Effect of Elevated Levels of Arsenic on Phosphorus and Sulfur Contents of Soil
1. Introduction
Phosphorus is a non-renewable source and a major plant nutrient for crop yield. Many soil constituents react with phosphorus to convert it into unavailable forms and the dominance of individual fraction is largely controlled by soil properties. Phosphorus is an essential element for plant growth and it has long been recognized as necessary to maintain profitable crop production. It occupies a key place among the major nutrients because of its relative scarcity among the light elements and its essential role in energy transformations in all forms of life. It is needed most by the young fast growing tissues and performs a number of functions related to growth, development, photosynthesis and utilization of carbohydrate. It is a constituent of genetic materials of all living organisms. The inorganic phosphates in soils have been classified into easily soluble phosphate (ES-P), aluminum phosphates (Al-P), iron phosphates (Fe-P), reductant soluble phosphates (Rs-P) and calcium phosphates (Ca-P) (Chang and Jackson, 1957). Phosphorus in adsorbed and/or minerals forms is often coated (occluded) by relatively insoluble oxides and hydroxides of Fe and Al.
Soil Phosphorus is an increasingly important consideration in the development of phosphorus based nutrient management strategies (Daniels el al., 2000). The significant role of phosphorus in sustaining and build up of land fertility particularly under intensive system of agriculture has been amply demonstrate from the results of large number of studies carried out all over the world.
The Phosphorus cycle in soil is a dynamic system involving soils, plants and micro-organisms. Major processes include uptake of soil by plants, recycling through return of plant and animals residues, biological turnover through mineralization-immobilization, fixation reactions at day and oxide surfaces and solubilization of mineral phosphates through the activities of micro- organisms. In natural state, essentially all the phosphorus consumed by plants is returned to the soil in plant and animal residues, under cultivation, some phosphorus is removed in the harvest and only part is returned. Losses of soil phosphorus occur through leading and erosion. Phosphorus is needed or favorable for seed formation, root development, strength of straw in cereal corps and crop maturity.
Plant contain about the same amounts of sulfur as phosphorus, the usual range being 0.2 to 0.5% on a dry weight basis. Sulfur occurs in soils in both organic and inorganic forms. The amounts of sulfur in the two forms vary widely, depending on the nature of the soil (drainage status, organic matter content, mineralogical composition) and depth in the profile. Sulfur is one of the essential elements required by all forms of life. In plants, among the sulfur compounds are gluta-thionine, thiamine, vitamin B, biotin, ferredoxin, coenzyme A, and the sulfur amino acids (Cysteine, Cystine and methionine). Sulfur has long been known as secondary element, but it ranks in importance with nitrogen and phosphorus in formation of protein (Sulfur Institute, 1967).
Sulfur cycling in soil is closely related to organic turnover. Decomposition of biological residues returns some sulfur back to the mineral. Sulfur can be added to the soil in fertilizers, in irrigation water, through adsorption of sulfur gases, in additional organic matter, dry deposition and in rainfall. The main sulfur bearing minerals in rocks and soils are gypsum (CaSO4.2H2O), epsomite (MgSO4.7H2O), pyrite (FeS2) etc.
Arsenic when withdrawal with groundwater and release on the soil surface, the surface soil which is very much important for growing plants, crops etc contaminated. It has been reported that phosphate and arsenical compounds have the same fixation sites in soil. Infect arsenic also interact with other essential nutrients, such as sulfur. These interactions might be antagonistic or synergistic (Jacobes et al., 1970).
It is very important to determine the interaction of arsenic with essential nutrients (Phosphorus, sulfur) in soils, which are very important for the growth of the crops.
Arsenic toxicity can be decreased or increased by interaction with phosphorus, and sulfur, consequently the availability phosphorus and sulfur can be decreased or increased by interacting with arsenic. The following objectives are investigate
a. to see the effect of elevated levels of arsenic on phosphorus availability in soil.
b. to see the effect of elevated levels of arsenic on sulfur availability in soil.
c. to find a relationship of arsenic effect on phosphorus and sulfur nutrients in soil.
2. Review of Literature
Phosphorus is a second demandable major or primary element after nitrogen. Phosphorus has more widespread influence on both natural and agricultural ecosystems than any other essential element. The concentration of phosphorus in the soil solution is very low. Generally ranging is 0.0001mgL-1 in very infertile soils to about 1 mgL-1 in rich, heavily fertilized soils. Plant roots absorb phosphorus dissolved in the soil solution, mainly as phosphate ions (i.e., HPO42- and H2PO4–).
Sulfur is present in soils in both inorganic and organic forms. In well leached surface soils, much of the sulfur is combined with organic matter. Soils high in free iron oxides tend to contain SO42- ions substituting for hydroxyl in basic sulfate complexes. Plant build up their sulfur-containing components primarily through the assimilation of sulfates from soil, although small amounts may be obtained through the absorption of S-amino acids or the direct assimilation of sulfur dioxide from the atmosphere.
The levels of arsenic may be much higher in soils contaminated by human activities. Soil is an important natural resource for mankind but it also serves as an important medium for the accumulation, transformation, and migration of toxicants (Pacyna, 1987). In Bangladesh about 33% of total arable land is now under irrigation facilities. The use of arsenic contaminated irrigation water may cause accumulation of arsenic in soils and plants and vegetables. It may create hazard both in soil environment and in crop quality (Imamul Huq et al., 2001a). This as contaminated ground water is being used for irrigation purpose, leaving a risk of soil accumulation of this toxic element and eventual exposure to the food chain through plant uptake and animal consumption (Imamul Huq et al., 2005a).
Arsenic is present in soils because arsenic compounds have been used extensively as pesticides, herbicides and fungicides. It can cause toxic effects to plants or may accumulate in plants and thereby enter the animal and human food chain. The total amount of arsenic in soil and its chemical forms have an important influence on plant growth and animal and human health.
