Soil aluminium toxicity

Aluminium, boron, silicon Lead in soil slurries Toxic organic compounds... 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Acid sulfate soils are an especially difficult class of acid soil formed in former marine sediments that have been drained. The acidity is generated from the oxidation of pyrite in the soil resulting in acute aluminium toxicity, iron toxicity, and deficiencies of most nutrients, especially phosphate which becomes immobilized in ferric oxide. The development and management of acid sulfate soils are discussed in detail in Dost and van Breemen (1983) and Dent (1986). 

Weatherley N. S., Rutt G. P., Thomas S. P., and Ormerod S. J. (1991) Liming acid stream aluminium toxicity to fish in mixing zones. Water Air Soil Pollut. 55, 345-353. 

Acid soils, with a pH of 5.5 or lower, significantly limit crop production worldwide approximately 50% of the world s potentially arable soils are acidic, and the production of staple food crops is particularly affected. Twenty percent of the worldwide production of maize and 13% of rice is on acid soils, while 60% of the acid soils in the world are in the tropics and subtropics. Thus, acid soils hmit crop yields in many developing countries, while in developed countries such as the United States, the extensive use of ammonia fertihzers causes further acidification of agricultural soils. The primary limitations of acid soils are toxic levels of aluminium and manganese, and suboptimal levels of phosphorous. Acidification of soils leads to acidification of rivers and streams, increasing the solubility of aluminium, with direct consequences on fish populations, and eventually of water supplies to the general population. 

Hesse, P.R. (1963) Phosphorus relationships in a mangrove-swamp mud with particular reference to aluminium toxicity. Plant and Soil, 19, 205-218. 

From an environmental standpoint, the recognition that acid rain mobilizes aluminium from poorly buffered soils into the aquatic environment has increased the awareness and concern about aluminium toxicity to aquatic organisms. The acidification of fresh water lakes and rivers in the USA, Canada and particularly the Scandinavian countries, and the subsequent rise in dissolved aluminium levels, has been linked to the decline in fish numbers and, in some cases, to the total elimination of entire fish populations [159,160]. In aquatic systems, it has been demonstrated that the A1(0H)2 species seems to be most toxic to fish [159]. Other workers [161], using thermodynamic calculations in conjunction with fish toxicological experiments, have pointed to the Al(OH) species as also toxic to fish. 

Worldwide, it has been estimated that 40 % of arable soils and perhaps as much as 70 % of land that can be cultivated are acidic enough to have an aluminium toxicity problem (World Food Nutrition Study, 1977). 

Forestry research embraces both the monitoring of environmental impacts on forest ecosystems as well as the impact of forestry on surrounding landscapes, in particular on water catchments. Studies carried out have analysed soil to monitor the effects of acid rain on forest nutrition in relation to observed disease sus ceptibility in trees from different geographical loca tions. It is postulated that immobilization of essential plant nutrients such as calcium and magnesium oc curs in soil where toxic aluminium becomes increas ingly mobile owing to the reduced soil pH attributed to acid rain. Routine monitoring of water catchments that surround forestry follows the level of movement of both nutrient elements (N and P) and other forest exudates (Mn, Cu, Zn, Sr and Ba) in run-off. Both F AAS and GF AAS are used in these analyses and offer acceptable sensitivity and precision. 

Acidic environments have always been characterised by restricted biological populations, but only relatively recently was it realised that the main cause of this is aluminium toxicity. The extensive literature that has developed on the biological effects of A1 has been reviewed by Foy (1984) for crop plants and by Howells et al (1990) for fisheries. It is surprising that the most abundant metal in the earth s crust is highly toxic to organisms, but fortunately in most environments it is completely insoluble and is therefore innocuous. Only in acidic environments is Al sufficiently soluble to be harmful, and increased mobilization of Al in soils, surface waters and groundwaters is therefore an important consequence of acid deposition. A useful detailed review on Al in the environment is given by Driscoll and Schecher (1988), and on the chemistry involved in its mobilization by Bache (1986). 

Soil pH is easily tested for and determines the availability of nutrients and the success of white clover. Very acid soils (below pH 5.0) will cause a deficiency of the trace elements iron, boron, copper and molybdenum and conversely will cause injury to plant growth by increasing the availability of aluminium and manganese to toxic levels. Over-liming, on the other hand, which can raise the pH above 6.5, will reduce the availability of certain essential elements such as phosphorus, manganese and boron. 

The solubilized aluminium is the main toxic agent to plants in acidic soils, and acid tolerant (calcifuge) plants are usually also aluminium tolerant. The ECEC method determines the levels of AP+, H+, Ca + and Mg + extracted by 1 M potassium chloride and is described in Method 5.2. 

Hoyt, EB. and Nyborg, M. (1971a) Toxic metals in acid soils. I. Estimation of plant-available aluminium. Soil Sci. Soc. Am. Proc., 35, 236-240. 

Research has continued on the speciation of aluminium in water and in soil related particularly to the effects of acid precipitation. Species of particular concern are Alm, Al(OH)2+ and Al(OH)4. These species are the most toxic with regard to fish and plants. The presence of fluoride, sulfate and organic compounds that can form complexes with aluminium result in a lower degree of toxicity. Consequently, the objectives of a number of investigations have been the relationship of... 

There are several factors however which are related to water acidity (low Ca" ", high content of heavy metals and aluminium) and other abiotic factors (temperature, transparency) which mask or enhance the pH effect. It now seems proven that aluminium is a real toxic agent in lakewater in acidified catchments, this metal being leached in high amounts from soils under acidification. Aluminium buffer system replaces the normal bicarbonate buffer system when lakes are acidified and A1 concentrations... 

Acidification of soil combined with lack of nutrients and release of aluminium ions which act toxic to plant roots was the other line of explanation. Wieler (1905, 1933), Ulrich et al. (1979, 1983 a, b) and Drablos and Tollan (1980) can be used to support this theory, while it is more critically discussed in VDI (1983). 

The enriched acidity of the rain (mentioned above) means that when it falls on the ground it dissolves essential ions, including magnesium, out of the soil. Magnesium ions are essential for photosynthesis, and so plant growth is affected. Acid rain also dissolves some of the more toxic ions, such as aluminium, copper, lead and zinc, out of the soil. These metals under normal conditions remain fixed in the soil, but acid rain makes these toxic elements more available to plant life. These elements stunt the growth of plants. The water run-off from the soil into rivers and lakes means a build-up of toxic ions, and aquatic plants and fish are affected. 

One of the best present examples of the description of a toxic effect without much awareness or proof of tlie causality of the possible agents is the so-called acid rain wliich results in some Northern European countries in the acidification of poorly buffered soil and lakes, and to a decreased productivity in forest wood and lake-fisb. This toxic effect has been attributed to the acidity and mobility of aluminium salts resulting from the input of large quantities of SO ions in systems previously out of l each of pollutants from the dense industrial zones... 

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