Evaluation of various chemical extraction methods to estimate plant-available arsenic in mine soils

Elevated levels of bioavailable As in mining soils, agricultural areas and human habitats may cause potential toxicity to human health, plants and microbe. Therefore, it is essential to determine proper soil chemical extraction method in order to estimate plant-available As in mining soils and protect agricultural and environmental ecosystems by evaluation of environmental risk and implementation of remediation measures. In this study, six single soil chemical extraction processes and four-step sequential chemical extraction protocol were used to determine the relative distribution of As in different chemical forms of soils and their correlations with total As in plants grown in mining areas and greenhouse experiments. The strongest relationship between As determined by single soil chemical extraction and As in plant biomass was found for sodium acetate and mixed acid extractant. The mean percent of total As extracted was: ammonium oxalate (41%)>hydroxylamine hydrochloride (32%)>mixed acid (16%)>phosphate (6%)>sodium acetate (1.2%)>water (0.13%). This trend suggests that most of the As in these soils is inside the soil mineral matrix and can only be released when iron oxides and other minerals are dissolved by the stronger chemical extractant. Single soil chemical extraction methods using sodium acetate and mixed acids, that extract As fractions complexed to soil particles or on the surface of mineral matrix of hydrous oxides of Fe, Mn and Al (exchangeable+sorbed forms) can be employed to estimate and predict the bioavailable As fraction for plant uptake in mining affected soils. In sequential chemical extraction methods, ammonium nitrate and hydroxylamine hydrochloride may be used to provide closer estimates of plant-available As in mining soils.     

Evaluation of new surfactant expanded zirconium and titanium phosphates for polycyclic aromatic hydrocarbons extraction from waters

A newly synthesized family of materials prepared with surfactant as organic template were tested for the extraction of polycyclic aromatic hydrocarbons (PAHs) from water, using pyrene (Pyr) and benzo[a]pyrene (B[a]P) as PAHs representatives. Particular attention was paid to the evaluation of the recovery factors with dichloromethane as eluent in order to estimate their potential as adsorbing solid phases for PAH remediation or analysis. Eleven lamellar MCM-50 type materials incorporating n-alkyl- (n = 12, 16, 18) trimethylammonium bromide molecules with different concentrations and chain lengths and two hexagonal MCM-41 type materials incorporating octadecyl-trimethyl ammonium bromide were tested. Best results were obtained by preparing lamellar MCM-50 zirconium and titanium phosphates in the presence of n-dodecyl-trimethylammonium at a relative molar concentration (surfactant/phosphate) of 1.     

Evaluation of copper availability to plants in copper-contaminated vineyard soils

The repeated use of copper (Cu) fungicides to control vine downy mildew has led to long-term accumulation of Cu in vineyard soils which now raises the issue of the potential bioavailability of Cu for various living organisms including plant species. The bioavailable Cu can be defined as the portion of soil Cu that can be taken up by roots, for a given plant species. In order to evaluate the bioavailability of Cu to plants, a pot experiment was conducted in glasshouse conditions with a crop species (maize) and 12 soils sampled in the upper horizon of 10 vineyard plots (total Cu ranging from 38 to 251 mg kg-1) and two woodland plots (control soils that had not received any Cu application; total Cu amounting to 20-26 mg kg-1). These soils were selected for their diverse physical (large range of particle size distribution) and chemical (from acid to calcareous soils) properties. After 35 days of growth, plant shoots were harvested for analysis. The roots were separated from soil particles for further analysis. The concentrations of Cu in the roots and aerial parts of the maize were then compared with the amounts of Cu extracted from the soil by a range of conventional extractants. Observed Cu concentrations in maize roots which have grown in contaminated vineyard soils were very high (between 90 and 600 mg kg-1), whereas Cu concentrations in the aerial parts varied only slightly and remained low (< 18 mg kg-1). Root Cu concentrations observed for maize increased with increasing total Cu content in the soil and with decreasing soil CEC. Cu accumulation in maize roots may be as high in calcareous soils as in acid soils, suggesting that soil pH had little influence. In the case of the vineyard soils studied, the lack of correlation found for maize between Cu concentrations in roots and in the aerial parts, suggests that an analysis of the aerial parts would not be a good indicator of plant Cu uptake, as it provides no insight into the real amount of Cu transferred from the soil to the plant. For maize, our results show that extraction with organic complexing agents (EDTA, DTPA) and extraction with ammonium acetate seem to provide a reasonably good estimate of root Cu concentration.     