2.1. Nature in phosphorus
The influence of organic matter of phosphorus supply to plants is somewhat more difficult to evaluate than in the case of nitrogen since plants derive a significant proportion of their phosphorus from inorganic sources. Thompson et al., (1954) observed that the quantity of organic-P mineralization was correlated positively with release of organic-C, organic-N and with soil. Herron et al., (1965) reported a substantial increase in available phosphorus in the manure soil. Halstead and Sowden (1968) the effect of added straw, alfalfa, leaves and manure on the phosphorus content of the soil and found that the addition of manure increased the soluble phosphorus as well as the total and inorganic phosphorus contents of the soils.
The decomposition of added organic matters effect the available phosphorus content of soil not only through the release of phosphorus they contained but also enhanced the solubilization and mobilization of native soil phosphorous. It has been found that the addition of inorganic phosphorus increased the rate of release of available phosphorus present in organic form.
Sinha (1975) commented that for the enhancement of biological mobilization of mineral phosphates, application of readily available sources of energy to the native soil micro-organisms is greater significance than seed or soil inoculation with specific micro-organisms, dissolving mineral phosphates. The added organic materials influence the availability of phosphorus in soil in many other ways. Flaig (1977) reported that the effect of organic fertilizers and soil organic matter on the availability of phosphate is mainly due to the complex formation of di or trivalent cat ions which inhibited the formation of non-available forms of inorganic phosphates. The products of organic decay, such as organic acids (Struthers and Sieling, 1950; Buckman and Brad, 1971) and humus (Swenson et al., 1945; Graham, 1955; Okuda and Hori 1957; Williams, 1960)phenolics (Appelt et al.,1975) and several sugars (Bradley and Sieting, 1953) prevent the precipitation of phosphates by iron and aluminum and thereby increased the availability of phosphates.
Islam and Islam (1973) reported thatphosphate concentration under submerged condition first increased, reached maximum and then decreased. The increase in solubility of water-soluble phosphorus may be attributed to (a) release of phosphorus from organic matter, (b) increase in solubility of calcium phosphates associated with the decrease in pH caused by accumulation of carbon dioxide in the calcareous soils (c) displacement of phosphate from ferric and aluminum phosphates by organic anions (Ponnamperuma, 1964).
2.2. Forms of Phosphorus in soil
Both inorganic and organic forms of phosphorus occur in soils, and both are important to plants as sources of this element.
2.2.1. Organic phosphorus in soils
The Three groups are:
a) Inositol phosphates or phosphate esters of a sugar like compound, inositol [C6H6(OH)6]
b) Nucleic acids and
c) Phospholipids.
Inositol phosphates are the most abundant of the known organic phosphorus compounds, making up 10 to 50% of the total organic phosphorus (Brady, 1984).
2.2.1.1. Mineralization of Organic-P
Phosphorus held in organic form can be mineralized and immobilized by the same general processes that release nitrogen and sulfur from soil organic matter.
? Immobilization ?
Organic P forms H2P Fe3+, Al3+, Ca2+ Fe, Al, Ca phosphates
Soluble insoluble fixed P
phosphate
? Mineralization ??
(Brady, 1984)
2.2.2. Inorganic phosphorus in soils
Two phenomena tend to control the concentration of phosphorus in the soil solution and the movement of phosphorus in soils:
(a) the solubility of phosphorus-containing minerals; and
(b) the fixation of adsorption of phosphate ions on the surface of soil particles.
Most inorganic phosphorus compounds in soils fall into one of two groups:
(1) those containing calcium and
(2) those containing iron and aluminum (and less frequently, manganese)
(Brady, 1984)
These reaction are given below –
1. Ca (H2PO4)2.H2O+ 2H2O 2(Ca HPO4. 2H2O) + CO2? Ca3(PO4)2+ CO2??5H2O
2. Al3++H2PO4– + 2H2O = 2H+ + Al(OH)2H2PO4.
2.2.2.1. Content of inorganic phosphorus in soils
The phosphorus content of soils in their natural varies considerable, depending on the nature of the parent materials, degree of weathering and extent to which phosphorus has been lost through leaching. The usual range of phosphorus in soils is the orders of 500 to 800 ug/g (dry weight basis). Of the inorganic forms, large quantities occur in minerals where the phosphorus is part of the mineral structure, as insoluble calcium, iron on aluminum phosphates. The calcium salt predominant in neutral or alkaline condition, the iron and aluminum salts in acid surrounding (Alexander, 1977). Islam and khan (1966) found that inorganic phosphorus content of Bangladesh soils ranged between 102-509ug/g and the content was related to clay and phosphorus content of soils. The distribution of inorganic in soil was found to measure the degree of weathering.
2.2.3. Transformation of phosphorus in soils
Phosphorus in most fertilizer is present largely in water soluble forms. When this water soluble phosphorus is added to soils, it dissolved in the soil water and quickly reacts with compounds of fertilizer reaction products are formed depending upon the fertilizer and soil properties (Goswami and sahrmawa, 1982). The transformation of added fertilizer phosphorus is mainly governed by the properties of the soils. The availability of phosphorus to plants is determined to no small degree by the ionic form of this element.
The availability of inorganic phosphorus is largely determined by
(a) soil pH
(b) Soluble iron, aluminum and manganese
(c) Presence of iron, aluminum and manganese containing minerals
(d) available calcium and calcium minerals
(e) amount and decomposition of organic matter and
(f) activities of microorganism.
The transformation and factor affecting availability of phosphorus in soils are:
Influence of soil texture, mineral types, aging, organic matter, moisture, temperature, liming, microorganism and practical significance of phosphorus fraction.
2.3. Forms of Sulfur in soil
Sulfur exists in two distinct categories: Inorganic and organic.
2.3.1. Inorganic-S in soils
Inorganic-S is generally much less abundant than is organic-S and often accounts for considerably less than 25 percent of the total S content in most agricultural soils(Burns, 1967; Wainwrignt, 1984). In terms of microbial oxidative and reductive reactions, the most important forms of-S are sulphide (S2-), thiosulphate (S2O32-) sulphite (SO32-), sulphate (SO42-) and the polythionates including possibly di-thionate (S2O52-).
