion electrode (less than 0.01 μg g^?1). Maximum Cu retention (98.6%) was observed at 2800 μg Cu g^?1, above which fractional retention decreased. In a calcareous, organic rich, sediment of pH 7.1, Cu⁺ was essentially unvailable to exert a toxic effect on respiration.     

Yield,Nutrient Uptake,and Soil Chemical Properties as Influenced by Liming and Boron Application in Common Bean in a No‐Tillage System

Abstract

In Oxisols, acidity is the principal limiting factor for crop production. In recent years, because of intensive cropping on these soils, deficiency of micronutrients is increasing. A field experiment was conducted on an Oxisol during three consecutive years to assess the response of common bean (Phaseolus vulgaris L.) under a no‐tillage system to varying rates of lime (0, 12, and 24 Mg ha^?1) and boron (0, 2, 4, 8, 12, 16, and 24 kg ha^?1) application. Both time and boron (B) were applied as broadcast and incorporated into the soil at the beginning of the study. Changes in selected soil chemical properties in the soil profile (0- to 10‐ and 10- to 20‐cm depths) with liming were also determined. During all three years, gain yields increased significantly with the application of lime. However, B application significantly increased common bean yield in only the first crop. Only lime application significantly affected the soil chemical properties [pH; calcium (Ca²⁺); magnesium (Mg²⁺); hydrogen (H⁺)+ aluminum (Al³⁺); base saturation; acidity saturation; cation exchange capacity (CEC); percent saturation of Ca²⁺, Mg²⁺, and potassium (K⁺); and ratios of exchangeable Ca/Mg, Ca/K, and Mg/K] at both soil depths (0–10 cm and 10–20 cm). A positive significant association was observed between grain yield and soil chemical properties. Averaged across two depths and three crops, common bean produced maximum grain yield at soil pH_w of 6.7, exchangeable (cmol_c kg^?1) of Ca²⁺ 4.9, Mg²⁺ 2.2, H⁺+Al³⁺ 2.6, acidity saturation of 27.6%, CEC of 4.1 cmol_c kg^?1, base saturation of 72%, Ca saturation of 53.2%, Mg saturation of 17.6%, K saturation of 2.7%, Ca/Mg ratio of 2.8, Ca/K ratio of 25.7, and Mg/K ratio of 8.6. Soil organic matter did not change significantly with addition of lime.     

Unlocking fixed soil phosphorus upon waterlogging can be promoted by increasing soil cation exchange capacity

Phosphorus (P) adsorbed by iron (Fe) oxyhydroxides in soil can be released when the Fe(III) minerals are reductively dissolved after soil flooding. However, this release is limited in tropical soils with large Fe contents and previous studies have suggested that P sorbs or precipitates with newly formed Fe(II) minerals. This hypothesis is tested here by scavenging Fe²⁺ in flooded soils by increasing the cation exchange capacity (CEC) of soil through resin application (30 cmol_c kg^?1; Na‐form). Three soils from rice paddies with contrasting properties were incubated in aerobic and anaerobic conditions with or without resin and with or without addition of organic matter (OM) to stimulate redox reactions. Dissolved Fe was 0.1–1.1 mm in unamended anaerobic soils and decreased to less than 0.07 mm with resin addition. Anaerobic soils without resin and aerobic soils with or without resin had marginal available P concentrations (<2 mg P kg^?1; anion‐exchange membrane P). In contrast, available P increased 3‐ to 14‐fold in anaerobic soils treated with resins, reaching 16 mg P kg^?1 in combination with extra OM. Application of Ca‐forms of resin did not stimulate P availability and dissolved Ca concentrations were larger than in unamended soils. Resin addition can increase P availability, probably by a combination of reducing solution Fe²⁺ (thereby limiting the formation of Fe(II) minerals) and increasing the OM solubility and availability through reducing dissolved Ca²⁺. The soil CEC is a factor controlling the net P release in submerged soils.     

