Cadmium contamination of soils and rice plants caused by zinc mining

Characteristics of heavy metal contamination in paddy soils were discussed with respect to the soil and field conditions influencing the metal contents in rice.

1. In contaminated areas along the Kuzuryu River, the heavy metal contents of rice (expressed as the average of each area) were largely related to contents of paddy soils in 0.1 N HCl soluble forms as well as in total contents. A correlation was also found for the ratio of Cd to Zn in their soluble forms in the soil.

2. Within a given contaminated field plot, the heavy metal contents were not uniform, i.e: they were high around the irrigation inlet and decreased towards the outlet. However, these changes seemed to alter the Cd rice level much less than expected.

3. Soil pH and exchangeable Ca were negatively correlated with the Cd content of rice at a highly significant level. An un-drained field condition with a low soil Eh remarkably lowered the Cd content, particularly during rainy weather after the heading of the plant.

4. Based on a comparison of the extractable heavy metal contents in the waste ores and their contaminated soils, sulfide minerals derived from the mining appeared to be weathered more rapidly in the soil. With some extraction methods, the Cd origin was distinguished as the solubility of the metal in the soil among the waste products from the zinc mining and refinery.     

Long-term impacts of zinc and copper enriched sewage sludge additions on bacterial, archaeal and fungal communities in arable and grassland soils

Long-term impacts of metal contamination derived from sewage sludge on soil microbial communities have been widely evaluated, but confounding effects have made it difficult to draw firm conclusions and thus to advise on safe metal limits. Here we used Multiplex-terminal restriction length fragment polymorphism (M-TRFLP) to assess the long-term impact of sludge-borne Zn and Cu contamination on the structure of bacterial, fungal and archaeal communities across seven different soils at metal levels relevant to current guideline limits. Despite strong effects of site on microbial community structure, analysis of similarity (ANOSIM) demonstrated a small but significant effect of Zn on bacteria (P < 0.001), archaea (P < 0.001), and fungi (P < 0.001). Significant effects of Cu on bacteria (P < 0.001), archaea (P < 0.001) and fungi (P < 0.001) were also observed. Several bacterial and fungal T-RFs were identified as responding to Zn or Cu. For example the bacterial T-RF 72 was negatively correlated with Zn and Cu, and T-RF 259 was positively correlated with Zn. Attempts to identify these bacterial markers of Zn and Cu contamination suggest a negative impact of Cu on Acidobacteria in arable soils. These results demonstrate for the first time, that despite a strong influence of site on microbial community structure, effects of Zn and Cu derived from sewage sludge can be detected as shifts in bacterial, fungal and archaeal communities indicating a common response more than 11 years after sludge addition.     

施用有机物料对土壤镉形态的影响

采用室内培养试验,研究作物新鲜秸秆和腐熟猪粪对模拟镉（Cd）污染的土壤中Cd形态转化的动态影响。结果表明,各处理土壤交换态Cd含量随培养时间均逐渐降低。碳酸盐结合态和铁锰氧化物结合态Cd含量先增加后降低, 而有机质结合态和残渣态Cd含量则逐渐增加。添加秸秆可增加土壤交换态Cd含量,但随时间延长,增幅逐渐降低, 猪粪则可降低土壤交换态Cd含量。添加有机物后土壤交换态Cd含量的变化主要是由有机质结合态或残渣态Cd含量的变化而引起。秸秆和猪粪对土壤Cd形态的转化与土壤胡敏酸（HA）和富里酸（FA）的变化有关。秸秆对能活化土壤Cd的FA增加幅度大于对能钝化土壤Cd的HA增加幅度,降低HA/FA比,但降幅随时间逐渐减少; 猪粪在整个培养阶段对HA增加幅度均大于FA的增加幅度,增加HA/FA比。秸秆和猪粪均可降低潮土pH而提高红壤pH,但只有猪粪可通过提高红壤pH降低Cd向交换态转化。添加秸秆和猪粪后,Cd由低活性态向交换态转化与HA/FA呈显著负相关。     

