Extractability of available boron and its sequential fractionation in alkaline calcareous soils of India

The evaluation of different extractants for boron (B) estimation and the study of different boron pools is of utmost importance for the effective use of native and soil applied B. For evaluation of different extractants in diverse soils, twenty-one soil samples varying in soil properties were analyzed to estimate chemical pools of B and available B by using extractants (NH₄OAc, AB-DTPA, hot and cold CaCl₂, tartaric acid, HCl, mannitol, and hot water soluble). Available B extracted by the most commonly used method (HWS-B) was significantly and positively correlated with that extracted by other reagents (r = 0.772** to 0.905**) and the maximum value was observed with HCC-B (r = 0.917**) followed by Mann-B (0.905**). The amount of B in readily soluble, specifically adsorbed, oxide bound, organically bound and residual mineral fraction varied from 0.17 to 2.71, 0.14 to 1.77, 0.31 to 5.88, 0.56 to 7.42 and 0.17 to 2.71% of total B in soils, respectively. Thus, HCC and Mann methods can be used as extracting reagent of B in alkaline calcareous soils instead of HWS.     

Influence of soil applied boron on yield of berseem (Trifolium alexandrium L.) and soil boron fractions in calcareous soils

A greenhouse experiment was conducted in North-west India to study the effect of soil applied boron on yield of berseem (Trifolium alexandrium L.) and soil boron fractions in boron deficient calcareous soils. Three soils with varying calcium carbonate content viz. 0.75% (Soil I), 2.6% (Soil II), and 5.7% (Soil III) were collected from different sites of Ludhiana, Bathinda, and Shri Muktsar Sahib districts, Punjab, India. The treatments consisted of six levels of soil applied boron viz. 0.5, 0.75, 1.0, 1.25, 1.5, and 2.0 mg B kg^?1 along with control. The green fodder yield and dry matter yield increased significantly at 0.75 mg B kg^?1 soil treatment level in the first cutting, while these were significant at 1.0 mg B kg^?1 soil treatment level in all soils at second, third, and fourth cuttings. Among all three calcareous soils, Soil I with lower calcium carbonate was the best soil in respect of mean yield in comparison to Soil II and Soil III. Combined effect of boron level and soils had significant effect on yield of berseem. There was a significant increase in mean dry root biomass at 1.0 mg B kg^?1 soil level over control and then remained non-significant with further high levels of soil applied boron. The mean dry root biomass decreased significantly for the soils having 0.75%, 2.6%, and 5.7% calcium carbonate levels. Readily soluble fraction is considered to be easily available fraction of B for plant uptake and consisted of 0.47–0.62% in Soil I, 0.31–0.43% in Soil II, and 0.24–0.34% in Soil III of the total boron. Among all B fractions, mean readily soluble, specifically adsorbed, and oxide-bound fractions got increased significantly with increase in B levels. Readily soluble and organically bound B fractions were more in Soil I as compared to Soil II and Soil III. Specifically adsorbed boron, oxide bound fraction, residual and total boron were more in Soil III in comparison to Soil I and Soil II. Among all fractions, residual fraction accounted for the major portion of the total B. It comprised of 92.71–93.90% in Soil I, 94.51–95.40% in Soil II, and 94.91–95.25% in Soil III of the total boron.     

Soil Boron Fractions and Response of Green Gram in Calcareous Soils

A greenhouse experiment with green gram on boron (B) deficient calcareous soils was conducted for two years at Ludhiana (Punjab), India to study soil B fractions and response of green gram to B application. Three soils with calcium carbonate (CaCO₃) content 0.8 (Soil I), 2.1 (Soil II) and 4.6 (Soil III) percent were collected from different sites of Ludhiana and Bhatinda districts, Punjab, India. The treatments comprised of five levels of soil applied B (0, 0.5, 0.75, 1.0, and 1.5 mg B kg^?1). The experiment was laid out in completely randomized design (CRD) factorial design with three replications. Readily soluble B comprised 0.39 to 0.76 percent in Soil I, 0.32 to 0.54 percent in Soil II, and 0.21 to 0.34 percent in Soil III of the total B, taking into account of all the levels of B applied at both stages of crop growth. Readily soluble B increased with increasing application rates of B and decreased from grand growth stage (40 days after sowing) to maturity of the crop. Specifically adsorbed, oxide bound, residual, and total B was higher in Soil III as compared to Soil II or Soil I. At maturity, specifically adsorbed B converted into other fractions to maintain equilibrium in soil solution. Organically bound B was greater than oxide bound B. Among all fractions, the residual fraction accounted for the major fraction of the total B. Soil application of lowest level of B was adequate to cause significant increase in dry matter yield and seed yield of green gram regardless of type of soil.     