Effects of long-term soil amendment with sewage sludges on soil humic acid thermal and molecular properties

Sewage sludges are frequently used as soil amendments due to their high contents of organic matter and nutrients, particularly N and P. However, their effects upon the chemistry of soil humic acids, one of the main components of the soil organic matter, need to be more deeply studied in order to understand the relation between organic matter structure and beneficial soil properties. Two sewage sludges subjected to different types of pre-treatment (composted and thermally dried) with very different chemical compositions were applied for three consecutive years to an agricultural soil under long-term field study. Thermal analysis (TG–DTG–DTA) and solid-state ¹³C NMR spectroscopy were used to compare molecular and structural properties of humic acids isolated from sewage sludges, and to determine changes in amended soils. Thermally dried sewage sludge humic acids showed an important presence of alkyl and O/N-alkyl compounds (70%) while composted sludge humic acids comprised 50% aromatic and carbonyl carbon. In spite of important differences in the initial chemical and thermal properties of the two types of sewage sludges, the chemical and thermal properties of the soil humic acids were quite similar to one another after 3 years of amendment. Long-term application of both sewage sludges resulted in 80–90% enrichment in alkyl carbon and organic nitrogen contents of the soil humic acid fraction.     

Effects of long-term soil amendment with sewage sludges on soil humic acid thermal and molecular properties

Sewage sludges are frequently used as soil amendments due to their high contents of organic matter and nutrients, particularly N and P. However, their effects upon the chemistry of soil humic acids, one of the main components of the soil organic matter, need to be more deeply studied in order to understand the relation between organic matter structure and beneficial soil properties. Two sewage sludges subjected to different types of pre-treatment (composted and thermally dried) with very different chemical compositions were applied for three consecutive years to an agricultural soil under long-term field study. Thermal analysis (TG–DTG–DTA) and solid-state ¹³C NMR spectroscopy were used to compare molecular and structural properties of humic acids isolated from sewage sludges, and to determine changes in amended soils. Thermally dried sewage sludge humic acids showed an important presence of alkyl and O/N-alkyl compounds (70%) while composted sludge humic acids comprised 50% aromatic and carbonyl carbon. In spite of important differences in the initial chemical and thermal properties of the two types of sewage sludges, the chemical and thermal properties of the soil humic acids were quite similar to one another after 3 years of amendment. Long-term application of both sewage sludges resulted in 80–90% enrichment in alkyl carbon and organic nitrogen contents of the soil humic acid fraction.     

Carbon deposition in soil rhizosphere following amendments with compost and its soluble fractions, as evaluated by combined soil-plant rhizobox and reporter gene systems