Navy bean responses to zinc fertilizers

Abstract

Experiments were conducted to study the effects of various Zn carriers and rates on the growth and Zn uptake of Sanilac navy beans. Plants were grown under greenhouse and growth chamber conditions for 2, 4, 6 or 8 weeks on a Wisner silty clay loam soil low in available Zn. Plant growth responded to the various Zn fertilizers except at 4 weeks where no response was obtained. Only ZnZnEDTA and ZnNa₂EDTA increased growth at 2, 6 and 8 weeks while ZnSO₄, ZnHEIDA, ZnNTA and Zn(NO₃)₂ did not give consistent responses at all of these periods. The Zn carriers did not influence uptake until 6 weeks after planting.

Zinc rates began to influence plant growth at about the bloom stage, 6 weeks after planting with 2.0 kg/ha Zn as ZnNa₂EDTA and 3.3, 6.6 and 13.2 kg/ha Zn as ZnSO₄ giving higher responses than 0.7 or 1.3 kg/ha Zn as ZnNa₂EDTA. The most consistent increase in Zn uptake at 2, 6 and 8 weeks was obtained with 6.6 and 13.2 kg/ha Zn applied as ZnSO₄. No differences in uptake were obtained at 4 weeks between all the rates of Zn that were used. The data herein indicate that the stage of growth at which plant response to Zn fertilizers is evaluated may be very critical.     

Cadmium and zinc availability in soil irrigated with sludge containing a cationic conditioner

Tomato plants (Lycopersicum esculentum Mill, ‘Red Cherry Small’), grown in 15 cm diameter plastic pots with a standard greenhouse medium (1:1:1, by volume, soil:peat:sand) were irrigated for 15 weeks with liquid sewage sludge containing a liquid cationic conditioner (Petroset SB, Phillips Petroleum Company, Bartlesville, Oklahoma) to determine the effect of the conditioner on Cd and Zn availability. Half of the plants received 50 ml week^?1 liquid digested sludge with no conditioner and half of the plants received 50 ml week^?1 sludge containing 0.25 ml conditioner (200:1, by volume, sludge:conditioner). Plant height was measured weekly. Plants were harvested 3, 6, 9, 12, and 15 weeks after sludge treatments began and separated into roots, shoots, and fruits for dry weight determination and Cd and Zn analyses. Soil and sludge crusts were sampled at the same times and analyzed for extractable concentrations of Cd and Zn. Dry weights of plants grown with conditioned sludge were similar to those grown with nonconditioned sludge. Plants with conditioner flowered and fruited one and two weeks earlier, respectively, than plants without conditioned. Six weeks after treatments began, when the plants had grown to their greatest height, Cd concentrations in sludge crusts, soil, and roots receiving conditioner were 2.0, 1.5, and 2.1 times greater, respectively, than crusts, soil and roots not receiving conditioner. After the six weeks sampling time, Cd concentrations in crusts, soil, and roots receiving conditioned sludge were similar to those in crusts, soil, and roots receiving nonconditioned sludge. At the third-week sampling time, shoots of plants grown with conditioner had 2.6 times more Cd than shoots of plants grown without conditioner. Cadmium concentrations in shoots from both treatments were similar at later sampling dates. Cadmium content of fruits was the same for both treatments all sampling times. Zinc content of roots, shoots, fruits soil, and sludge crusts was no affected by the conditioner. Results showed that a cationic conditioner, added to sludge, increased the availability of Cd, but not of zn, for tomato plants until maximum height was reached.     

