Geochemical features and prevalence of aluminum in soil cover

In terms of prevalence in nature, aluminum ranks third after oxygen and silicon and first among metals. Its clarke in the earth's crust is 7.45%, in soil – 7.12, in seawater – 1·10–6, in plants – 0.02, and in livestock animals – 1·10–5%. Due to its chemical activity, aluminum is not found in a free state. Several hundred aluminum minerals are known. The most important of these are bauxite, alunite, kaolin, and nepheline. It is a component of numerous feldspars: orthoclase, albite, as well as zeolite and micas. Aluminum is characterized by a clearly defined concentration differentiation depending on the rock type. Ultramafic rocks contain an average of about 0.45% aluminum, while in sedimentary rocks (clays, shales) its amount increases to 10-11%. Acidic, intermediate, and basic rocks (granites, diorites, basalts) occupy an intermediate position – 7-9%.

The geochemical features of aluminum are determined by its high affinity for oxygen (in minerals, aluminum enters oxygen octahedra and tetrahedra), constant valence (+3), and low solubility of most natural compounds. In endogenous processes during magma solidification and the formation of igneous rocks, aluminum enters the crystal lattice of feldspars, micas, and other minerals – aluminosilicates. In the biosphere, aluminum is a weak migrant; there is little of it in organisms and the hydrosphere. The scheme of the biogeochemical cycle of aluminum is shown in Figure 67 (Orlov D.S., Bezuglova O.S., 2000).

Fig. 67. Biogeochemical cycle of aluminum

In humid landscapes, aluminum is part of organic-mineral soil colloids; Al3+ is adsorbed by clays and humus. Under the protection of organic colloids, aluminum also migrates in swamp waters. The bond with silicon is partially broken in this process, which leads to the formation of aluminum hydroxides, the minerals boehmite and hydrargillite. Nevertheless, a significant part of aluminum remains in the form of kaolinite and other clay minerals. The low mobility of this element determines the residual accumulation of its hydroxides in the weathering crust of humid tropics and the formation of bauxite. Aluminum migration depends less on redox conditions because it has a constant valence. At the same time, the amphoteric nature of this element ensures its dependence on the acid-base conditions of the environment: in a strongly acidic environment, it behaves as a cation, while in a strongly alkaline one, it forms anionic complexes. In the neutral and slightly alkaline waters of steppes and deserts, it practically does not migrate; aluminum mobility is highest in the strongly acidic waters of volcanic regions and sulfide oxidation zones. In strongly alkaline thermal and cold soda waters, this element migrates intensively in the anionic form AlО2–. The formation of dawsonite 3)(OH2)] and other aluminum minerals, which sometimes form large deposits, is associated with these processes.

The aluminum content in soils is mainly due to the presence of feldspars and clay minerals, and partly to epidotes, garnet, corundum, and other aluminum-rich primary minerals. Free alumina may also be present in the form of various aluminum hydroxides (boehmite, diaspore, hydrargillite) in amorphous or crystalline form. The total aluminum content in soils usually ranges from 1-2 to 15-20%, and in ferrallitic soils of the tropics and bauxite, it can exceed 40%.

The soil-forming process leaves a definite imprint on the aluminum content. Depending on the soil type, both the average content of this element in the soil profile and its distribution along genetic horizons change. The significance of aluminum in soil formation and soil fertility, according to D.S. Orlov (1985), is determined by the following provisions: 1) the high content of aluminum in soils and its participation in the formation of aluminosilicates determines its leading constitutional role, along with Si, O, and C. It should be mentioned that aluminosilicates are the most common minerals, accounting for up to 85% of the earth's crust mass. At the same time, any specific influence of aluminum on the morphological features of soils is not pronounced; 2) aluminum has sufficiently high reactive and migration ability and forms diverse types of compounds. It actively participates in the redistribution of substances throughout the soil profile, and its compounds and the nature of their distribution can be used for the diagnosis of soils and certain processes; 3) aluminum participates in the formation of potential (exchange and hydrolytic) soil acidity; 4) an increased content of mobile aluminum compounds is not indifferent to plants; in their presence, poorly soluble aluminum phosphates are formed, the phosphorus of which becomes less accessible to plants upon aging and crystallization of precipitates. Furthermore, aluminum is toxic to many plants: already at a concentration of 2 mg/L in the solution, a sharp deterioration in the development of the root system is observed, and carbohydrate, nitrogen, and phosphate metabolism are disrupted. Higher concentrations of aluminum cause a sharp decrease in the harvest of grain crops and even their death.