We determined the organic carbon released by roots of maize plants (Zea mays L.) when grown in soils amended with compost and its soluble fractions. In rhizobox systems, soil and roots are separated from the soil of a lower compartment by a nylon membrane. Treatments are applied to the upper compartment, while in the lower compartment luminescent biosensors measure the bioavailable organic carbon released by roots (rhizodeposition). The rhizobox-plants systems were amended with a compost (COM), its water extract (TEA), the hydrophobic (HoDOM) and hydrophilic (HiDOM) fractions of the dissolved organic matter (DOM) extracted from the compost. After root development, the lower untreated compartments were sampled and sliced into thin layers. The bioavailable organic carbon in each layer was assessed with the lux-marked biosensor Pseudomonas fluorescens 10586 pUCD607, and compared with total organic carbon (TOC) analyses. The TOC values ranged between 8.4 and 9.6 g kg(-1) and did not show any significant differences between bulk and rhizosphere soil samples in any treatment. Conversely, the biosensor detected significant differences in available C compounds for rhizosphere soils amended with various organic materials. Concentrations of available organic compounds in the first 2 mm of soil rhizosphere were 1.69 (control), 1.09 (COM), 2.87 (HiDOM), 4.73 (HoDOM) and 2.14 (TEA)micromol Cg(-1) soil g(-1) roots. The applied rhizobox-biosensor integrated method was successful in detecting and quantifying effects of organic amendments on organic carbon released by maize plant roots. This approach may become important in assessing the carbon cycle in agricultural soils and soil-atmosphere compartments.     

Dolomite phosphate rock (DPR) application in acidic sandy soil in reducing leaching of phosphorus and heavy metals—a column leaching study

A column leaching study was designed to investigate the leaching potential of phosphorus (P) and heavy metals from acidic sandy soils applied with dolomite phosphate rock (DPR) fertilizers containing varying amounts of DPR material and N-Viro soils. DPR fertilizers were made from DPR materials mixing with N-Viro soils at the ratios of 30, 40, 50, 60, and 70 %, and applied in acidic sandy soils at the level of 100 mg available P per kilogram soil. A control and a soluble P chemical fertilizer were also included. The amended soils were incubated at room temperature with 70 % field water holding capacity for 21 days before packed into a soil column and subjected to leaching. Seven leaching events were conducted at days 1, 3, 7, 14, 28, 56, and 70, respectively, and 258.9 mL of deionized water was applied at each leaching events. The leachate was collected for the analyses of pH, electrical conductivity (EC), dissolved organic carbon (DOC), major elements, and heavy metals. DPR fertilizer application resulted in elevations up to 1 unit in pH, 7–10 times in EC, and 20–40 times in K and Ca concentrations, but 3–10 times reduction in P concentration in the leachate as compared with the chemical fertilizer or the control. After seven leaching events, DPR fertilizers with adequate DPR materials significantly reduced cumulative leaching losses of Fe, P, Mn, Cu, and Zn by 20, 55, 3.7, 2.7, and 2.5 times than chemical fertilizer or control. Even though higher cumulative losses of Pb, Co, and Ni were observed after DPR fertilizer application, the loss of Pb, Co, and Ni in leachate was <0.10 mg (in total 1,812 mL leachate). Significant correlations of pH (negative) and DOC (positive) with Cu, Pb, and Zn (P?<?0.01) in leachate were observed. The results indicated that DPR fertilizers had a great advantage over the soluble chemical fertilizer in reducing P loss from the acidic sandy soil with minimal likelihood of heavy metal risk to the water environment. pH elevation and high dissolved organic carbon concentration in soils after DPR fertilizer application are two influential factors.     

Effect of bone meal (calcium phosphate) amendments on metal release from contaminated soils--a leaching column study

Metal-contaminated soil may be remediated in situ by the formation of highly insoluble metal phosphates if an appropriate phosphorus (P) source can be found. Leaching column experiments have been carried out to assess the suitability of bone meal as such a source. Bone meal additions reduced metal release from a contaminated soil, increased soil and leachate pH and decreased soil leachate toxicity. Minimal P leaching occurred from the soil. The data are consistent with a proton consuming bone meal (calcium phosphate) dissolution reaction followed by the formation of metal phosphates. Although, no metal phosphates were observed to form using X-ray diffraction of scanning electron microscopy this could be due to their low concentration. Relatively low (1:50 bone meal:soil) concentrations of fine (90-500 microns) bone meal would appear to be an effective treatment for metal-contaminated soils.     