Cadmium,copper, and zinc adsorption by a forest soil in the presence of sludge leachate

Samples from three genetic horizons of an acidic forest soil were equilibrated with solutions containing Cd, Cu, and Zn in the presence and absence of a municipal sewage sludge leachate. Copper adsorption was greater than Cd and Zn in all three horizons, while Cd and Zn adsorption was quite similar. Relative to a NaN0₃ background solution, sludge leachate reduced Cu adsorption in all horizons; however, in the presence of leachate Zn adsorption increased in the B2 and C horizons, while Cd adsorption decreased in the Al and was unaffected in the B2 and C horizons. Distribution coefficients generally increased as solution concentration and adsorption increased. In all horizons additions of Cd and Zn were able to reduce apparent surface charge. Copper not only reduced surface charge in the Al horizon, but caused a charge reversal in the 132 and C horizons. Adsorption data were best fit by linear or Freundlich equations. Differences in adsorption between sludge leachate and NaN0₃ solutions could not be explained simply by differences in metal activities as calculated using the GEOCHEM program. Competition from cations and organics present m the sludge leachate appears to contribute to lower metal adsorption.     

Cadmium and zinc accumulation by the hyperaccumulator Thlaspi caerulescens from soils enriched with insoluble metal compounds

Abstract

The clay mineralogy of seven Dystrandepts developed on basalts in Northland (New Zealand), the French Massif Central and Western Oregon (U.S.A.) was determined by selective dissolution—differential infrared spectroscopy in combination with chemical, X-ray diffrac tion, electron microscopic and thermogravimetric analyses. Of 14 soil samples, 6 from Northland and Cantal (French Massif Central) contained allophane and imogolite, whereas the remaining 8 did not. Allophane-like constituents and/or “alumina” were found in all the samples, and opaline silica was present in three A₁ horizons. The contents of 2 : 1–2 : 1 : 1 layer silicates and their intergrades varied somewhat among the samples. Predominant volcanic glass shards in the Northland Dystrandepts and quartz in the Western Oregon Dystrandepts indicated that their parent materials were not restricted to basalt.     

Cadmium and zinc accumulation in willow and poplar species grown on polluted soils

Woody plant species that produce high biomass have been proposed for use in phytoremediation technology. We investigated the accumulation of cadmium (Cd) and zinc (Zn) in Salix babylonica, S. caprea, S. dasyclados, S. matsudana × alba, S. purpurea, S. smithiana, Populus tremula, and P. nigra clones grown in a pot experiment on a Calcaric and a Eutric Cambisol (pH 7.2 and 6.4) of different levels of contamination (total metal concentrations in mg kg^–1 in soil A: 32.7 Cd, 1760 Zn; soil B: 4.34 Cd, 220 Zn). Generally, the tested clones tolerated large metal concentrations in soils and had larger Cd and Zn concentrations in leaves compared to the roots. The largest Cd concentrations in leaves were found in two clones of S. smithiana (440 mg kg^–1 on soil A; 70 mg kg^–1 on soil B). One of the S. smithiana clones had also the largest Zn concentrations (870 mg kg^–1) on soil B but accumulated slightly less Zn than a S. matsudana × alba clone (2430 mg kg^–1) on soil A. The Cd concentrations in leaves of both S. smithiana clones on soil A are the largest ever reported for soil‐grown willows. The bioconcentration factors of the best performing clone reached 15.9 for Cd and 3.93 for Zn on the less contaminated soil B. Also based on the metal contents in leaves, this clone was identified as the most promising for phytoextraction. The metal concentrations in leaves observed in the pot experiment do not reflect those found in a previous hydroponic study and the leaf‐to‐root ratios are clearly underestimated in hydroponic conditions. This demonstrates the need for testing candidates for phytoextraction crops on soils rather than in hydroponics. Our data also show that the phytoextraction potential should be tested on different soils to avoid misleading conclusions.     

