Health and Sustainable Agriculture
Editor: Christine Jakobsson
Sustainable Agriculture
1
Combatting Soil Degradation
Trace elements are introduced into soils from various sources, including atmospheric deposition of metal/met- alloid-bearing particles, application of sewage sludge, phosphate fertiliser, pig slurry and pesticides, where they exist in several chemical forms. Their fate in soil depends on the chemical state of the element in the contaminating material.
Risks associated with polluted soils are contamination of the food chain. They are closely related to the bioavail- ability of toxic elements (i.e. ability to enter the differ- ent compartments of the food chain) and primarily to the phytoavailability (i.e. availability to plants). Plants are essential components of natural ecosystems and agroe- cosystems, and are the first compartment of the terrestrial food chain. When grown on polluted soils they become a potential threat to human and animal health, as they may accumulate toxic elements (e.g. metals) in their tissues, as dramatically illustrated by the Itai-Itai disease that affect- ed farmers on a long-term diet of cadmium-contaminated rice. Plants may also have their growth sharply reduced by high levels of toxic elements in their tissues, causing a decrease in crop yields and further economic damage to farmers, as can be observed near metal smelters or mine spoils. On the other hand, some elements, toxic when present at high concentration in tissues, are also essen- tial to plants, and their deficiency induces loss in biomass production and physiological disorders in plants.
It is necessary to determine the pathways of transfer of trace elements from soil to plants in order to properly manage polluted soils:
Plants take up trace elements from the soil solution, where ions are in equilibrium with those located in the sol- id phase through various reactions, including adsorption, exchange, complexation with organic and inorganic lig- ands, redox reactions, and precipitation-dissolution (Zyrin et al., 1985; Morel, 1997). The extent of the reactions, and hence the solubility of trace elements, is a function of soil mineral content (e.g. silicate layers, carbonates, ox- ides and hydroxides), soil organic matter (e.g. humic and fulvic acids, polysaccharides and organic acids), soil pH, redox potential and soil temperature and humidity.
The risks of heavy metal transfer into the food chain are dependent on the mobility of the heavy metal species and their availability in the soil (Richards et al., 2000).
Different kinds of extractants are used for the extraction of the mobile forms of heavy metals. 1.0 M mineral acids extract most heavy metals and the species extracted are considered to represent a pool closely related to the total concentration, which can be mobilised potentially. Heavy metals extracted by an acetate-ammonium buffer solution characterise these mobile pools. Even more mobile is the exchangeable form of the elements extracted by neutral
Contamination of Agricultural Soils with Heavy Metals
Marina Efremova
St Petersburg State Agrarian University, St Petersburg, Russia
Alexandra Izosimova
Agro-Physical Research Institute, Pushkin, Russia
35
salts, which is also considered the most available fraction for plants (Gorbatov and Zyrin, 1987). The other forms of elements are more or less immobile. Mobilisation of metals from these forms or transformation from mobile fractions into immobile are very slow processes, which are controlled mainly by kinetic factors.
Haq et al. (1980) evaluated the effectiveness of strong and weak acids, as well as chelates, in extracting Ni and revealed the following order of effectiveness:
DTPA (Diethylenetriaminepentaacetic acid) > EDTA (Ethylenediaminetetraacetic acid) > NTA (Nitrilotriacetic acid) > CH3COOH (acetic acid) > H2O.
Obviously chemical forms do not show the real metal distribution in agrocoenosis and allow the contribution of one certain compartment to be deduced. They only reflect the combined contribution of several compartments in forming one or another metal fraction in soil. In measur- ing the metal availability for plants, it is necessary to take into account a wider range of soil properties.
Uptake and accumulation of trace elements by plants are affected by several soil factors, including pH, Eh, clay content, organic matter content, cation exchange capaci- ty, nutrient balance, concentration of other trace elements in soil, and soil moisture and temperature.
Liming is considered to be an economically acceptable measure that generally helps to reduce the transport of heavy metals into the food chain. Liming has two effects.