The Determination of Soil Boron Fractions,Their Relationships to Soil Properties and the Availability to Olive (Olea europea L.) Trees

This study was conducted to investigate the various boron fractions in olive tree grown soils. The correlations between boron fractions in leaves, fruits and soil properties were examined. For this purpose cv “Gemlik” olive (Olea europaea L.) orchards were visited. Soil samples from 0–30 cm and 30–60 cm deep, the leaf and fruit samples were collected. The greatest proportion of total soil boron is present in residual form (85–88%). It is followed by organically bound B (2.84–4.50%), specifically adsorbed on soil colloid surfaces (0.93–1.31%), oxides (manganese oxyhydroxides, amorphous Fe and Al oxides, crystalline Fe and Al oxides) bound B (7.27–8.31%). The smallest one readily soluble (extracting plant available) boron values were composed of only 0.40–0.50% of total boron ranging. To determine readily soluble boron five different extraction solutions were in the order Hot water ? 0.01 M CaCl₂ ? 1 M NH₄OAc ? 0.1 M KCl ? 0.005 M DTPA. Fruit boron concentration and soil boron fractions showed close correlations than leaf boron concentration.     

Manganese extracted from different chemical fractions of bulk and rhizosphere soil as affected by method of sample preparation

Abstract

Rhizosphere soils had higher amounts of ‘readily soluble’, ‘weakly adsorbed’, ‘carbonate bound’ and ‘specifically adsorbed’ Mn, but had lower amounts of ‘oxide‐Mn’, than did bulk soils. This observation was true regardless of whether the comparison was based on values within moist or air‐dried treatments. Observed trends in Mn distribution between different soil fractions were qualitatively similar regardless of method of sample preparation. However, there were substantial quantitative differences depending on the method of sample preparation. Air‐dried samples increased significantly in the ‘oxide‐Mn’ fraction and decreased in its soluble and adsorbed fractions relative to moist soil samples. There was a significant effect of method of air‐drying on the distribution of Mn in rhizosphere samples. Samples that were extracted moist at first and then air‐dried accumulated more adsorbed Mn and were depleted in ‘oxide‐Mn’ relative to samples that were air‐dried initially. There was a significant rhizosphere x air‐drying interaction. Air‐drying of some rhizosphere samples resulted in a significant underestimation of the ‘readily soluble’, ‘specifically adsorbed’, and ‘oxide‐Mn’ fractions beyond the overall effect of air‐drying. The results of this study suggested that soil samples used for Mn analyses be extracted immediately in a moist condition rather than air‐dried, particularly for analyses of rhizosphere soil samples.     

Fractionation and colorimetric determination of boron in soils

We attempted to modify and evaluate existing sequential fractionation schemes for B involving the use of chemicals, which subsequently do not interfere with the measurement of B by colorimetry. Also evaluated was the contribution of various soil B fractions to the amount of B extracted by hot CaCl₂, CaCl₂‐mannitol, salicylic acid, ammonium acetate, HCl, and tartaric acid. For this purpose, 17 soils with diverse properties were used. The extraction scheme proposed here partitioned B into five pools, (i) readily soluble, (ii) specifically adsorbed, (iii) oxide bound, (iv) organically bound, and (v) residual boron, respectively extracted with 0.01 M CaCl₂, 0.05 M KH₂PO₄, 0.175 M NH4‐oxalate (pH 3.25), 0.5 M NaOH, and HF + H₂SO₄ + HClO₄. The procedure of elimination of color from extracts of oxide bound, organically bound, and residual B fractions was also evolved. Relationships of individual B fractions with physicochemical properties of the experimental soils confirmed the general validity of the proposed fractionation scheme. The relationships of different B fractions with extractable B in soils suggest that hot CaCl₂ and salicylic acid may be better extractants for available B in soils.     