Cadmium accumulation and partitioning in Ocimum basilicum as influenced by the application of various potassium fertilizers

Potassium (K) is one of the major essential nutrient elements whose application of organic or nano-chelate-fertilizers has received increased attention recently. Cadmium (Cd) contamination in agricultural soils and environment is increasing due to the over-application of Cd-containing phosphate fertilizers. But few studies have been carried out on the environmental influences of K-nano-chelate fertilizers especially on Cd-polluted soils. Therefore, the effects of K-fertilizer application in different rates (0, 100 and 200 mg kg^?1 soil) and forms (KCl, K₂SO₄ and K-nano-chelate) on Cd content and partitioning in Ocimum basilicum grown on an artificially Cd-contaminated calcareous soil (with 40 mg Cd kg^?1 soil) were studied under greenhouse conditions. Cadmium decreased shoot dry weight (SDW), but did not affect root dry weight (RDW) and no consistent trend was observed with applied K. Cadmium increased shoot and root Cd concentration or uptake. KCl and K₂SO₄ increased shoot Cd concentration compared to that of control, whereas K-nano-chelate did not affect it. In Cd-treated soils the mean value of Cd translocation factor (ratio of Cd concentration in shoots to that of roots) decreased by 60% as compared to that of the control. Application of 100 mg K-K₂SO₄ and 100 and 200 mg K-nano-chelate increased the Cd translocation factor by 49, 59 and 112% in Cd-treated soils, respectively. In Cd-treated soils, greater amounts of Cd accumulated in roots. K-nano-chelate could mitigate the adverse effect of Cd on SDW and Cd accumulation in plants grown on Cd-polluted soils, so the risk of Cd entrance to the food chain is reduced (however, in Cd-untreated soils, K-nano-chelate increased the Cd translocation factor higher than other K sources). In Cd-polluted soils KCl was the most inappropriate fertilizer that may intensify Cd accumulation in plants. However, it may be useful in the phytoremediation of Cd-polluted soils.     

Ammonia volatilization from five nitrogen compounds used as fertilizers following surface application to soils

Measurements were made of the volatilization of ammonia from mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP), ammonium sulphate (AS), ammonium nitrate (AN) and urea, applied to the surface of five contrasting soils. The compounds were applied as solids, at a rate equivalent to 100 kg N ha^?1, to samples of moist soil packed into columns (48 mm diameter) and placed individually in jars through which a stream of air was passed for a period of 8 d. Volatilization ranged from nil to 53% of the N applied, with both the nature of the compound and soil type having large effects. Taking all combinations into account, there was a close relationship between the extent of volatilization, expressed as a percentage of the ammonium or urea N, and the pH attained after 24 h by the corresponding mixtures of soil and compound. Using the results of these and other experiments, the proportion of fertilizer N volatilized as ammonia is estimated to be about 3.4% over the UK as a whole.     

Evolution of mercury content in agricultural soils due to the application of organic and mineral fertilizers

Purpose

Mercury pollution in agricultural soils associated to the use of fertilizers and its influence on crops is a cause of major concern. The purpose of this work was to investigate the impact of the application of different organic and mineral fertilizers on the Hg concentration in the agricultural soils and its uptake by barley.

Materials and methods

Hg concentration was studied through a field test in an agricultural land located in the province of Palencia (Spain) over a 5-year period. The impact of irrigation and of four different fertilizers (a mineral one and three different organic waste materials, namely municipal solid waste compost, sewage sludge, and dehydrated sewage sludge) was assessed. The amounts of the mineral and organic fertilizers added to the soil were determined according to agricultural fertilization needs. The experimental crop was barley (Hordeum vulgare L.), planted as an annual crop. Mercury analyses were conducted using a direct mercury analyzer and validated according to EPA Method 7473. BCR-141R was used as a certified reference material.

Results and discussion

After 5 years, whereas the application of the mineral fertilizer did not increase the mercury content in the agricultural soils, the application of the organic residues led to Hg contents 1.7–7.6 times higher than that of the control soil. The treatment with solid municipal waste compost (MSWC) led to the largest increase in Hg content in the soil, followed by composted sewage sludge (CSS) and by dehydrated sewage sludge (DSS). No significant differences were observed in the Hg content in the barley grains, although the highest values were associated to the sludge-treated plots.