Effects of acid irrigation and liming in a norway spruce stand (Picea abies [L.] KARST.)

Acid irrigation (pH 2.7 to 2.8; mean annual input 4.1 kmol H⁺ ha^?1 as H₂SO₄) has caused significant changes in the chemistry of the soil of a mature Norway spruce stand (Picea abies [L.] KARST.) after 4 years of treatment. In the surface humus layer around 20% of the exchangeable Ca, Mg, K and Mn ions were leached. This was connected with a decrease of pH and cation exchange capacity. In the mineral soil no changes of pH and cation adsorption were observed. However there was a significant increase of Al³⁺ ions in the soil solution, exceeding 20 mg L^?1, mainly caused by dissolution of Al-hydroxides and Al-hydroxosulphates. Also the concentrations of ionic Cu, Zn and Cd were nearly doubled. Manganese concentrations are fluctuating according to periods with and without acid irrigation, showing reduction and oxidation phases. In contrast to microorganisms, certain moss species and Oxalis acetosella, the mature spruce stand was not severely damaged up to now. It is hypothesized that Ca/Al and Mg/Al ratios of single horizons are insufficient for characterizing Al stress in the field. Liming (4 Mg ha^?1) led to a significant increase of dissolved organic C, which is associated with mobilization of metals such as Pb, Cu and Al in organic complexation. Also nitrification increased in the surface humus layer. As a consequence the nitrate concentrations in the seepage water exceeded 250 mg NO₃ L^?1.     

Effect of dissolved organic matter on the precipitation and mobility of the lead compound chloropyromorphite in solution

Chloropyromorphite, CPM, Pb₅(PO₄)₃Cl, is one of the most insoluble lead minerals. Inducing the formation of CPM by application of phosphate to soil has been suggested for immobilizing Pb at contaminated sites. We have examined the effect of organic matter on the completeness and the rate of CPM precipitation and on the particle size and the mobility of CPM crystals. We did experiments at pH 3–7 and with varying content of dissolved organic C, 0–72 mg C l^?1, mixing Pb(NO₃)₂ (0.5 mmol l^?1) and phosphate (2 mmol l^?1) solutions. The organic matter was extracted from samples of a forest floor. The precipitates were identified by X‐ray diffraction, and their size and shape were analysed by scanning electron microscopy and by photon correlation spectroscopy. The presence of organic matter in the solutions did not affect the mass of CPM that precipitated within 30 minutes at pH 5, 6 and 7. At pH 3 and 4, however, organic matter strongly inhibited the precipitation. The particles were markedly smaller in solutions containing organic matter than without at all pHs and passed through water‐saturated columns filled with calcareous sand, whereas the precipitates from the carbon‐free solutions did not. We suggest that the organic matter blocked the surfaces of crystal seeds and impaired crystal growth. At high pH, organic matter may additionally decrease the crystal size of the individual crystals by increasing the number of crystal seeds. We conclude that organic matter in the solution might limit the potential of phosphate to immobilize Pb in soil because it favours the formation of mobile colloids.     

Short‐term response of the Ca cycle of a montane forest in Ecuador to low experimental CaCl2 additions

The tropical montane forests of the E Andean cordillera in Ecuador receive episodic Sahara‐dust inputs particularly increasing Ca deposition. We added CaCl₂ to isolate the effect of Ca deposition by Sahara dust to tropical montane forest from the simultaneously occurring pH effect. We examined components of the Ca cycle at four control plots and four plots with added Ca (2 × 5 kg ha^–1 Ca annually as CaCl₂) in a random arrangement. Between August 2007 and December 2009 (four applications of Ca), we determined Ca concentrations and fluxes in litter leachate, mineral soil solution (0.15 and 0.30 m depths), throughfall, and fine litterfall and Al concentrations and speciation in soil solutions. After 1 y of Ca addition, we assessed fine‐root biomass, leaf area, and tree growth. Only < 3% of the applied Ca leached below the acid organic layer (pH 3.5–4.8). The added CaCl₂ did not change electrical conductivity in the root zone after 2 y. In the second year of fertilization, Ca retention in the canopy of the Ca treatment tended to decrease relative to the control. After 2 y, 21% of the applied Ca was recycled to soil with throughfall and litterfall. One year after the first Ca addition, fine‐root biomass had decreased significantly. Decreasing fine‐root biomass might be attributed to a direct or an indirect beneficial effect of Ca on the soil decomposer community. Because of almost complete association of Al with dissolved organic matter and high free Ca²⁺ : Al³⁺ activity ratios in solution of all plots, Al toxicity was unlikely. We conclude that the added Ca was retained in the system and had beneficial effects on some plants.     