Effect of biochar amendment on sorption and leaching of nitrate,ammonium, and phosphate in a sandy soil

When applied to soils, it is unclear whether and how biochar can affect soil nutrients. This has implications both to the availability of nutrients to plants or microbes, as well as to the question of whether biochar soil amendment may enhance or reduce the leaching of nutrients. In this work, a range of laboratory experiments were conducted to determine the effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil. A total of thirteen biochars were tested in laboratory sorption experiments and most of them showed little/no ability to sorb nitrate or phosphate. However, nine biochars could remove ammonium from aqueous solution. Biochars made from Brazilian pepperwood and peanut hull at 600 °C (PH600 and BP600, respectively) were used in a column leaching experiment to assess their ability to hold nutrients in a sandy soil. The BP600 biochar effectively reduced the total amount of nitrate, ammonium, and phosphate in the leachates by 34.0%, 34.7%, and 20.6%, respectively, relative to the soil alone. The PH600 biochar also reduced the leaching of nitrate and ammonium by 34% and 14%, respectively, but caused additional phosphate release from the soil columns. These results indicate that the effect of biochar on the leaching of agricultural nutrients in soils is not uniform and varies by biochar and nutrient type. Therefore, the nutrient sorption characteristics of a biochar should be studied prior to its use in a particular soil amendment project.     

Effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil

When applied to soils, it is unclear whether and how biochar can affect soil nutrients. This has implications both to the availability of nutrients to plants or microbes, as well as to the question of whether biochar soil amendment may enhance or reduce the leaching of nutrients. In this work, a range of laboratory experiments were conducted to determine the effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil. A total of thirteen biochars were tested in laboratory sorption experiments and most of them showed little/no ability to sorb nitrate or phosphate. However, nine biochars could remove ammonium from aqueous solution. Biochars made from Brazilian pepperwood and peanut hull at 600 °C (PH600 and BP600, respectively) were used in a column leaching experiment to assess their ability to hold nutrients in a sandy soil. The BP600 biochar effectively reduced the total amount of nitrate, ammonium, and phosphate in the leachates by 34.0%, 34.7%, and 20.6%, respectively, relative to the soil alone. The PH600 biochar also reduced the leaching of nitrate and ammonium by 34% and 14%, respectively, but caused additional phosphate release from the soil columns. These results indicate that the effect of biochar on the leaching of agricultural nutrients in soils is not uniform and varies by biochar and nutrient type. Therefore, the nutrient sorption characteristics of a biochar should be studied prior to its use in a particular soil amendment project.     

Analysis of volatile and semivolatile organic compounds in municipal wastewater using headspace solid-phase microextraction and gas chromatography.

The aim of this work was to develop a simple and fast analytical method for the determination of a wide range of organic compounds (volatile and semivolatile compounds) in municipal wastewater. The headspace-solid-phase microextraction (HS-SPME) and gas chromatography (with mass spectroscopy) was used for determination of the organic compounds. In this study, 39 organic compounds were determined, including 3 sulfur compounds, 28 substituted benzenes, and 8 substituted phenols. The extraction parameters, such as types of SPME fiber, extraction temperature, extraction time, desorption time, salt effect, and magnetic stirring, were investigated. The method had very good repeatability, because the relative standard deviations ranged from 0.5 to 12%. The detection limit of each compound was at or below the microgram-per-liter level. This method was applied for determination of the organic compounds in raw wastewater, primary effluent, secondary effluent, and chlorinated secondary effluent samples from the Chania Municipal Wastewater Treatment Plant (Crete, Greece).     