2.3.2. Organic-S in soils
Most of the sulfur in surface horizons of well-drained agricultural soils of humid, semiarid, temperate and subtropical regions is present in organic forms. It is generally agreed that sulfur associated with these organic compounds accounts for well over 90 percent of the total sulfur in most non-calcareous surface soils (Tisdale et al., 1985). The nature and properties of the organic-S fraction in soils are important since they govern the release of plant available (Bettany and Steward, 1982).
(i) HI-reducible S- fraction
The HI-reducible sulfur fraction contains sulfur compounds that are not directly bonded to carbon and are believed to be largely in the forms of sulphate esters and ethers with C-O-S linkage (Tisdale et al., 1985).
(ii) C-Bonded S- fraction
Sulfur directly bonded to carbon is another major component of organic-S. It is determined by reduction to sulfide with Raney nickel (Lowe and DeLong, 1963).
(iii) Residual or inert- S
Organic sulfur that is not reduced by either hydriodic acid or Raney nickel is considered to be inert or residual. The presence of this fraction of soil organic-S was first suggested by Lowe (1964), and later supported by results from other studies which indicate that this fraction accounts for between 3 and 59 per cent of the total organic-S in mineral soil (Biederbeck, 1978).
2.4. Transformations of organic sulfur in soil
Soil sulfur can be transformed form inorganic to organic (immobilization) or in the reverse direction (mineralization). These procedures are essentially microbiological and then are influenced by those factors that affect the growth of microorganisms. The forward and reverse process occurs concurrently, so that what can be observed in soils is the balance of the two. Without losing sight of the fact that such transformations are occurring, it is easier to comprehend the effects of soil forming processes if we consider the organic and inorganic forms of sulfur separately.
Most of the sulfur in agricultural soils is present in complex organic form and cannot be directly assimilated by crops or pasture plants. The release in inorganic form (mineralization) and the subsequent availability of this sulfur seems to depend mainly on the dynamics of the soil microorganisms, which is governed by the availability of energy sources in soil (Willams, 1964).
2.5. Mineralization
Mineralization can be defined as the conversion of an element from organic state to more mobile inorganic state. When sulfur is the element of interest, it is called sulfur mineralization.
McGill and Cole (1981) hypothesized that sulfur may be mineralized by two related but independent processes. Biological mineralization, defined as “release of organic forms of sulfur from organic materials during oxidation of carbon by soil organisms to provide energy, and is driven by the search for energy” (McGill and Cole, 1981).
Biochemical mineralization defined as “release of inorganic ions of sulfur from organic form through enzymatic catalysis external to the cell membrane, and is strongly controlled by the supply of and need for the element released rather than the need for energy. The net mineralization of sulfur is then the result of the combination of the two mechanisms (McGill and Cole, 1981).
During this process organic sulfur is mineralized; some is used for the synthesis of new cell material (mineralization) and not required for synthesis is released as inorganic sulfur. Immobilization and mineralization occur simultaneously in soils wherever organic debris is undergoing microbiological decomposition (Maynard, 1982).
2.6. Formation of sulfate
Because the opposing reactions immobilization and mineralization occur simultaneously, differing patterns of inorganic SO42- release would be expected, depending on the availability of energy materials for microorganisms. The patterns which have been observed are:
I. Immobilization of sulfur during the initial stages of the study followed by SO42- release (Barrow, 1961; Nelson, 1964; Haque and Walmsley, 1972; Nor, 1981).
II. A steady, linear release of SO42- over the whole period of study (Willams, 1967; Tabatabai and Al-Khafaji, 1980).
III. A rapid release of SO42- during the first few days followed by a slower linear release (William, 1967; Nor, 1981).
IV. A rate of release which decreases with time (Williams, 1967).
V.
The pattern of SO42- release has not yet been related to any specific soil property, but may be caused by adjustment of the microbial population to the incubation conditions imposed as well as to the initial levels of available substrates.
2.7. Factors affecting of Sulfate chemistry in soil
It is believed that the conversion of organic sulfur to inorganic sulfate is mainly carried out by soil micro-organisms (Alexander, 1961) and thus any variable which affects the growth of micro-organisms will affect the mineralization of sulfur. Thus temperature, moisture, pH (Chaudry and Cornfield, 1967; Williams, 1967) and availability of food supply (Barrow, 1960a, b, c) have all been shown to affect the mineralization of sulfur (Williams, 1967)
Addition of calcium carbonate increased the amounts of sulfur mineralized. With three soils the amount of sulfur mineralized was directly proportional to pH up to a value of 7.5. With three other soils mineralization was proportional to the amount of calcium carbonate added and not to the pH attained (Williams, 1967).
It appears that addition of CaCO3 to soils may affect the inorganic SO42- concentration in a number of ways:
I. Sulfate may be released from organic-S compounds by microorganisms growing better in more favorable pH environment.
II. Sulfate may be released from soil organic matter by chemical hydrolysis under alkaline conditions (Barrow, 1960b).
III. Adsorbed SO42- may be desorbed from soil exchange sites; because of the increased pH (Chao et al., 1964; William and Steinbergs, 1962).
IV. Sulfate may be added in the CaCO3 (Williams and Steinberge, 1962, 1964).
Better predictions may be obtained when the main sulfur compounds in soil are identified and their turnover rates are determined. Freney and Spencer (1960)showed that mineralization of sulfur in the presence of plants was greater than that which occurred in the unknown soil ( Cowling and Jones, 1970 ; Nicolson, 1970 ) and suggested that the increased mineralization may have been due to the greater proliferation of organisms under plants.