Forms and Uptake of Manganese in Relation to Soil Taxonomic Orders in Alluvial Soils of Punjab,India

Different forms of manganese (Mn) were investigated, including total, diethylenetriamine penta-acetic acid (DTPA) extractable, soil solution plus exchangeable (Mn), Mn adsorbed onto inorganic sites, Mn bound by organic sites, and Mn adsorbed onto oxide surfaces, from four soil taxonomic orders in northwestern India. The total Mn content was 200–950 mg kg^?1, DTPA-extractable Mn content was 0.60–5.80 mg kg^?1, soil solution plus exchangeable Mn content was 0.02–0.80 mg kg^?1, Mn adsorbed onto inorganic sites was 2.46–90 mg kg^?1, and Mc adsorbed onto oxide surfaces was 6.0–225.0 mg kg^?1. Irrespective of the different fractions of Mn their content was generally greater in the fine-textured Alfisols and Inceptisols than in coarse-textured Entisols and Aridisols. The proportion of the Mn fractions extracted from the soil was in the order as follows: Adsorbed onto oxide surfaces > adsorbed onto inorganic site > organically bound > DTPA > soil solution + exchangeable. Based on coefficient of correlation, the soil solution plus exchangeable Mn, held onto organic site and oxide surface (amorphous) and DTPA-extractable Mn, increased with increase in organic carbon of the soil. The two forms, adsorbed onto inorganic site (crystalline) and DTPA extractable, along with organic carbon, increased with increase in clay content of the soil. DTPA-Mn and Mn adsorbed onto oxide surfaces and held on organic site decreased with increased with an increase in calcium carbonate and pH. Total Mn was strongly correlated with organic carbon and clay content of soil. Among the forms, Mn held on the organic site, water soluble + exchangeable and adsorbed onto oxide surface were positively correlated with DTPA-extractable Mn. DTPA-extractable Mn seems to be a good index of Mn availability in soils and this form is helpful for correction of Mn deficiency in the soils of the region. The uptake of Mn was greater in fine-textured Inceptisols and Alfisols than in coarse-textured Entisols and Aridisols. Among the different forms only DTPA-extractable Mn was positively correlated with total uptake of Mn. Among soil properties Mn uptake was only significantly affected by pH of the soil.     

Soil boron status: impact of lime and fertilizers in an Indian long-term field experiment on a Typic Paleustalf

In Indian agriculture, nitrogen (N) and phosphorus (P) fertilizers are predominantly used by the farmers, often ignoring secondary and micronutrients. Significance of boron (B) in nutrient management studies has been increasingly underlined under intensive cropping systems particularly in acid soils. In order to understand the distribution of soil native B in different fractions and their contribution to plant B uptake as influenced by nutrient management, soil samples collected after wheat (2009–2010) from a long-term experiment (LTE) continuing since 1972–1973 on Typic Paleustalf of Ranchi were subjected to sequential fractionation of soil B. Treatments included N alone, NP, NPK, 150% of recommended NPK, NPK + farmyard manure (FYM), NPK + lime, and an unfertilized-control. Five soil B fractions were determined along with hot CaCl₂-extractable (available) B. Averaged across the treatments, the soil had low organic carbon (C), pH and cation exchange capacity (CEC), and high free sesquioxides. Total B content was 21.7 mg kg^?1. Among different B fractions, residual B was the major contributor to total B and other fractions collectively shared 7% of total B only. Application of N alone depleted readily soluble, specifically adsorbed and organically bound B bringing the contents even below unfertilized-control. Conjoint use of lime or FYM with NPK increased significantly these fractions, whereas a decrease in oxide bound B was noticed under these treatments. Available B was positively correlated with these fractions indicating their significance in controlling B availability in the soil. The study revealed that use of lime or FYM helped modifying the distribution of soil B in different fractions by way of changing soil pH and organic C content, resulting in enrichment of plant available pool. A drastically low available B content in different treatments receiving fertilizers alone, however, suggested the necessity of B fertilization at prescribed rates for maintaining soil B fertility as also high crop yields.     

Assessment of Cadmium Distribution in Some Australian Krasnozems by Sequential Extraction

A sequential extraction procedure was used to investigate the influence of long-term phosphate fertilization on the distribution of cadmium throughout the major components of krasnozemic soils in the potato growing district located to the East of Ballarat, Victoria, Australia. The soil fractions investigated consisted of the water soluble, exchangeable, weakly bound to metal oxides, strongly bound to metal oxides, bound to organic material and residual. Other soil parameters, such as pH_H2O, pH_CaCl2, electrolytical conductivity and total organic material were investigated. The concentrations of Ca, Mg, Fe, Mn and Zn in each soil fraction were also measured. The total concentration of cadmium in the farmed soils had increased from the background level of 100.5 μg/kg to an average of 210 μg/kg. The proportion of cadmium available or potentially available for plant uptake was very low. On average, 89% of the cadmium in the farmed soils had been adsorbed by amorphous metal oxides and organic material, or was associated with silicates, and is therefore unavailable for plant uptake. The metal oxides adsorbed the highest proportion of cadmium.     