Conclusions

The application of organic fertilizers such as sewage sludges and municipal solid wastes led to an increase in the mercury concentration in the agricultural soils, noticeable for soils with low initial Hg concentrations (similar to background levels). This increase differed depending on the type of waste and on the intra-organic matter diffusion mechanisms, as well as on the type of irrigation of the agricultural land. Conversely, no significant differences in the Hg content in grains were found among the soils with the different fertilization treatments, although the highest values were observed for those treated with sewage sludge. The resulting Hg levels in both soils and grains were within legal limits, posing no danger to the environment or to human health.

     

Effect of zinc application methods on apparent utilization efficiency of zinc and potassium fertilizers under rice-wheat rotation

Apparent utilization of zinc (Zn) and potassium (K) fertilizers was examined in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) using combinations of no K; soil applied K levels and no Zn; soil and foliar applied Zn. Application of 33.2 kg K ha^?1 in rice and 24.9 kg K ha^?1 in wheat along with foliar spray of 2 kg Zn ha^?1 at 30 and 60 days gave the highest mean grain yields. Foliar application of zinc increased Zn concentration in flag leaves, grain, and straw of rice and wheat and K concentration in flag leaves of rice and straw of wheat significantly. Potassium application increased Zn concentration in rice grain and straw and K concentration in wheat straw significantly. Zinc and K increased the uptake of each other in grain; straw and total uptake by both crops significantly. Zinc fertilizer enhanced the utilization of soil K. Potassium fertilizer enhanced the utilization of applied Zn.     

Sorption of Cd and Pb by selected temperate and semi-arid soils: effects of sludge application and ageing of sludged soils

Five soils from semi-arid regions of India and 5 soils from England were compared with regard to their ability to sorb Cd and Pb when mixed with sewage sludge and as unsludged controls. The application of sewage sludge at 150t ha^?1 significantly increased the amount of Cd retained by the soils. The sorption data were statistically best-fit to the linearized Freundlich equation and the slopes of the isotherms were steeper with sludge application, indicating an increased affinity for Cd in the soil-sludge mixtures. A similar trend in Pb sorption was also observed for the English soils following the sludge treatment. In contrast, most of the Indian soils showed a decline in Pb sorption following the sludge application. Liming an acidic English soil to pH 7.0 was shown to increase its metal sorption capacity. The longer-term persistence of these observed effects of sewage sludge addition on metal sorption by soils was investigated in two sludge-soil mixtures maintained under experimental conditions for up to 450 days. Samples of these soils taken 1,60 and 450 days after the mixing with the sludge were batch equilibrated with Cd and Pb and it was found that the metal retention in both soils decreased significantly over this time period.     

杂交油菜施用锌钼肥的效果

供试土壤为下蜀系黄土发育而成,pH值6.5,有机质2.3%,全氮(N)0.14%,速效磷(P2O5)5mg/kg,速效钾(K2O)51mg/kg,有效硼(B)0.4mg/kg,有效锌(Zn)0.6mg/kg,有效钼(Mo)0.1mg/kg.试验地点:孔城赵山.     

Distribution of zinc forms in highly calcareous soils as influenced by soil physical and chemical properties and application of zinc sulfate