2.8. Arsenic
Arsenic (As) occur in both solids and liquids states and is found in mineral such as arsenopyrites (FeAsS2), realger (As2S2), orpiment (As2S2), and arsenolite (As2O2). Arsenic is obtained primarily as a by product from smelting Copper, Lead, Zinc, Gold and other oars. Soil as a primary recipient, can accumulate arsenical compounds from different sources and once these material enter the soil, they become part of a cycle that affects all forms of life. Higher arsenic accumulations are generally associated with the following environments (Khan, 1997) (a) Alluvial deposits particularly: in semiarid areas, (b) Geothermal system (c) Copper and lead smelting and d) Uranium and gold mining. Hence arsenic can originate in soil and other environments from both natural and anthropogenic sources. Another source of arsenic may be aquifer recharge by concentrated irrigation water (Hussain, 1997).
Arsenic has been recognized as a toxic and carcinogen. The toxicity of arsenic depends mainly on the chemical and physical forms of arsenic, its concentrations and duration of exposure. Arsenic present in soil, they are arsenite [As (III)], arsenate [As (v)], monomethyl arsenic acid (MMAA) and dimethyl arsenic acid (DMAA) (Marin et al., 1992). Inorganic arsenics have been found to be more toxic than the organics, while arsenite is four times more hazardous than arsenate (Khan, 1997).
Human beings are affected with arsenic hazard mainly through contaminated drinking water and foods. Arsenic has been found to be carcinogenic and teratogenic and chronic poisoning of arsenic may cause a number of diseases.
The different retention levels of As (III) and As (V) may also explain the differences in their toxicities. Trivalent As is significantly more toxic than its metabolities, As (v), or the organic arsenicals. The ability of arsenic to inhibit ATP production suggested that the organ functions will increase rapidly, especially in the event of acute poisoning. Cases of skin cancer, hyper-pigmentation and keratosis have predominated in areas where the drinking water contains 0.8.26 mgL-1 As (Chen et al., 1985).
WHO (1995) set the maximum permissible limit for arsenic concentration in drinking water at 0.01 mgL-1. The set limit value for arsenic contents in fruit, crops and vegetables is 2.6 mg/kg As fresh weight (US public health service). The average human intake is about 300 mg day, but may be higher, if fish is a large of part of diet. Human skin tissues are known to contain 0.12 mgL-1 As, while in nail and hair, its concentrations are 0.36 and 0.65 mg/g respectively (Weekly Jai Jai Din, 26.11.96).
2.9. General properties of Arsenic
Arsenic (atomic no. 33) is a stel-gray, brittle, crystalline metalloid with three allotropic forms that are yellow, black, and gray. It tarnishes in air and when heated is rapidly oxidized to arsenous oxide (As2O3) with the odor of garlic. It belongs to group V-A, has an atomic weight of 74.922, and closely resembles to phosphorus chemically. Gray As, the ordinary state form, has a density of 5.73 g/cm3, a melting point of 817?C, and sublimes at 613?C. The more common oxidation states available to arsenic are- 0, III, and V. Arsenic compounds compete with their phosphorus analogs for chemical binding sites. Arsenic bonds covalently with most non metals and metals and forms stable organic compounds in both its trivalent and pentavalent states. The most important compound are white As (As2O3), Paris Green [3 Cu (As2O2)2. Cu (C2H3O2)], calcium arsenate, and lead arsenate, the last three being use as agricultural pesticides and poisons (Buat-Menard et al.,1987).
Arsenic in the natural environment occurs in soils at an average concentration of about 5 to 6 mg/kg, but this varies among geographic regions (Peterson et al., 1981). Soils close to or derived from sulfide ore deposits may contain up to 8000 mg/kg As. Levander
(1977) reported that the association of arsenic with sulfur results in relatively high concentrations of arsenic in mud, gases, ground-water, and soils adjacent to geothermal eruptions (Lancaster et al., 1971).
2.10. Sources of Arsenic in the Environment
Arsenic in soil may originate from the materials that form the soil and from industrial waste discharge or agricultural use of pesticides (Tanake, 1988).
2.10.1. Natural sources
Arsenic is found in naturals reservoirs, such as ocean soil and atmosphere. The concentration of arsenic in naturals reservoirs should that more than 99% of the total arsenic in the environment is present in rocks ranging from 0.5 to 2.5mg/kg. It is a major constituent of more than 245 minerals and is found in high concentration in sulfide deposits: arsenides (27 minerals), sulfides (13 minerals) sulfosalts (65 minerals) and others (Adriano, 1986). Arsenic transport from continents to the ocean arises from natural process, such as weathering and volcanism. About 45000 metric tons of arsenic is weathered per years 73% of which is in dissolved form (Fergus and Gavis 1972). According to Chilvers and Peterson (1987), volcanic activity and low temperature volatilization (biological methylation) are the two dominant natural sources. From their study, it appeared that nearly 60% of the total natural flux comes from low temperature volatilization and the remainder from volcanoes. They found a low temperature volatilization to be 26200 tons As/year, with volcanoes, on average contributing 17150 tons/year As and arsenic emissions from natural sources of 45480 tons As (Livesey et al., 1981).
According to Chilvers and Peterson (1987), volcanic sources includes explosive volcanic activity and continuous emission, magma degassing, fumarolic activity, geothermal activity and dissolution of minerals within the volcanic rocks e.g. arsenopyrite orpirnent, realgar.
2.10.2. Anthropogenic sources
Another source of arsenic in soil, water and atmosphere is due to human activities. At least 75% of the global atmospheric arsenic has been reported as anthropogenic and elevated concentration in water result from anthropogenic activities (Nordstrom, 1998). Soil contaminated with arsenic from anthropogenic sources has increased as a result of the following factors (Brady, 1984; Buat-Menard et al., 1987; Chilvers and Peterson, 1987).
a) Mining and smelting of metal, e.g. Copper, Lead and Zinc.
b) Coal combustion
c) Insecticides, extensively used on cotton, tobacco and fruit crops, e.g. lead arsenate.
d) Weed killers, e.g. sodium arsenate
e) Cattle dip, e.g. arsenic trioxide
f) Feed additives e.g. arsanillic acid
g) Wood preservation e.g. arsanillic acid
h) Agricultural burning
i) Industrial wastes, e.g. wastes from industries of glass and ceramics cements, pigments, enamels, antifouling, paints iron and steel production, textiles and fire works.
j) Sewage sludge (Epstein, 1962).