A New Method of Preparing Gel for DGT Technique and Application to the Soil Phosphorus Availability Test

There are different methods for soil phosphorus (P) availability assessment. Researchers have recently introduced a new method called diffusive gradients in thin films (DGT) to evaluate P availability in soils. The aim of this study was manufacturing DGT gel using copolymerization of acrylamide and allylic double bonds agarose to replace the patented agarose as a cross-linker. We evaluated the efficacy of the prepared gel and used the assembled DGT with that gel in 10 calcareous soils. We compared the available P measured with the assembled DGT in those soils to Olsen’s P measured in the same soils. Corn plant (Zea mays L.) was also grown in those soils in a greenhouse for two months. Results showed that the diffusion coefficient of P ions in the gel with 0.3% of allylagarose at 25 °C was 6.9 * 10^–6 cm².s^?1, implying that P ions were allowed to diffuse freely through the prepared diffusive gel. Measured P by the assembled DGT showed a close linear correlation (r² = 0.98) with the P concentrations measured in soil solutions extracted by the Olsen method. This demonstrates the prepared allylagarose cross-linked gels’ efficiency and thus the reliability of the assembled DGT in easily and rapidly measuring the soil’s available P. The P, as measured by the assembled DGT in the examined soils, indicated a little lower correlation coefficient (r = 0.77) than those measured by the Olsen method (r = 0.88). However, the CE values measured by DGT showed a good correlation with P uptake by the roots (r = 0.88, p ≤ 0.01). The best correlation between CE and P content in corn occurred for the measurements made after 48 h of DGT placement in soils. The 20-h measurements had about the same r value, indicating that 20 h is sufficient for the placement of DGT assembly in the soil before its removing and measuring the available P. According to our findings, acrylamide—allylagarose gel as a diffusive gel in DGT was found to be suitable for P measurements in both aqueous solutions and soil.     

Distribution of Boron Forms in Relation to Soil Characteristics,Chemical Fertilizers,and Amendments in an Acid Alfisol of Northwestern Himalayas

Different forms of boron (B) were studied under long-term effect of chemical fertilizers and amendments in an acidic Alfisol of northwestern Himalayas. Residual B was dominant, comprising 71.5% of total B. Different B fractions correlated among themselves, showing interdependency of pools of B. On an average, all pools except for organically bound B exhibited depletion. Application of recommended doses of nitrogen (N), phosphorus (P), and potassium (K) with farmyard manure (FYM) in general showed less depletion in comparison to other treatments with the exception of liming treatment in oxide-bound B. Soil reaction (pH) had a significant positive relationship with most fractions. Except for residual and total B, significant positive relationships with other fractions of B with cation exchange capacity (CEC) were observed. Available B had a significant positive relationship with nonspecifically and specifically adsorbed B, indicating their importance in replenishing available B in soil. Imbalanced use of fertilizers had the most deleterious effects on soil properties.     

Distribution of Forms of Copper and their Association with Soil Properties and Uptake in Major Soil Orders in Semi-arid Soils of Punjab,India