S.V. Zonn and A.P. Travleev (1992), while examining the role of aluminum in pedogenesis, draw three conceptual conclusions: 1) the rates of release of aluminum oxides from minerals are determined by a decrease in humidity and heat, down to desert carbonate-alkaline and gley-acidic soils of the tundra; 2) one of the leading factors influencing the release of aluminum oxides is organisms and their detritus. With a decrease in heat and an increase in atmospheric moisture, the production of humic acids increases, the formation of alumino-fulvic compounds intensifies, which acquire mobility and increased leaching, which determines the relative accumulation of humus-ferruginous compounds of quartz and silica in soils below the humus-accumulative horizon. With an increase in hydrothermal indicators, the leaching of all compounds, including SiO2, increases; Al- and Fe-oxides reduce mobility despite relatively greater moisture. Under high thermal (steppe, desert) regimes, aluminum oxides are the least active. Their release from minerals fades, and they lose their diagnostic significance; 3) the altitudinal (vertical) factor within the subtropical and tropical zones enhances allitization and intensifies the impact of humic and other acids. In polar and subpolar conditions, the leaching of aluminum and iron intensifies, which determines the differences in the relative increase of silicon dioxide in the upper soil profile.

Forms of aluminum compounds in the soil

Aluminum in soils is represented by compounds with sharply different solubility, and for soils of humid regions (podzolic, sod-podzolic, zheltozem), the presence of not only readily soluble but also exchangeable aluminum is characteristic. Possible forms of aluminum compounds in soils are very diverse. These are aluminum oxides and hydroxides; aluminum-containing mineral salts; simple and complex aluminum compounds with organic substances, and, finally, aluminosilicates.

S.V. Zonn and A.P. Travleev (1992) distinguish the following groups and forms of aluminum compounds in soils:

• A. Basic, or primary, groups of aluminum compounds:
  • Silicate, which are part of primary minerals.
  • Non-silicate (free), released during weathering from primary minerals, which consist of: a) crystallized oxides and hydroxides; b) amorphous – primary oxides.
  • Extractable, or active, subdivided into: a) organo-aluminum compounds or complexes; b) exchangeable; c) non-exchangeable.

  The content of certain groups and forms of aluminum in soils and their ratios are of great importance for establishing the direction and intensity of the soil-forming process. Aluminum compounds in some soils can be, as it were, preserved by carbonate-calcium-magnesium shells and remain in an unchanged state as long as their protection exists.

  Other soils contain predominantly various degrees of crystallized aluminum oxides and hydroxides, as well as weakly crystallized and amorphous forms of the oxide type in various proportions. The latter constitute mainly the silty and colloidal fractions of soils. In the process of soil formation, mobile ionic fractions of aluminum are formed in them: extractable, exchangeable, and non-exchangeable forms, which are in physico-chemical bonds with the dispersed part of the soils. Non-silicate forms of aluminum compounds pass into the soil solution as a result of the destruction of minerals or the replacement of aluminum ions by other cations under the influence of neutral and highly acidic solutions.

  Migration and accumulation of aluminum in the soil

  Depending on the physico-chemical conditions of the environment and the form of the compounds, aluminum migrates and accumulates in soils in different ways. The release of aluminum during mineral degradation can occur either as a result of the increased leaching of all other chemical elements that are more mobile under the given conditions, or due to the destructive effect on aluminosilicates by microbial metabolites that form complex compounds with aluminum. In the first case, aluminum can accumulate in the form of a mineral residue, whereas in the latter, it immediately undergoes chelation, binds into an organometallic complex, and acquires mobility.