2.11. Use of arsenic
Arsenic is used in lead and copper alloys to increase the strength at elevated temperatures. The primary commercial sources of arsenic are copper lead ores. Arsenic compound are use as insecticides, herbicides, fungicides, algaecides, wood preservatives etc. All naturally occurring arsenic consists of the stable isotope arsenic-75; the radioactive isotopes arsenic- 72, 74 and 76 have been used in medicals diagonistic procedure. Arsenic oxides are used as decolorized in the manufacture of glass, minerals, ornament etc. Some uses ofarsenic are listed in the below (Nriagu, 1994).
Table-2.1: Uses of arsenic in various sectors
| Sector | Uses |
| Agriculture | Pesticides, insecticides, wood preservatives, soil disinfectants |
| Livestock | Feed additives, disease prevention, heart warm infections, cattle and sheep dips etc |
| Medicine | Antisyphilic drugs, treatment of trypanosomiasis |
| Electronic | Solar cells, opto- electronic devices |
| Industry | Glassware, catalysts, ceramics |
| Metallurgy | Alloys e. g. radiators, automotive body solder. |
Table-2.2: Major arsenic chemical species evolved during fossil fuel combustion and industrial processes (Pacyna, 1987)
| Process | As species |
| Coal combustion | As2O3, As2S3 |
| Oil combustion | As(O), As2O3, Organic arsines |
| Non-ferrous metal production | As2O3 |
| Refuse incineration | As(O), As2O3, AsCl3 |
2.12. Arsenic Concentrations in Natural Soils
Under reducing conditions, arsenate dominates in soils, but elemental arsenic can also be present (Walsh and Keeney, 1975). Arsenic would be present in well-drained soils as H2AsO41- if the soil is acidic or as HAsO42- if the soil is alkaline.
Typical amounts of arsenic in natural uncontaminated soils varied from 5 to 6 mg/kg in Austria (Aichberger and Hofer, 1989; Horvath and Moller, 1980) up to 11 mg/kg in the Netherlands (Wiersma et al., 1985) and Canada (MacLean and Langille, 1981). Also under flooded conditions As (III) would dominate whereas aerobic conditions would favor the oxidation of As (III) to As (V) (Haswell et al., 1985). Total arsenic in natural soils was evenly distributed with depth (Aichberger and Hofer, 1989). Soils derived from shales and granites have been shown to have elevated arsenic concentrations of up to 250 mg/kg (Colbourn et al., 1975) and Australian soils derived from quartzite contained from 100 to 200 mg/kg, resulting in reduced growth and toxicity symptoms on leaves of banana palms (Fergus, 1955).
2.13. Forms of Arsenic in Soils
In soils, arsenic forms solids with Fe, Al, Ca, Mg, and Ni; however, there are no arsenic solids, other than As2 S3, than have solubilites <0.05 mg/l (Gupta and Chen, 1978). Retardation of arsenic movement in soils is related to the concentrations of phosphate present from fertilizers or wastes disposed on land, but in not related to variations in concentrations of Cl–, NO3– or SO42-(Livesey and Huang, 1981). Soluble arsenic concentrations are usually controlled by redox conditions, pH, biological activity, and adsorption reactions, but not by solubility equilibrium. In both soil and water systems, arsenic species are subject both chemically and microbiologically to oxidation and reduction. At high Eh values, As (V) exists as H3AsO4– ,H2AsO42- and AsO4, whereas at low Eh values, the corresponding As (III) species is present along with AsS2–. Soil components that contribute to sorption and retention of arsenic are oxides of Al, Fe, and Mn, soil mineralogy and organic matter (Pacyna, 1987).
2.14. Speciation of Arsenic in the Soil Environment
Arsenic (III): Arsenite [As (III)], the reduced state of inorganic arsenic is a toxic pollutant in natural environments. It is much more toxic (Ferguson and Gavis, 1974; Webb, 1976) and more soluble and mobile (Deuel and Swoboda, 1972b) than the oxidized state of inorganic arsenic, arsenate [As (V)].
Arsenic (V): Arsenate can be adsorbed onto clays, especially kaolinite and montmorillonite (Frost and Griffin, 1977). In a montmorillonitic, calcareous clay, arsenate was highly adsorbed onto kaolinite and montmorillonite at low pH with a maximum near pH 5.0 and became less adsorbed at high pH (Goldberg and Glaubig, 1988). Adsorption of As (V) by calcite increased from pH to 10, peaked at pH 10 to 12, and decreased above pH 12 (Kitagishi et al., 1981)
Organic arsenic: A ubiquitous, volatile, arsenic compound, dimethyl- arsinic acid (cacodylic acid) seems to be present in all soils and may dominate in many (Braman, 1975).
2.15. Arsenic Concentrations in Arsenic-Contaminated Soils
Arsenic has been added to soils in many ways, but about 41% has come from commercial product wastes of the total arsenic added to soils, about 23% comes from coal fly ash and bottom ash, 14% from atmospheric fallout, 10% from mine tailings, 7% from smelter, 3% from agriculture, and 2% from manufacturing, urban, and forestry wastes (Nriagu and Pacyna, 1988).
2.16. Behaviour of arsenic in soil
The chemistry of arsenic in an aqueous or soil system is complex and knowledge of arsenic recycling in the environment is limited. Once entered in a soil system, arsenic may precipitate in the form of insoluble arsenic minerals, be adsorbed on the soil exchange sites or be adsorbed by the plants (Sadiq, 1967).