Profiles of semi-arid-zone soils in Punjab, northwestern India, were investigated for different forms of copper (Cu), including total Cu, diethylenetriaminepentaacetic acid (DTPA)–extractable Cu, soil solution plus exchangeable Cu, Cu adsorbed onto inorganic sites, Cu bound by organic sites, and Cu adsorbed onto oxide surfaces. When all soils were considered, total Cu content ranged from 7 to 37 mg kg^?1, while DTPA-extractable and soil solution plus exchangeable Cu contents ranged from 0.30 to 3.26 mg kg^?1 and from 0.02 to 0.43 mg kg^?1, respectively. Copper adsorbed onto inorganic sites ranged from 0.62 to 2.6 mg kg^?1 and that onto oxide surfaces ranged from 2.0 to 13.2 mg kg^?1. The Cu bound by organic sites ranged from 1.2 to 12.2 mg kg^?1. The magnitudes of different forms of Cu in soils did not exhibit any consistent pattern of distribution. Organic matter and size fractions (clay and silt) had a strong influence on the distribution of different forms of Cu. The content of all forms of Cu was generally greater in the fine-textured Alfisols and Inceptisols than coarse-textured Entisols. Soil solution plus exchangeable Cu, Cu held onto organic sites, and and Cu adsorbed onto inorganic sites (crystalline) had significant positive correlations with organic carbon and silt contents.The DTPA Cu was positively correlated with organic carbon, silt, and clay contents. Total Cu content strongly correlated with silt and clay contents of soils. Among the forms, Cu held on the organic site, water soluble + exchangeable Cu, and Cu adsorbed onto oxide surface were positively correlated with DTPA-extractable Cu. The DTPA-extractable Cu and soil solution plus exchangeable Cu seems to be good indices of Cu availability in soils and can be used for correction of Cu deficiency in the soils of the region. The uptake of Cu was greater in fine-textured Inceptisols and Alfisols than coarse-textured Entisols. Among the different forms only DTPA-extractable Cu was positively correlated with total uptake of Cu.     

Measuring and simulating pH buffer capacity of calcareous soils using empirical and mechanistic models

ABSTRACT

We studied (i) the pH buffer capacity (pHBC) of calcareous soils varied widely in calcite and texture, (ii) the contribution of soil properties to pHBC and (iii) the significance of using a model based on calcite dissolution to estimate the pHBC of calcareous soils. The pHBC of soils was measured by adding several rates of HCl to soils (100–6500 mM H⁺ kg^–1), in a 0.01 M CaCl₂ background and an equilibration time of 24 h. The pHBC (mM H⁺ kg^–1 pH^?1) varied from 55 to 3383, with the mean of 1073. The pHBC of the soils was strongly correlated with soil CaCO₃ equivalent (calcite) (r = 0.94), sand (r = ?0.72), silt (r = 0.60), EC (r = 0.63), pH (r = 0.55), and weakly (r = 0.37) but significantly with clay content. The attained pHBC values indicated that calcite was probably the main buffer system in these soils. The chemical equilibrium model successfully predicted pH titration curves based on calcite dissolution, indicating buffering of acid inputs in the calcareous soils is dominated by calcite dissolution. The model can be used to simulate acidification of calcareous soils and to provide information for making environmental management decisions.     

Soil Quality Changes Resulting from Long-Term Fertilizer Application Under Intensive Cropping System in Alluvial Soils

Soil quality is one of the most important factors which reveal the soil–environment functionality for identifying whether soil quality is improving, remain constant, or declining. This paper evaluated change in soil quality after 40 years of chemical fertilizer use and continuous cultivation of multiple crops (jute–rice–wheat) on alluvial soils. The concepts of relative soil quality index (RSQI) and Cumulative Rating Index (CRI) were used in the evaluation. It was observed that soil of class III reduced to class II after 20 years and to class I after 40 years of fertilizer application with manure, nitrogen, phosphorus, and potassium (NPK_100+FYM). There was a strong correlation between SQI and CRI (r = 0.82**) which showed more promising effects on soil sustainability. Significant positive relationship between ?RSQI and crop yield of jute (r = 0.89**) and wheat (r = 0.90**) was found. However, rice yield declined up to 44% of its initial yield (r = 0.20).     

Soil biological properties and carbon dynamics subsequent to organic amendments addition in sodic black soils

The field experiments on calcareous sodic Vertisols were conducted on farmer’s fields in Purna valley of Vidarbha region of Maharashtra. The treatments comprised of different green manures (GMs); crop residues (CRs); gypsum. The chemical and biological properties after 2 years experiment showed that the application of gypsum recorded significant drop in pH and exchangeable sodium percentage (ESP) as compared to organic amendments. But later has outperformed with respect to biological activities viz., dehydrogenase activity (DHA) and microbial respiration and carbon sequestration by enhancing soil organic carbon (SOC), soil organic carbon (SOC) stock, soil microbial biomass carbon (SMBC) and labile carbon pool (POXC). Among the different organic amendments the application of dhaincha improved SMBC by 90%, microbial respiration by 104%, POXC by 59% and DHA by 265% as compare to control. High ESP of these soils showed negative relationship with microbial respiration and POXC (r = 0.48 and r = 0.43, p = < 0.05). While addition of biomass showed positive relationship with SMBC, microbial respiration, POXC and DHA (r = 0.93, r = 0.81, r = 0.83 and r = 0.91 p = < 0.01). The results of study showed green manuring in sodic black soil found to be alternative choice to gypsum, which besides gradual reclamation also enhance biological properties and carbon sequestration.     