Abstract

Water‐soluble zinc (Zn) fertilizers are rapidly converted to insoluble forms in calcareous soils resulting in lower efficiency of such fertilizers. A knowledge of distribution of native and applied Zn in such soils is necessary for understanding the fate of applied Zn fertilizers and finding ways to increase their efficiency. This experiment was conducted to obtain such information in selected highly calcareous soils of Iran. A sequential extraction method was used to fractionate the Zn forms of surface horizons (0–20 cm) of 20 highly calcareous soils [16 to 58% calcium carbonate (CaCO₃) equivalent; pH 7.9 to 8.5] which had previously received 0, 10, or 20 mg Zn/kg as zinc sulfate (ZnSO₄) and had been under one corn (Zea mays L.) crop in the greenhouse. The forms determined were exchangeable (EXZN), sorbed (SRZN), organic (ORZN), carbonate (CRZN), residual (RSZN), and sum of forms (SMZN). The native SMZN ranged from 32.4 to 66.7 mg/kg with a mean of 49.9 mg/kg. Application of 10 and 20 mg Zn/kg as ZnSO₄ increased the mean to 57.7 and 62.7 mg/kg, respectively. Concentration of different forms of Zn in the soils was determined to be in the following order: RSZN >>> CRZN > SRZN > EXZN > ORZN. The concentration of native EXZN+SRZN+ORZN forms constituted less than 5% of SMZN, while concentration of CRZN alone ranged from 4.37 to 16.05% with a mean of 8.36%. Application of ZnSO₄, while significantly increased the concentration of all forms of Zn, had a pronounced effect on CRZN. Averaged over all soils, 58 and 60% of the applied ZnSO₄ was converted to CRZN for the 10 and 20 mg Zn/kg, respectively. Regression equations relating different Zn forms to soil physical and chemical properties indicated that the Zn forms are significantly influenced by soil properties.

It was concluded that conversion of applied ZnSO₄ to CRZN was mainly responsible for retention of this fertilizer in highly calcareous soils, making it temporarily unavailable to plants, and therefore decreasing its apparent recovery by the first crop.     

Cadmium speciation as influenced by soil water content and zinc and the studies of kinetic modeling in two soils textural classes

Background and aimsSince few studies have existed in the literature about the effect of zinc (Zn) on cadmium (Cd) chemical forms in soils. Therefore, this study has been performed to determine the impact of Zn on cadmium Cd chemical forms in two soil textural classes in Fars province-Iran at two soil water content (SWC) (flooded soil water content (FSWC) and field capacity soil water content (FCSWC)) and study the kinetic modeling of Cd.Methods and materialsVariables were three levels of Cd (0, 30 and 60 mg kg^-1 of soil as CdSO₄·8H₂O), three levels of Zn (0, 5 and 10 mg kg^-1 of soil as Zn-EDTA) three level Incubation times (2, 4 and eight weeks), two soil textural classes (clay and sandy clay loam) and two SWC. The randomized completed block design (RCBD) was used for this experiment. The Tessier sequential extraction method was used to determine the Cd concentration in (WsEx), (Fe-MnOx), (Car), (Om) and (Res) chemical forms.ResultsIn the FSWC, Zn reduced the Cd concentration in Fe-MnOx, Car and Om forms and increased the WsEx but had no significant effect on the Res form. Changes in the Cd chemical forms under the influence of Zn in both soils followed a similar trend. In the FCSWC, Zn reduced the Cd concentration Car and Om forms and increased the Cd concentration in the Fe-MnOx and WsEx forms while had no significant effect on Res form in the sandy clay loam soil. In the clay soil adding Zn reduced the Cd concentration in Car and Om fractions and increased the Fe-MnOx and Res forms while has no significant effect on WsEx form. The competitive transport and adsorption Interactions between these two ions caused the changing in the Cd concentration in its chemical forms. Zn reduces the Cd concentration in the forms which are easily released into the soil solution from where they can be absorbed by plants. The power function kinetic mode is the best fitted model which can describe the Cd adsorption in our soil samples. The clay and organic compounds control the Cd adsorption in soils. The higher rate of Cd adsorption in almost all shaking times shows that Cd has more ability to occupy the adsorption sites in soils.     

Analysis of iron chelates in commercial iron fertilizers by gel chromatography

A gel chromatographic method for the quality control of iron chelate fertilizers is described. The iron chelates are separated on a column of Sephadex G-10 and the eluates are analysed for iron. Using a sample quantity of 25 mg in a volume of 5 ml water and eluting with 0.15 M soidum chloride solution, a separation was achieved of commercial products of Fe-EDDHA or Fe-EDDHMA. The chromatographic analysis of Fe-EDTA or Fe-DTPA calls for a better resolution. This was obtained by decreasing the sample quantity and eluting with 0.035 M calcium chloride solution of pH 7.0. In this way it is possible to eliminate the interference of iron chelates of moderate stability which can be present in commercial products containing Fe-EDTA or Fe-DTPA.     

