The ability of humic substances to form complexes with cationic micronutrients depends on their content of electron donor functional groups (Stevenson, 1994). Either weak bonds such as water bridges, electrostatic attraction due to cation exchange capacity, or strong bonds involving coordination with single groups or formation of ring structures (chelates) with carboxyl, alcohol, and amino groups can thereby be formed. The formation of different types of bonds depends on the latters' degree of saturation; weaker bonds play a more important role when stronger ones become saturated. The formation of more than one bond between the metal and the organic molecule usually results in a higher stability of the complex. The stability of the metal chelate complex depends on the number of atoms that form a bond with the metal ion, the number of rings that are formed, the nature and concentration of the metal ion, and pH (Stevenson, 1994). The stability order of the complexes formed between metals and humic acids has been determined through potentiometric titration and follows the Irwing-Williams series:
Pb2+ > Cu2+ >Ni2+ >Co2+>Zn2+ >Cd2+ >Fe2+>Mn2+ >Mg2+
On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni and Zn, whereas Pb, Cu and Fe were more strongly bound (Schnitzer and Kahn, 1972); this behavior indicates that at different pH values, metal-humic substances complexes of different stability are formed in the soil. Due to the heterogeneity of humic substances concerning molecular weight, content of functional groups, variety of bonding sites and changes in conformation of these macromolecules with pH and salt concentration, it is difficult to reach an unequivocal state of knowledge (Chen and Stevenson, 1986). Attempts have been made to determine the apparent stability constants (Kapp) for Fe3+ with two hydrophobic fractions of dissolved organic matter obtained from a manure compost, after sorption onto XAD-8 resin (Chen et al., 2004). The Kapp values at pH 5.0 and an ionic strength of 0.1 M were 7.91 for the fraction desorbed with NaOH and 6.76 for that desorbed with methanol. Garcia-Mina et al. (2004) investigated the stability of different metal-humic complexes (NaOH extracted) in the pH range 6-9 and ionic strength of 3 mM, and found maximal Kapp values (4.11) at pH 8.0. These values are somewhat lower than the maximal stability constant values determined for complexes between Fe and synthetic chelating agents (e.g. EDTA, EDDHA) (Lucena, 2003) or organic compounds of biological origin (e.g. organic acids, siderophores, phytosiderophores) (Von Wiren et al., 2000; Crowley, 2001; Ryan et al., 2001). However, it should be kept in mind that for compounds like EDTA, citrate and oxalate the ability to maintain Fe in solution above pH 6.0 is very limited. Furthermore, the stability of high levels of organic acids or (phyto)siderophores in soils is unlikely, due to their rapid microbial decomposition. In this context, the contribution of humic substances to Fe availability is conceivably significant, based on the amount of molecules present in the soil and soil solution, the high resistance to microbial degradation, and the relatively high stability constant at pH values typically encountered in alkaline and calcareous soils.
Humic substances are known to be redox reactive and capable of chemically reducing metals including Fe3+ (Skogerboe and Wilson, 1981; Struyk and Sposito, 2001). Standard redox potentials for fulvic and humic acids have been evaluated to be around 0.5 and 0.7 V, respectively. It has been shown that reduction of Fe3occurs at significant levels at pH values lower than 4; at higher pH values reduction is decreased by formation of complexes between Fe3+ and humic molecules.
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