Manganese [Mn, 25]
191
During weathering processes, Mn compounds are oxidized and Mn oxides are
reprecipitated and form readily secondary Mn minerals. The behavior of Mn in surface deposits is very complex and governed by different factors, of which Eh–pH
conditions are the most important. Presence of Mn in soils is the key to the entire
status of soil redox potential (Sparks vide Kabata-Pendias 2011).
Manganese compounds readily leach down from the upper soil layers under cold
and humid climate. Under tropical and subtropical conditions, is may be concentrated in soils, mainly in the form of concretions and nodules. Manganese is mobile
in the most soil media, and occurs in soil solution in various species (Kabata-Pendias
and Sadurski 2004):
2
2
3
r Cationic forms: Mn , MnOH , MnCl , MnHCO , Mn (OH)
3
2 , and Mn OH
2
2
2
r Anionic forms: MnO , HMO , Mn(OH) , and MN(OH)
4
2
3
4
Because of a low mobility of Mn compounds in oxidizing systems at pH levels near
neutrality, any changes in the Eh–pH conditions are important, and have an impact on
its concentration in the soil solution. Concentration of Mn in soil solutions vary highly
from 25 to 2000 μg/L. Solubility of Mn always increases with the increase of soil acidity.
However, the ability of Mn to form anionic complexes, and to combine with organic
ligands, may contribute to increased Mn mobility in the alkaline pH range. Among abiotic and biotic soil parameters, fixation by root exudates and cross-interactions with Fe
hydroxides play a crucial function in the Mn mobility, and thus in phytoavailability.
Also, SOM, and especially fulvic acid, has a high impact on Mn behavior in soils.
All Mn forms, minerals, concretions, nodules, exhibit a great adsorption capacity to various metals, which usually increases with increasing pH. In the case of Pb,
this increase is from 20% to 80% of the total Pb concentrations in solution, at 5 and
8 pH, respectively (Wilson et al. vide Kabata-Pendias 2011). Manganese oxides have
a relatively high total surface (30–300 m 2 /g) and cation exchangeable capacity value
(150–320 cmol/kg). The most readily sorbed metals by Mn oxides are Cu, Co, and Pb,
which may be unavailable to plants. However, due to the reducing and oxidizing properties, Mn oxides can increase the mobilization of some metals, under specific soil
conditions. Biologically mediated processes are the most significant in the Mn–redox
cycling in soils (Perelomov et al. 2013). They increase formation of Mn oxides, as
well as stimulate the release of Mn from some compounds. The microbial dissolution
of Mn compounds, especially due to the enzymatic reduction of Mn (+3 and +4), and
due to the production of CO 2 , and organic acids, is of real importance.
Some microorganisms (bacteria and fungus), on the other hand, may precipitate
Mn by oxidizing Mn 2+ to Mn 3+ and Mn 4+ , or stimulating the precipitation of carbonates, sulfides, and so on. Some organic acids secreted by microorganisms may
release Mn from several MnO 2 compounds. Biologically mediated processes are the
most significant in the Mn–redox cycling in soils. Colloidal Mn oxides reveal a great
affinity for adsorption of cationic and anionic species of various elements, as well
as for OM. However, due to both reducing and oxidizing properties, Mn oxides may
increase the mobilization of some metals under specific soil conditions.
191
During weathering processes, Mn compounds are oxidized and Mn oxides are
reprecipitated and form readily secondary Mn minerals. The behavior of Mn in surface deposits is very complex and governed by different factors, of which Eh–pH
conditions are the most important. Presence of Mn in soils is the key to the entire
status of soil redox potential (Sparks vide Kabata-Pendias 2011).
Manganese compounds readily leach down from the upper soil layers under cold
and humid climate. Under tropical and subtropical conditions, is may be concentrated in soils, mainly in the form of concretions and nodules. Manganese is mobile
in the most soil media, and occurs in soil solution in various species (Kabata-Pendias
and Sadurski 2004):
2
2
3
r Cationic forms: Mn , MnOH , MnCl , MnHCO , Mn (OH)
3
2 , and Mn OH
2
2
2
r Anionic forms: MnO , HMO , Mn(OH) , and MN(OH)
4
2
3
4
Because of a low mobility of Mn compounds in oxidizing systems at pH levels near
neutrality, any changes in the Eh–pH conditions are important, and have an impact on
its concentration in the soil solution. Concentration of Mn in soil solutions vary highly
from 25 to 2000 μg/L. Solubility of Mn always increases with the increase of soil acidity.
However, the ability of Mn to form anionic complexes, and to combine with organic
ligands, may contribute to increased Mn mobility in the alkaline pH range. Among abiotic and biotic soil parameters, fixation by root exudates and cross-interactions with Fe
hydroxides play a crucial function in the Mn mobility, and thus in phytoavailability.
Also, SOM, and especially fulvic acid, has a high impact on Mn behavior in soils.
All Mn forms, minerals, concretions, nodules, exhibit a great adsorption capacity to various metals, which usually increases with increasing pH. In the case of Pb,
this increase is from 20% to 80% of the total Pb concentrations in solution, at 5 and
8 pH, respectively (Wilson et al. vide Kabata-Pendias 2011). Manganese oxides have
a relatively high total surface (30–300 m 2 /g) and cation exchangeable capacity value
(150–320 cmol/kg). The most readily sorbed metals by Mn oxides are Cu, Co, and Pb,
which may be unavailable to plants. However, due to the reducing and oxidizing properties, Mn oxides can increase the mobilization of some metals, under specific soil
conditions. Biologically mediated processes are the most significant in the Mn–redox
cycling in soils (Perelomov et al. 2013). They increase formation of Mn oxides, as
well as stimulate the release of Mn from some compounds. The microbial dissolution
of Mn compounds, especially due to the enzymatic reduction of Mn (+3 and +4), and
due to the production of CO 2 , and organic acids, is of real importance.
Some microorganisms (bacteria and fungus), on the other hand, may precipitate
Mn by oxidizing Mn 2+ to Mn 3+ and Mn 4+ , or stimulating the precipitation of carbonates, sulfides, and so on. Some organic acids secreted by microorganisms may
release Mn from several MnO 2 compounds. Biologically mediated processes are the
most significant in the Mn–redox cycling in soils. Colloidal Mn oxides reveal a great
affinity for adsorption of cationic and anionic species of various elements, as well
as for OM. However, due to both reducing and oxidizing properties, Mn oxides may
increase the mobilization of some metals under specific soil conditions.