Determination of organic carbon in soils and fertilizers

Abstract

Many methods have been proposed to determine the total amount of organic carbon in soil; some of them determine only a percentage of the actual carbon content and therefore the results need the application of a correction factor. Methods for the determination of organic carbon in soil have been rarely extended to organic fertilizers and amendments.

We propose a rapid method based on a modification of the original Springer and Klee¹⁰procedure. Samples are oxidized for exactly 10 minutes with a mixture of 20 ml 2N K₂Cr₂O₇and 26 ml conc. H₂SO₄at 160±2°C; the excess dichromate is titrated either potentiometrically or manually with FeSO₄.

The method is fast, accurate, and more reliable than other commonly used procedures. The procedure can be easily adopted for serial determination of carbon in both soils and organic fertilizers or amendments     

Nitrogenous fertilizers and earthworm populations in agricultural soils

The influence of various inorganic and organic fertilizers was assessed in three long-term “classical” experiments and two short-term field experiments, one on grass and one on wheat. The long-term experiments included Broadbalk which had grown continuous wheat since 1843, Barnfield, continuous root crops since 1843 and Park Grass, continuous grass since 1836. Annual fertilizer treatments were farmyard manure (48 and 96 kg N ha^?1), various forms of inorganic nitrogen (48, 96, 144 and 192 kg N ha^?1), liquid and solid sewage sludge and sewage cake in a wide range of doses.In the three arable experiments, all species of earthworms were more numerous in plots treated with organic fertilizers than in untreated plots.There was a strong positive correlation (r = 0.9825) between amounts of inorganic N applied and populations of earthworms. Plots receiving both inorganic and organic N had the largest populations of earthworms.The effects of both inorganic and organic N were much less on earthworm populations in grassland than on those in arable crops, even in the long-term experiments, and there was some evidence of adverse effects when an excessive amount of liquid sludge was applied in a single dose.Effects of organic fertilizers were greater on populations of Lumbricus terrestris than on those of Allolobophora longa, A. caliginosa or A. chorotica.     

Cadmium sorption by two acid soils as affected by clearing and cultivation

Abstract

The objective of this study was to determine the effect of clearing and cultivation on the sorption of cadmium (Cd) by two acid soils from Zimbabwe with differing cultivation stories. In their original state, not cleared‐not cultivated (virgin soils), the two soils exhibited noticeable and similar capacities to sorb Cd. The Mazowe soil contains the highest level of organic matter (40 g kg^‐1) and a effective cation exchange capacity (ECEC) of 144 mmol_c kg^‐1. Yet, Bulawayo soil (23.5 g kg^‐1 organic matter and ECEC of 146 mmol_c kg^‐1) has higher pH and Mn and Fe oxide content and these characteristics seemed to counteract the effect of lower organic matter. After 50 years of cultivation, The Mazowe soil has lost 60% of its organic matter and ECEC, and consequently the ability of its soil matrix to bind Cd has proportionally decreased. In Bulawayo (cleared in 1983 and first ploughed in 1984), on the contrary, the organic matter and ECEC of the cultivated soil remains over 95% of the values on its virgin counterpart. In this soil, the retaining ability for Cd has not still been affected. In the two soils Cd sorption was highly pH‐dependent. The extent of sorption was minimal under acidic conditions and increased sharply as the pH was raised. The immediate reversibility of the sorption process proved to be very low. When sorption and desorption data were compared it was clear that soil characteristics like high organic matter and oxide content which showed to enhanced Cd sorption, contributed at the same time to slow down the backward reaction.