Separating the effects of organic matter-mineral interactions and organic matter chemistry on the sorption of diuron and phenanthrene

Even though it is well established that soil C content is the primary determinant of the sorption affinity of soils for non-ionic compounds, it is also clear that organic carbon-normalized sorption coefficients (K(OC)) vary considerably between soils. Two factors that may contribute to K(OC) variability are variations in organic matter chemistry between soils and interactions between organic matter and soil minerals. Here, we quantify these effects for two non-ionic sorbates-diuron and phenanthrene. The effect of organic matter-mineral interactions were evaluated by comparing K(OC) for demineralized (HF-treated) soils, with K(OC) for the corresponding whole soils. For diuron and phenanthrene, average ratios of K(OC) of the HF-treated soils to K(OC) of the whole soils were 2.5 and 2.3, respectively, indicating a substantial depression of K(OC) due to the presence of minerals in the whole soils. The effect of organic matter chemistry was determined by correlating K(OC) against distributions of C types determined using solid-state (13)C NMR spectroscopy. For diuron, K(OC) was positively correlated with aryl C and negatively correlated with O-alkyl C, for both whole and HF-treated soils, whereas for phenanthrene, these correlations were only present for the HF-treated soils. We suggest that the lack of a clear effect of organic matter chemistry on whole soil K(OC) for phenanthrene is due to an over-riding influence of organic matter-mineral interactions in this case. This hypothesis is supported by a correlation between the increase in K(OC) on HF-treatment and the soil clay content for phenanthrene, but not for diuron.     

Research and application of method of oxygen isotope of inorganic phosphate in Beijing agricultural soils

Phosphorus (P) in agricultural ecosystems is an essential and limited element for plants and microorganisms. However, environmental problems caused by P accumulation as well as by P loss have become more and more serious. Oxygen isotopes of phosphate can trace the sources, migration, and transformation of P in agricultural soils. In order to use the isotopes of phosphate oxygen, appropriate extraction and purification methods for inorganic phosphate from soils are necessary. Here, we combined two different methods to analyze the oxygen isotopic composition of inorganic phosphate (δ¹⁸O_P) from chemical fertilizers and different fractions (Milli-Q water, 0.5 mol L^?1 NaHCO₃ (pH = 8.5), 0.1 mol L^?1 NaOH and 1 mol L^?1 HCl) of agricultural soils from the Beijing area. The δ¹⁸O_P results of the water extracts and NaHCO₃ extracts in most samples were close to the calculated equilibrium value. These phenomena can be explained by rapid P cycling in soils and the influence of chemical fertilizers. The δ¹⁸O_P value of the water extracts and NaHCO₃ extracts in some soil samples below the equilibrium value may be caused by the hydrolysis of organic P fractions mediated by extracellular enzymes. The δ¹⁸O_P values of the NaOH extracts were above the calculated equilibrium value reflecting the balance state between microbial uptake of phosphate and the release of intracellular phosphate back to the soil. The HCl extracts with the lowest δ¹⁸O_P values and highest phosphate concentrations indicated that the HCl fraction was affected by microbial activity. Hence, these δ¹⁸O_p values likely reflected the oxygen isotopic values of the parent materials. The results suggested that phosphate oxygen isotope analyses could be an effective tool in order to trace phosphate sources, transformation processes, and its utilization by microorganisms in agricultural soils.     

Sorption and desorption of glyphosate,MCPA and tetracycline and their mixtures in soil as influenced by phosphate

Phosphate fertilizers and herbicides such as glyphosate and MCPA are commonly applied to agricultural land, and antibiotics such as tetracycline have been detected in soils following the application of livestock manures and biosolids to agricultural land. Utilizing a range of batch equilibrium experiments, this research examined the competitive sorption interactions of these chemicals in soil. Soil samples (0-15 cm) collected from long-term experimental plots contained Olsen P concentrations in the typical (13 to 20 mg kg^?1) and elevated (81 to 99 mg kg^?1) range of build-up phosphate in agricultural soils. The elevated Olsen P concentrations in field soils significantly reduced glyphosate sorption up to 50%, but had no significant impact on MCPA and tetracycline sorption. Fresh phosphate additions in the laboratory, introduced to soil prior to, or at the same time with the other chemical applications, had a greater impact on reducing glyphosate sorption (up to 45%) than on reducing tetracycline (up to 13%) and MCPA (up to 8%) sorption. The impact of fresh phosphate additions on the desorption of these three chemicals was also statistically significant, but numerically very small namely < 1% for glyphosate and tetracycline and 3% for MCPA. The presence of MCPA significantly reduced sorption and increased desorption of glyphosate, but only when MCPA was present at concentrations much greater than environmentally relevant and there was no phosphate added to the MCPA solution. Tetracycline addition had no significant effect on glyphosate sorption and desorption in soil. For the four chemicals studied, we conclude that when mixtures of phosphate, herbicides and antibiotics are present in soil, the greatest influence of their competitive interactions is phosphate decreasing glyphosate sorption and the presence of phosphate in solution lessens the potential impact of MCPA on glyphosate sorption. The presence of chemical mixtures in soil solution has an overall greater impact on the sorption than desorption of individual organic chemicals in soil.     