The behavior of mercury in the soil with special emphasis on complexation and adsorption processes - A review of the literature

The behavior of Hg in the soil is mainly controlled by adsorption and desorption processes depending on complexation, the most important ligands in solution being OH^?, Cl^?, and organic anions. Since the solubility of HgCl₂ and Hg(OH)₂ is rather high, the affinity of Hg to these ligands leads to an increased mobility. This is especially true for HgCl₂, whereas the hydrolysis of Hg²⁺ may result in the specific adsorption of Hg on mineral colloids. The high affinity of Hg to S explains the strong binding of Hg to soil organic matter and also the stability of HgS. Further precipitation products than HgS are unlikely to occur, since the activity of Hg²⁺ remains too low to exceed the solubility product of any other defined Hg compound. It is mainly the physical fractioning of soil organic matter (dissolved vs adsorbed) that determines the behavior and distribution of Hg in soils.     

Soil solarization: Effects on soil properties,crop fertilization and plant growth

Summer solarization of six wet field soils of four different textures raised soil temperatures by 10–12°C at 15cm depth. Soil solarization increased concentrations of NO^?₃N and NH⁺₄N up to six times those in nontreated soils. Concentrations of P, Ca²⁺, Mg²⁺ and electrical conductivity (EC) increased in some of the solarized soils. Solarization did not consistently affect available K⁺, Fe³⁺, Mn²⁺, Zn²⁺, Cu²⁺, Cl^? concentrations, soil pH or total organic matter. Concentrations of mineral nutrients in wet soil covered by transparent polyethylene film, but insulated against solar heating, were the same as those in nontreated soil. Increases in NO^?₃N plus NH⁺₄N were no longer detected in fallowed soils 9 months after solarization. No significant correlation between mineral-nutrient concentration in plant tissue and plant growth was found. Fresh and dry weights of radish, pepper and Chinese cabbage plants usually were greater when grown in solarized soils than in nontreated soils. Fertilization of solarized soils sometimes resulted in greater plant growth responses than observed in solarized but nonfertilized soils.     

Effects of Ca,Mg, K,Na, and pH on Copper Toxicity to Pakchoi (Brassica chinensis L.)

The influence of calcium, magnesium, sodium, and potassium (Ca²⁺, Mg²⁺, Na⁺, K⁺) ions and pH on copper (Cu) toxicity to pakchoi (Brassica chinensis L.) was independently estimated by measuring root elongation in nutrient solutions. Increases in Ca²⁺, Mg²⁺, and hydrogen (H⁺) significantly increased the 5-d EC50Cu_T (expressed as total soluble Cu) by a factor of 12 for all treatments, which clearly demonstrated the limitation of using total Cu concentration to predict Cu toxicity to pakchoi. EC50{Cu²⁺} (expressed as free Cu²⁺ activity) was not significantly influenced by changing the Ca²⁺, Mg²⁺, and H⁺ activities. The nonlinear relationship between EC50{Cu²⁺} and cations indicated that competition for binding sites between Cu²⁺ and cations was not a significant factor in determining toxicity of Cu²⁺ for pakchoi. The lower variation of EC50{Cu²⁺} suggests that free Cu²⁺ activity was a better predictor of toxicity to pakchoi than EC50Cu_T.