Arsenic is found in soil both in inorganic and organic forms. Arsenic is subjected to chemically and/or microbiologically mediate oxidation-reduction and methylation (Brannon, 1983) reactions in soils. Both arsenic solubility (Massehelehn et al., 1991) and (Clements and Minson, 1974) toxicity to plants and animals depend on its chemical form. It has been suggested that arsenic behaves in soils very much like phosphorous. For the absorption solubility relationships it has also been suggested that arsenic solubility in soil solution depends on clay content and soil pH. Bradly (1948) pointed out that most of the applied arsenic is relatively unavailable for plant uptake. Being present in anionic form (e.g. arsenite AsO2 and arsenate AsO43-) arsenic is adsorbed by hydrous iron and aluminum oxide, especially in acid soils. This adsorbed arsenic is replaceable from these oxides by phosphate through the process of anion exchange. In calcareous soils the soluble forms of arsenic are found as H3AsO4, H2AsO4-, HAsO42- and AsO43- under normal oxidation reduction conditions.
Literatures suggest that arsenic does accumulate mainly in the surface layers of soil and it is not readily subjected to leaching though it can migrate into the ground water system through soil layers and contaminate water (Bignoli and Sabbioni, 1984). In an orchard soil treated with lead arsenate pesticide that the As concentration of the soils were in the order of 50-330 ?g/g. Arsenic concentrations decreased with increasing depth with maximum concentrations were found in the upper 5cm. This finding suggests that arsenic does not readily leach from surface layers of soil (Misra and Mani, 1991).
2.17. Levels of arsenic in soil and plant
Arsenic occurs in almost all soils in small amount (Misra and Shukla, 1990) and though there are reasonably heavy applications of arsenical pesticides over a period of years especially to orchard soils only in few cases, arsenic accumulation in soils reach to toxic levels. According to Misra and Mani (1991), arsenic concentration in soils average 5-6 mg/kg soil, but can vary considerably according to region with some Dantmoor soils reaching 250 mg/kg arsenic soil. Soils in the vicinity of industries and arsenic contaminated materials can also substantially elevated 5-6 mg/kg level up to 2500 mg/kg soil due to mining/smelting, 21-42 mg/kg arsenic soil due to pesticide use and 9-58 mg/kg arsenic soil due to wood preservatives.
Through the biogeochemical and biochemical pathways arsenic enters the living biota. It was suggested that, because of the so-called soil/plant barrier effect, elevated arsenic concentration in soils may well reduce crop production substantially before enhanced food chain accumulation occurred (Chaney, 1984). Moreover, the food chain can be affected directly through ingested soil which may also contain significant levels of arsenic contamination (Buat-Menard et al., 1987).
Poisonings from arsenic contaminated well water have occurred in many countries on a small scale (Buat-Menard, 1987). Hutton (1987) suggested that though the biological half lives of some arsenic compounds are relatively short and thus suggest no age dependent accumulation in man, inorganic arsenic does accumulate in hair, skin and nails. The most characteristics effects following chronic arsenic exposure are hyperkeratosis of the palm and sobs of the feet together with hyperpigmentation, particularly in areas not exposed to the sun. Skin tumours have also been commonly reported. Other severe effects of arsenic contamination in humans are haemangioendothelioma of the liver, peripheral vascular disturbance resulting in gangrene and disease termed ‘Black foot disease’(Tseng, 1977), Lung cancer (pershagen et al., 1977; Nordostom et al., 1978 Gastro intestinal disturbance and the symptoms shown by the arsenic affected patients can be grouped into two stages:
1. Pre- clinical stages: In this stage, patients do not show sign and symptoms.
2. Clinical stages: It includes three sub- stages.
(i) Primary stage: Rain drop pigmentation shown in the body and leucomelarnosis.
(ii) Second stage:
(i) Keratosis (ii) Bronchitis (iii) Neural disease
(iii) Third stage:
(i) kidney problem (ii) Liver problem (iii) Skin cancer
Source: Sangbad August 31, 1997.
Table-2.3: Effects on health in environmentally exposed populations attributed to arsenic (Hutton, 1987):
| Organ affected | Effects |
| Skin | Hyper pigmentation
Hyper keratosis Skin tumours |
| Lung | Lung cancer |
| Cardio vascular system | Peripheral vascular disturbance
Leading to gangrene |
| Nervous system | Peripheral neuropathy
Hearing defects |
| Haemotopoietic system | Disturbed erythropoiesis anaemia |
| Reproductive system | `Increased frequency of spontaneous abortions |
2.18. Effect of arsenic on plant growth
Arsenic is not essential for plants growth and appears not to be involved in specific metabolic reaction when supplied at low concentrations (Liebig, 1966; Marin, 1989). A higher concentration, however, arsenic has been reported to interfere with metabolic processes and to inhibit plant growth, sometimes leading to death (Baker et al., 1976; Marin et al., 1992; Reed and stugis, 1936; Schweizer, 1967).
Concentration of arsenic in plants grown on uncontaminated soils vary from 0.009 to 1.5 mg/kg (in the weight basis), with leafy vegetables being in the upper range and fruits in the lower (Kebata- pendias and pendias, 1985).
Several reports on the linear relationship between arsenic content of vegetation and concentrations in soil of both total and soluble arsenic suggest that plants take up arsenic passively with the water flow. Thorsby and Thornton (1979) described the ready uptake of arsenic by plant species apparently. Arsenic is Tran located in plants since its concentration in the gain also has been reported. With increasing soil arsenic, however, the highest arsenic concentrations were always recorded in old leaves and in roots
(Kabata-pendias and Pendias, 1985).
The toxicity effect of arsenic was partly reduced after three years of cultivation without any special treatment, but an application of sulfur greatly reduced the phytotoxicity of arsenic.
Kapustka et al., (1995) studied the toxic effects of arsenic on the growth of alfalfa lettuce and wheat in impacted area soils and found a substantial toxicity in plants.