First it induces an increase in soil pH and supplies Ca2+. The solubility and availability/toxicity to organisms of heavy metals Cd2+, Cr3+, Fen+, Pb2+, Mnn+, Hg2+, Ni2+, and Zn2+ decreases as soil pH increases (McLaughlin, 2002).
This is due to the increase in the negative charge on vari- able charge surfaces in soil (Bolan et al., 2003). Nebolsin and Sychev (2000) and Cho and Han (1996) reported a general decrease in Ni uptake with increasing lime doses from experiments with Vicia, barley and radish plants.
Increasing pH, induced by lime, activates microbio- logical processes in the soil. Weyman-Kaczmarkowa and Pedzivilk (2000) reported that alkalinisation has a very strong stimulatory effect on bacterial growth, espe- cially in loose sandy and sandy loam soils. The micro- bial biomass increases and can accumulate considerably high amounts of certain heavy metals. On the other hand microbiological increases in the heavy metal availability are caused by microorganisms capable of reducing cer-
tain compounds (generally Mn and Fe) and also by their variable bioaccumulation of heavy metals (Kovalskiy and Letunova, 1974).
In the pH range 7.1-8.5, carbonate acts as a pH buffer.
The surfaces of calcite are reactive and various ions may adsorb or interact at the crystal’s surface. For example Mg2+, Zn2+, Cu2+, Fe2+ and Al3+ may replace Ca2+ on ex- posed surface lattice sites. The reactive surfaces of car- bonates may adsorb soil contaminants such as Ba2+, Cd2+
and Pb2+ (Ming, 2002).
In some cases, however, an increase in soil pH may not necessarily result in a decrease in metal availabili- ty. Molybdenum in soil, which is in the form MoO4, is more soluble when the pH increases (Kabata-Pendias and Pendias, 1992). A surprisingly higher Cd uptake or even toxicity has been observed in high pH soils compared with low pH soils (Eriksson, 1989), but the mechanisms are not clearly elucidated.
Many findings confirm that the solubility of heavy metals in soil is directly correlated with the redox poten- tial (Patrick et al., 1990; Masscheleyn et al., 1991). Yaron et al. (1996) showed that under same pH values, metal solubility increases as redox potential decreases. As redox potential decreases, trace elements become less available.
The uptake of Cd by rice seedlings is at a minimum at low Eh values, where metals may have precipitated as sulphides (Reddy and Patrick, 1977). Availability of the metalloid As increases with increasing concentration in solution (lower soil pH) and with increasing amounts of soluble As (IV) (lower soil redox) compared with As (V) (Marin et al., 1993). Fe and Mn are particularly soluble under water-logged conditions, and precipitates may ap- pear on the root surface through the oxidation of metals supplied by the mass flow. Large amounts of metals (e.g.
Zn and Cu) can be adsorbed to those iron oxides, leading to an increase in metal uptake by roots (Morel, 1997).
Metals are more available in sandy soils than in clayey soils, where they are firmly retained on the surface of clay minerals. They may form types of complexes on clay sur- faces: outer-sphere ion-exchange complexes on the basal plane, and coordination complexes with SiOH or AlOH groups exposed at the edge of the silicate layers (Zachara et al., 1993). Other minerals, including amorphous hy- droxides and oxides, gibbsite and allophane clay, adsorb metals and reduce their mobility in soil. For example, up-
Combatting Soil Degradation
take of Cd by soybean is lower from soil low in organic matter and high in oxides of Fe and Mn compared with soil low in oxides and high in organic matter (White and Chaney, 1980).
Organic matter in soil, e.g. humic compounds, bears negatively charged sites on carboxyl and phenol groups, allowing for metal complexation (Stevenson, 1982).
Metals can be precipitated or adsorbed organic matter, or can be in the soil solution as soluble organic complexes with low molecular weight compounds (e.g. organic and fulvic acids). The presence of high amounts of insoluble organic matter in soil is negatively correlated with plant uptake, as often observed on peat soils with Cu.