Phosphorus sorption on marine carbonate sediment: phosphonate as model organic compounds

Organophosphonate, characterized by the presence of a stable, covalent, carbon to phosphorus (C-P) bond, is a group of synthetic or biogenic organophosphorus compounds. The fate of these organic phosphorus compounds in the environment is not well studied. This study presents the first investigation on the sorption of phosphorus (P) in the presence of two model phosphonate compounds, 2-aminothylphosphonoic acid (2-AEP) and phosphonoformic acid (PFA), on marine carbonate sediments. In contrast to other organic P compounds, no significant inorganic phosphate exchange was observed in seawater. P was found to adsorb on the sediment only in the presence of PFA, not 2-AEP. This indicated that sorption of P from phosphonate on marine sediment was compound specific. Compared with inorganic phosphate sorption on the same sediments, P sorption from organic phosphorus is much less in the marine environment. Further study is needed to understand the potential role of the organophosphonate compounds in biogeochemical cycle of phosphorus in the environment.     

Remediation of copper-contaminated topsoils from a wood treatment facility using in situ stabilisation

Five organic matters, three phosphate compounds, zerovalent iron grit (ZVIG, 2% by soil weight), two alkaline compounds, and two commercial formulations were incorporated, singly and some combined with ZVIG, into a highly Cu-contaminated topsoil (Soil P7, 2600 mg Cu kg⁻¹) from a wood treatment facility. Formulations and two composts were also singly incorporated into a slightly Cu-contaminated topsoil (Soil P10, 118 mg Cu kg⁻¹) from the facility surrounding. This aimed to reduce the labile pool of Cu and its accumulation in beans cultivated on potted soils in a climatic chamber. Lowest Cu concentration in soil solution occurred in P7 soils amended with activated carbon (5%) and ZVIG, singly and combined. Basic slag (3.9%) and compost of sewage sludge (5%) combined with ZVIG promoted shoot production and limited foliar Cu accumulation. For amended P10 soils, no changes occurred in soil solution and foliar Cu concentrations, but one compost increased shoot production.     

Phosphorus fractions in grassland soils

The partitioning of P among major chemical forms was studied in 110 non-fertilized, semi-natural grassland soils of southeastern Sweden, comprising a wide range of soil acidity and other chemical properties. The P fractions bound to Ca and Fe were closely related to soil acidity, expressed as pH-KCl, especially when calculated as % of the total pool of mineral P. The relationship was inversely linear for log10P-Fe, whereas it was strongly positively curvilinear for log10P-Ca, with a sudden decrease of this fraction below pH-KCl 4.5. Phosphate bound to Al was inversely related to pH-KCl, but the variability accounted for was rather low. Easily exchangeable phosphate, interpreted as the most labile form of P-Al, was more closely related to soil acidity, with very low values at pHg-KCl > 5.5. Soil concentration of organic P was mainly a function of its contents of organic matter. Data were treated by PCA and stepwise regression analysis.     