Distribution of Zinc Fractions in Some Major Soils of India and the Impact on Nutrition of Rice

Abstract

Speciation study of microelements in soils is useful to assess their retention and release by the soil to the plant. Laboratory and greenhouse investigations were conducted for five soils of different agro‐ecological zones (viz., Bhuna, Delhi, Cooch‐Behar, Gurgaon, and Pabra) with diverse physicochemical properties to study the distribution of zinc (Zn) among the soil fractions with respect to the availability of Zn species for uptake by rice plant. A sequential extraction procedure was used that fractionated total soil Zn into water‐soluble (WS), exchangeable (EX), specifically adsorbed (SA), acid‐soluble (AS), manganese (Mn)‐oxide‐occluded (Mn‐OX), organic‐matter‐occluded (OM), amorphous iron (Fe)‐oxide‐bound (AFe‐OX), crystalline Fe‐oxide‐bound (CFe‐OX), and residual (RES) forms. There was a wide variation in the magnitude of these fractions among the soils. The studies revealed that more than 90% of the total Zn content occurred in the relatively inactive clay lattice and other mineral‐bound form (RES) and that only a small fraction occurred in the forms of WS, EX, OM, AFe‐OX, and CFe‐OX. Rice (Oryza sativa L.) cultivars differ widely in their sensitivity to Zn deficiency. Results suggested that Zn in water‐soluble, organic complexes, exchange positions, and amorphous sesquioxides were the fractions (pools) that played a key role in the uptake of Zn by the rice varieties (viz., Pusa‐933‐87‐1‐11‐88‐1‐2‐1, Pusa‐44, Pusa‐834, Jaya, and Pusa‐677). Isotopic ally exchangeable Zn (labile Zn) was recorded higher in Typic Ustrochrept of Pabra soil, and uptake of Zn by rice cultivars was also higher in this soil. The kinetic parameters such as maximum influx at high concentrations (I_max) and nutrient concentration in solution where influx is one half of I_max (K_m) behaved differentially with respect to varieties. The highest I_max value recorded was 9.2×10^?7 µmol cm^?2 s^?1 at the 5 mg kg^?1 Zn rate for Pusa‐933‐87‐1‐11‐88‐1‐2‐1, and the same was lowest for Pusa‐44, being 4.6×10^?7 µmol cm^?2 s^?1 at the 5 mg kg^?1 Zn rate. The K_m value was highest for Pusa‐44 (2.1×10^?4µmol cm^?2 s^?1) and lowest for Pusa‐933‐87‐1‐11‐88‐1‐2‐1 (1.20×10^?4µmol cm^?2 s^?1). The availability of Zn to rice cultivars in Typic Ustrochrepts of Bhuna and Delhi soils, which are characterized by higher activation energy and entropy factor, was accompanied by breakage of bonds or by significant structural changes.     

Influences of lignin from paper mill sludge on soil properties and metal accumulation in wheat

The influences of lignin application on soil properties of three different soils, Jiangxi soil (Ultisol, Hapludult), Heilongjiang soil (Alfisol, Entioboralf) and Beijing soil (Alfisol, Haplustalf), and metal accumulation in wheat (Triticum aestivum L.) were studied in a pot experiment. By lignin amendment, soil pH, organic matter (OM) and cation exchange capacity (CEC) increased, except for CEC in the Beijing soil. Analysis showed that available P and K in lignin-amended soils were also elevated, except for P in the Jiangxi soil. A three-step sequential extraction procedure proposed by the Standards, Measurements and Testing Programme (formerly BCR) of the European Commission was used to investigate the fraction redistribution of heavy metals in soils with lignin application. The fractions were specified as B1: water soluble, exchangeable and carbonate bound, and weakly adsorbed; B2: Fe-Mn oxide bound; and B3: organic matter and sulfide bound. Generally, the heavy metal content of the B2 fraction decreased whereas that of the B3 fraction increased. Lignin application to arable soils can not only improve plant growth in vitro, but also reduce the accumulation of the heavy metals Cu, Zn, Cd, Pb, Cr and Ni in wheat plants.     