  Role of organo-mineral complexes in aluminum migration

  The mobility of aluminum in soils is largely determined by its ability to form organo-mineral complexes, in which it can migrate over a wide pH range. The mechanism of aluminum mobilization from the crystal lattices of aluminosilicates is often based on chelation reactions.

  The organo-aluminum compounds formed in this process are widespread in nature; their appearance in soils is the result of the impact of decomposition products of plant residues and specific humic acids on the aluminosilicates of the parent rock. In podzolic soils, they are present in the form of organo-mineral complexes formed by aluminum with humic acids. In the illuvial horizons of podzols, significant amounts of organo-aluminum compounds of fulvic acids become fixed and accumulate.

  Complex aluminum compounds are not only constantly formed in the soil profile as a result of the interaction between the transformation products of plant residues and the aluminosilicates of the parent rock, but also enter it in their ready-made form with plant litter.

  Biogenic transformation of aluminum compounds

  Depending on the specific ecological conditions, organo-aluminum compounds that have entered the soil or have been formed within it can:

  • undergo mineralization;
  • be leached beyond the soil profile;
  • be preserved and immobilized within humus substances.

  In the soils of the humid tropics, processes of mineralization and leaching of organo-aluminum complexes predominate, whereas in the soils of the podzolic zone, the processes of their preservation are quite pronounced. According to the author's views, the decomposition of low-molecular-weight complex aluminum compounds and organo-aluminum complexes of fulvic acids occurs under the influence of microorganisms.

  The participation of microorganisms in the aluminum transformation cycle involves several processes:

  • mobilization of aluminum from primary and secondary minerals;
  • decomposition (mineralization) of organo-aluminum compounds;
  • accumulation of aluminum hydroxide (Babjeva I.P., Zenova G.M., 1989).

  Fig. 68. Aluminum transformation cycle in soils

  The processes of mineralization of organo-aluminum complex compounds are linked to the vital activity of fungi and microorganisms of the mycoplasma group. In the presence of organo-mineral compounds of aluminum or iron, fungal hyphae in symbiosis become covered with deposits of the corresponding metal hydroxides. One of the important mechanisms for the mobilization of aluminum from the crystal lattices of aluminosilicates is chelation. This process involves, on one hand, aggressive products of microbial synthesis and microbial decomposition of plant residues, and on the other hand, specific organic soil substances – humic acids.

  The resulting organo-aluminum compounds are widely distributed in soils. These compounds not only form within the soil itself but also enter it with plant residues in the form of complexes of aluminum with organic acids, amino acids, and proteins. Subsequently, depending on ecological conditions, these compounds in the soil undergo various transformations: they are leached beyond the soil profile, mineralized, or immobilized within humus substances.

  When considering issues related to the aluminum content and the transformation of its compounds in soils, one cannot bypass the problem of soil acidity and the role of aluminum and hydrogen ions therein. Currently, the following forms of soil acidity are distinguished: 1) actual acidity; 2) potential acidity, which is subdivided into exchangeable and hydrolytic. Actual acidity is caused by the presence of carbonic acid, organic acids, hydrolytically acidic salts, and other components exhibiting acidic properties in the soil solution. Among the latter, aluminum ions have the greatest influence, and their acidic properties are comparable to those of acids such as carbonic and acetic. Exchangeable acidity is caused by exchangeable adsorbed hydrogen and aluminum cations and manifests upon the action of neutral salt solutions:

  [SEC]H + KCl = [SEC]K + HCl;

  [SEC]Al + 3KCl = [SEC]3K + AlCl3.

  In this process, aluminum chloride undergoes hydrolytic dissociation with the formation of a weak base and a strong acid, which also acidifies the soil solution. Thereby, aluminum also participates in the formation of hydrolytic acidity.