Cation exchange capacity (CEC), a function of clay and organic matter content in soil, controls the availability of trace elements. In general, an increase in CEC decreases uptake of metals by plants (Haghiri, 1974; Miller et al., 1976; Hinesly et al., 1982, Tyler and McBride, 1982).
Absorption of trace elements by roots is controlled by the concentration of other elements and interactions have often been observed. They may be positive or neg- ative, the uptake of a given element being improved or depressed by others present at high concentrations in the soil. Macronutrients interfere antagonistically with up- take of trace elements. Phosphate ions reduce the uptake of Cd and Zn in plants (Haghiri, 1974; Smilde et al., 1992), and diminish the toxic effects of As, as observed on soils treated with arsenic pesticides (Benson, 1953).
Calcium controls the absorption of metals, e.g. Cd, as a result of competition for available absorption sites at the root surface (Cataldo et al., 1983). Antagonism between micronutrients is quite frequent. Leaf chlorosis, a symp- tom due to Fe deficiency, can be induced by an excess of other metals such as Zn, Ni, and Cu, which depresses Fe uptake by plant roots. Conversely, Fe affects Cd ab- sorption, acting as a strong antagonist against the toxic metal. Cd and Zn, two metals chemically close similar- ity in electronic configuration and reactivity with organ- ic ligands, interact in the soil-plant system, causing the well-known Cd/Zn antagonism (Smilde et al., 1992). Zn depresses Cd uptake (Cataldo et al., 1983). On the other hand, at low concentrations the interaction is synergistic and the input of Zn increases Cd uptake (Haghiri, 1974).
Application of K fertilisers to the soil leads to increasing uptake of Cd, Zn, Cu and Ni by oats. This is assumed to
be the result of competition between K and microele- ments for exchange sites on the solid phase of the soil.
It is necessary to remember that fertilisers can contain considerable amounts of trace elements. Two major sources of soil contamination are sludges and phosphate fertilisers.
During the period of sludge decomposition, after applica- tion to soils trace elements may remain highly available to plants as a result of the release of soluble organic carbon and the decrease in pH following mineralisation/nitrifica- tion, which increases the solubility of heavy metals (Dudley et al., 1986; Alekseev, 1987). Long-term use of phosphate fertilisers can elevate the content of many trace elements (e.g. Cd, Hg and As) in soils. It has been shown that the use of these fertilisers significantly increases Cd in soil and the subsequent uptake of the metal by plant roots (Mulla et al., 1980; Rothbaum et al., 1986; Mineev, 1990).
Manifold environmental pollution with different risk po- tential has been produced by unfavourable waste manage- ment and intensive mining activities. Detailed investiga- tions of heavy metal mobility behaviour from waste dump material of typical smelting products (e.g. Mansfelder Land county, mining area of Eastern Thuringia) have been performed in Germany. From these investigations con- clusions have been drawn on the long-term behaviour of components and estimates of the risks have been made.
Based on different German studies, the transfer behaviour of heavy metals, including Ni, from soil to well-chosen food and forage plants has been analysed. On this basis, a concept for hazard assessment concerning the adverse effects of soil contamination to plants has been developed (Knoche et al., 1999; Schoenbuchner, 2005).
The availability to plants of trace elements from the soil is also controlled by plant micronutrient requirements and their ability to take up or exclude toxic elements. Some plants are well adapted for survival in stressful environ- mental conditions. They can hold in their tissues amounts higher than 1% of the metal and up to 25% on a dry mat- ter basis. These plants are called ‘hyperaccumulators’.
Grasses take up less trace elements than fast-growing plants, e.g. lettuce, spinach and carrots. When grown in the same soil, accumulation of Cd by different plant spe- cies decreases in the order: leafy vegetables > root vege- tables > grain crops (Morel, 1997). Screening of cultivars that exclude toxic elements should be a priority to protect food quality.
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