2.19. Phyto-toxicity of Arsenic
Phyto-toxicity symptoms include witting of new cycle leaves, followed by retardation of root and top growth of plant (Liebig, 1965). It is often accompanied by root discoloration and necrosis of leaf tips and margins. In rice plants, tillering is severely depressed, as in the phosphorus deficiency (Chino, 1981). These symptoms indicate restriction in the movement of water into the plant which may result in death (Woolson et al., 1971). Comparative sensitivity to as arsenic of various plants (Benson and Reisenaver, 1951; Liebig, 1965) is shown below:
Tolerant Moderately tolerant Low tolerant
Apples cherries peaches
Tomato Strawberries apricots
Potato vegetables and field crops Corns soybeans Corns
Cabbage Radish Rice
Carrots
2.20. Arsenic contamination in soils and groundwater of Bangladesh
At first Bangladesh Water Development Board (BWDB) and Director of Public Health and Engineering (DPHE) worked in some areas of the country mainly in Rajshahi, Kushtia, Jessore and Khulna districts. Analysis of 143 water samples was presented the concentration of arsenic and varied 0.021 to 0.388 mg/l. It showed of arsenic in the tested water samples in toxic amount.
The contamination of arsenic in groundwater was first detected in 1993 at the Barogharia Union of Chapainawabgonj district by the department of Public Health Engineering (Independent, 0.4.02.97). By now 240 water samples have shown high concentration (above 0.05 mg/l) and 176 other samples between 0.01 mg/l and 0.05 mg/l. So far 1328 samples have been analyzed.
In Ramgonj and Raipur thana of Laxmipur district water samples from 201 tube wells were analyses for arsenic (Itefaq, 03.03.97). It was fund that the tube wells had arsenic above the safe limit of WHO.
In sadder thana of Barokhada of Kushtia districted samples from tube-wells were analyzed for arsenic (Ittefaq, 03.03.97). The samples were found to have arsenic concentrations of about 0.12 m/l. In Daulatpur thana of Kushtia districts water from tube wells were analyzed for arsenic. The amount of arsenic was found 0.56 mg/l.
Although the exact reason for arsenic contamination has been established yet and whatever the sources, it is now the only a issue of public health concern. Most of the cases are with skin diseases. Patients with hyper keratosis, hyper pigmentation (brown and black pots in the sob of foot and hand palm, later they become hard) were mostly observed. Other observed diseases are gastrointestinal diseases, conjunctivitis, bronchitis etc (Khan, 1997).
2.21. Effect of Arsenic on plant Nutrient Elements in soils
Interaction of plant nutrients elements or factors occurs when the response of one or more of the elements or factors is modified by the effect of one or more of the other elements. Interaction may be positive or negative. There are not enough works in relation to the interactions between arsenic and one or more of the plant nutrient elements. It is however, evident that, arsenic shows interaction with almost all of the nutrient elements and in most cases, the effects are negative or antagonistic.
In an experiments with rice ( Oryza Sativa L.), Yamare (1989) a decreased in the nitrogen concentration in rice plant and a corresponding decreased in the vegetative growth due to arsenic application (>2.5 mg/kg).
Merry et al., (1986) also found antagonistic effects between arsenic and nitrogen in Silver beet grown on an arsenic contaminated orchard soil.
2.22. Phosphorus-Arsenic Interaction
Arsenic is chemically similar to phosphorus (Barrachina et al., 1994) and is taken up by roots in the same mechanism as phosphorous (Mcharg and Macnair, 1990). As a result in interaction between arsenic and phosphorus is very likely to occur.
Onken and Hassner (1995) investigated the species and concentrations of arsenic present in soil solution of flooded soils and correlated them to phosphorus concentration and growth rate of plants grown in As treated soils. Rice (Oryza Sativa L.) was grown in two soils for 60 days treated with 0, 5, 15, 25, 35 and 45 mg/kg arsenic soil added as either Na-arsenate or Na-arsenite. In both soils, plant phosphorus was well correlated to the amount of arsenic added rather than any arsenic in the soil solution.
Khattak et al., (1991) investigated the interaction between arsenic and phosphorus in alfalfa grown in sand culture with three levels of arsenic 0, 0.05 and 0.1 mgL-1As as Na2HAsO4 and two levels of phosphorus, 1.0 and 4.0 mg/l P as KH2PO4. Though not a very dramatic effect was found, yet a negative interaction between P and As was observed.
Yamare (1989) showed that the levels of > 2.5ppm arsenic significantly decreased the phosphorus concentration in rice plants (Oryza sativa L.).
Blatt (1990) in a sand culture experiment with cauliflower cultivars Fortuna and Idol original found that as the arsenic in solution increased, leaf phosphors decreased, though phosphorus deficiency symptoms was not observed in either cultivar even at 50 mgL-1As in solution. The antagonistic effect of arsenic with phosphorus in all plant parts of bush bean grown in solution culture has also been reported by Wallace et al., (1980).
2.23. Sulfur-Arsenic Interaction
Merry et al., 1986 point out that sulfur and arsenic exist in soil solution in similar anionic forms (e.g. sulfate and arsenate) and hence there should be a competition between these similar ions as has been reported for sulfate and selenate (Epstein, 1962). It has also been reported that the toxic effects of arsenic appear to be diminished by the presence of sulfur (Kitagishi and Yamare, 1981; Merry et al., 1986). Inspite of these findings, S-As interactions have not been investigated widely and the nature of the interaction between them has not been clearly understood (Wallace et al., 1980).
Kulich (1990) showed synergistic effect between arsenic and sulfur in a number of plants, i.e. oats, Vicia faba barley, Triofolium pratense and Medicago Sativa in pot trials. It was observed that dry matter yields of all the plants increased with the application of Arsenic+ Sulfur.
3. Materials and methods
A pot experiment was set up in the laboratory of the Department of Soil, Water and environment, University of Dhaka in 2008 to study the effect of elevated levels of arsenic on phosphorus and sulfur contents of soil.
3.1. Collection and preparation of soil sample
Soil samples representing Dhamrai series were collected from Dhaka district. The bulk of soil samples representing 0-15cm depth from the surface were collected by composite soil sampling method as suggested by the Soil Survey Staff of the USDA (1951). The samples were scraped from top to bottom with the help of an auger and mixed thoroughly. Samples were collected from the sampling sites and put in polythene bags, tagged with rubber band and labeled.