Degree of phosphorus saturation of an Oxisol amended with biosolids in a long-term field experiment

When applied to agricultural soils, phosphate fertilizers and the mineral or organic compounds present in solid and/or liquid waste may raise phosphorus (P) content and increase soil P saturation. The degree of phosphorus saturation (DPS) is a good indicator of potential P loss from agricultural soils. The purpose of this study was to calculate the DPS of samples from an Oxisol amended for 5 years with biosolids and mineral fertilizer. DPS was calculated based on P, iron, and aluminum extracted by ammonium oxalate and oxalic acid (DPS_ox) or by Mehlich-1 solution (DPS_M1). Treatments included NPK mineral fertilization (175 kg ha^?1 of P), B1?=?19.02 t ha^?1 of biosolids (350 kg ha^?1 of P), B2?=?38.17 t ha^?1 of biosolids (703 kg ha^?1 of P), B3?=?76.26 t ha^–1 of biosolids (1,405 kg ha^?1 of P), and a control (no P added). Water-extractable P (WEP) was also measured. Critical levels of DPS_ox and DPS_M1 (21 and 24 %, respectively) were only achieved in the topsoil (0–0.1 m) at the highest biosolid dose. Concentration of WEP was positively correlated to DPS_ox and DPS_M1. The DPS_M1 method may be an alternative to DPS_ox for assessing the environmental risk of P loss from soil into surface runoff.     

Sequestration of maize crop straw C in different soils: role of oxyhydrates in chemical binding and stabilization as recalcitrance

While biophysical controls on the sequestration capacity of soils have been well addressed with physical protection, chemical binding and stabilization processes as well as microbial community changes, the role of chemical binding and stabilization has not yet well characterized for soil organic carbon (SOC) sequestration in rice paddies. In this study, a 6-month laboratory incubation with and without maize straw amendment (MSA) was conducted using topsoil samples from soils with different clay mineralogy and free oxy-hydrate contents collected across Southern China. The increase in SOC under MSA was found coincident with that in Fe- and Al-bound OC (Fe/Al-OC) after incubation for 30 d (R(2)=0.90, P=0.05), and with sodium dithionate-citrate-bicarbonate (DCB) extractable Fe after incubation for 180 d (R(2)=0.99, P<0.01). The increase in SOC under MSA was found higher in soils rich in DCB extractable Fe than those poor in DCB extractable Fe. The greater SOC sequestration in soils rich in DCB extractable Fe was further supported by the higher abundance of (13)C which was a natural signature of MSA. Moreover, a weak positive correlation of the increased SOC under MSA with the increased humin (R(2)=0.87, P=0.06) observed after incubation for 180 d may indicate a chemical stabilization of sequestered SOC as humin in the long run. These results improved our understanding of SOC sequestration in China's rice paddies that involves an initial chemical binding of amended C and a final stabilization as recalcitrant C of humin.     

Correlations between experimental and predicted equilibrium distribution coefficient of chlorobenzene and chlorophenol compounds in soil-water systems using partial solubility parameters

Many pesticides are degraded to become chlorinated aromatic compounds in soils. Equilibrium distribution of chlorobenzene and chlorophenol compounds in soil-water systems of Yangmingshan loam, Pingcheng silty clay loam and Annei silty loam was studied with the integral distribution equilibrium equation involving the partial solubility parameters of the chemicals. If the adsorption of chemicals on soils is partitioning in soil organic matter surrounding the soil mineral particles, the absorption constant (Kd) of a chemical in soil-water system could be stated as the distribution coefficient (or partition constant, Koc) of the chemical in the two adjunct immiscible phases--water and soil organic matter. The distribution coefficient (Koc) of chemicals calculated from the integral distribution equilibrium equation agrees well with the experimental adsorption coefficient (Kd, or experimental Koc) of chemicals determined in this study, for all the three different types of soils in water according to multiple-regression analysis. Reference data of Karger or Tijssen are employed to estimate the Koc for both polar and non-polar chemicals. The integral distribution equilibrium equation can exactly describe the distribution behavior of nonionic compound of chlorobenzenes and chlorophenols in soil-water systems.