Zn fractionation and availability in different soil aggregate fractions from Isfahan region,Central Iran

Zinc (Zn) distribution in different soil aggregates can affect Zn availability. In this study, the effect of soil aggregate-size fractions on Zn distribution and availability was determined in some heavy metal-contaminated soils. Air-dried samples were fractionated into four different aggregate-size fractions (2.0–4.0, 0.25–2.0, 0.05–0.25 and <0.05 mm). Extraction efficiency of available Zn determined by using Mehlich-3, DTPA-TEA, Mehlich-1 and H₂O methods in different aggregates and relation between extracted Zn and corn indices were studied. Moreover, the Tessier fractionation scheme was applied to determine the partitioning of Zn in different aggregates. Among all the extractants, Mehlich-3 showed better extractability of Zn from soils and the highest amount of extracted Zn was found in the <0.05 (87.65 mg kg^?1) and 0.05–0.25 (80.86 mg kg^?1) mm fractions. Zn extracted by Mehlich-3 and DTPA-TEA in the <0.05 and 0.05–0.25 mm fractions had significant correlation with the amount of Zn in corn. Also, correlation coefficients between carbonate-bound and Fe–Mn oxide-bound Zn and available Zn in <0.05 and 0.05–0.25 mm aggregates was higher than other aggregates, suggesting that this two fraction of Zn in finer aggregates constituted the major available Zn pools in the studied soils.     

Effect of Oxalic and Citric Acids on Zinc Release Kinetic in Two Calcareous Soils

Rate of zinc (Zn) release from solid to solution phase by organic acids can influence Zn availability in calcareous soils. The objective of the present study was to investigate the effect of different concentrations (1.1, 2.2, and 3.3 mM) of oxalic acid and citric acid on the kinetic release of Zn from two calcareous soils from Eastern Iran. The two organic acids showed significant difference in Zn release from studied soils. Cumulative Zn release during 72 h ranged from 5.85 to 10.4 mg kg^?1 in soil 1 and ranged from 8.7 to 16.9 mg kg^?1 in soil 2 using different concentrations of oxalic acid. The amount of cumulative Zn release after 72 h in soil 1 ranged from 13.65 to 28.77 mg kg^?1 and from 17.63 to 23.13 mg kg^?1 when different concentrations of citric acid was used. In general, Citric acid released 38% more Zn from soils than oxalic acid. The release of Zn from soils increased with citric acid concentration but decreased with increasing of oxalic acid concentrations in the solution. The simplified Elovich equation best described Zn release as a function of time (r² = 0.93 and SE = 0.78). From the present study, Zn release from soils can be limited by the higher concentration of oxalic acid, while citric acid is suitable for enhancing soil lability of Zn.     

The change of heavy metals fractions during hydrochar decomposition in soils amended with different municipal sewage sludge hydrochars

Purpose

This study was to investigate the changes of heavy metals in the soils amended with different municipal sewage sludge hydrochars.

Materials and methods

Sewage sludge hydrochars prepared at either 190 or 260 °C, for 1, 6, 12, 18, or 24 h, respectively, were added to soil samples and then incubated for 60 days. Water-extractable organic carbon (WEOC) and CO₂ evolution were determined during the incubation. The total quantities of heavy metals and their different fractions were analyzed by inductively coupled plasma spectrometry (ICP).

Results and discussion

Hydrochar-amended soils had much higher water-extractable carbon and more CO₂ evolution than control soil, indicating that the added hydrochars contained a significant amount of WEOC and could be decomposed during the incubation. Hydrochar addition immediately and significantly increased the total heavy metals of the soil. Moreover, both oxidizable and residual fractions of all heavy metals were significantly higher in all the hydrochar-added soils than those in control soil. Both oxidable and residual fractions of heavy metals decreased in the hydrochar-amended soils during 60-day incubation. In contrary, both acid soluble and reducible fractions of heavy metals increased in the hydrochar-amended soils during incubation. It is thus obvious that the heavy metals in both oxidable and residual fractions may be released during hydrochar decomposition and then be adsorbed by soil matrix such as carbonates, iron oxides, and clays.

Conclusions

Municipal sewage sludge can be readily carbonized into hydrochar. However, it is watchful of applying the hydrochar into soil since hydrochar addition increases in both total and bioavailable heavy metals in soil. More work is particularly required to investigate the long-term impacts on soil and environment.