3.2. Preparation of soil sample
The collected soil samples were dried in air for three days (40oC) by spreading in a thin layer on a clean piece of paper. Visible roots and debris were removed from the soil sample and discarded. For hastening the drying process, the soil samples were exposed to sunlight. After air-drying, a portion of the larger and massive aggregates were broken by gently crushing them by wooden hammer. Ground samples were screened to pass through 2mm stainless still sieve. The sieved samples were then mixed thoroughly for making the composite sample. Soil samples were preserved in plastic containers and labeled properly showing soil number, sample number, date of collection. These soil samples were used for various physical analyses. Another portion of the soil samples was further ground and screened to pass through a 0.5mm sieve. The sieved sample where mixed thoroughly for making composite samples and preserved in the same way as described above. These soils were used for chemical and physicochemical analyses.
3.3. Experimental set-up
3.3.1. Treatment combination
The treatments of the experiment were designed as follows:
Soil only (control)
Soil + 5 ppm As (III)
Soil +20 ppm As (III)
Soil +40 ppm As (III)
Each Treatment was replicated thrice. The incubation period was 15, 30,
45 and 60days respectively.
3.3.2. Incubation tests
50gm of the composite sample was placed in pot (sized 100ml). The source were Sodium meta arsenite (NaAsO2). The dose of As [III] was 0 ppm, 5 ppm, 20 ppm, 40 ppm the background soil as recommended ( the recommended rate was 25kg/ha ) by fertilizer recommendation guide. Water was added to bring the pot soil approximately to field condition and waterlogged condition was kept in this condition throughout the incubation period at room temperature for determining the effect of elevated levels of arsenic on phosphorus and sulfur contents of soil.
3.4. Method for soil analyses
The soil samples were analyzed in the laboratory for there physical, physico-chemical and chemical properties. The following methods are as follows:
3.4.1. Physical analysis of the soil
3.4.1a. Particle size distribution
Particle size distribution of the soils were made by hydrometer method as Imamul Huq and Alam (2005) and the textural classes were determined by Marshall’s Triangular co-ordinates as Devised by the United Department of Agriculture (1951).
3.4.1b. Moisture percentage
The percentage of moisture present in the air dried soil was determined by drying know amount of soil in an electric oven at 105oC for 24 hours until constant weight was obtained and the moisture percentage was calculated from the loss of moisture from the sample as described by Imamul Huq and Alam (2005).
% moisture = (weight of air dry soil- weight of oven dry soil)100/weight of oven dry soil.
3.4.1c. Water holding capacity
Water holding capacity of the soil was determined by the method described as (Imamul Huq and Alam).
3.4.2. Chemicals analyses of the soils
3.4.2a. Soil pH
The pH of the soil was measured electrochemically using Griffin pH meter Model 40. The ratio of soil to water was 1:2.5.
3.4.2b. Soil organic carbon and organic matter
Soil organic carbons of the samples were determined by Walkley and Black’s wet oxidation method as outlined by Imamul Huq and Alam (2005). Organic matter was calculated by multiplying the percent value of organic carbon with the conventional Van-Bemmelene’s factor of 1.724 (piper, 1950).
3.4.3. Total nutrient contents of the soils
3.4.3a. Nitrogen
Total nitrogen of the soils was determined by Micro-Kjeldahl’s method following H2SO4 acid digestion as suggested by Imamul Huq and Alam (2005).
3.4.3b. Phosphorus
Total phosphorus of the samples was determined from the HNO3 and HCl (1:3 ) acid digestion. The digest was spectrophotometrically analyzed; for phosphorus using a spectrophotometer at 490nm wavelength after developing yellow color with vandomolybdate as described by Imamul Huq and Alam (2005).
3.4.3c. Arsenic
The arsenic of the soil samples was determined from the HNO3 and HCl (1:3) acid
digestion. Total arsenic was analyzed by using a Varian Spectra-220AAS.
3.4.3d. Sulfur
The total sulfur of the soil samples was wet digested with nitric acid and hydrochloric acid and was determined by turbidimetric method (Imamul Huq and Alam, 2005)
3.4.4. Available nutrient contents of the soils
3.4.4a. Nitrogen
The available nitrogen of the soil was determined by Micro- Kjeldhel distillation method using Devarda’s alloy as described by Imamul Huq and Alam (2005).
3.4.4b. Phosphorus
Available phosphorus was extracted by Bray-1(Bray and Kartz, 1945) solution and was determined by ascorbic acid blue color method. The ratio of sample and extractant was 1:5.
3.4.4c. Sulfur
Available sulfur was extracted by calcium di-hydrogen phosphate [Ca (H2PO4)2H2O] and was determined turbidemetrically (Hunt, 1980). The ratio of soil sample to extract ant was 1:5.
3.4.4d. Arsenic
Available arsenic was extracted by deionized water and analyzed by using a Varian
Spectra-220AAS. The ratio of sample and extract ant was 1:5.
3.5. Data analysis
Microsoft Excel and MINITAB programs were used for data manipulation, graphing and statistical analysis. The statistical analysis performed by ANOVA.
4. Results and Discussion
The common physical, physico-chemical and chemical properties of the soil series were analyzed and the results of the analyses are presented in the table
Table 4.1: Particle size distribution and textural class of the soil
| Properties | Values % |
| Sand | 9.75 |
| Silt | 52.12 |
| Clay | 37.14 |
| Texture | Silty clay |
Table 4.2: Various physical, physico-chemical and chemical properties
| Properties | Values |
| Total N | 0.12% |
| Total P | 0.09% |
| Total S | 0.1% |
| Available N | 0.03% |
| Available P | 10.08 ppm |
| Available S | 1.31 ppm |
| Total and available arsenic | 2.5 ppm and 0 ppm |
| Soil pH |