Soil organic matter-metal interactions in Chilean volcanic soils under different agronomic management

The organic matter existing in volcanic soils must be investigated not only in quantity, but also in quality in order to understand its role in soils and thus ensure adequate levels of stable organic polymers contributing to soil humification. The meaningful contribution proposed by this investigation is based on the study of the interaction of soil organic matter (SOM) in its fractions humic acid and fulvic acid with metallic elements that are macro- and micronutrients and also contaminants. The study was carried out in Chilean soils of the type Andisols and Ultisols. To this end, humic acids and fulvic acids were fractionated to determine phenolic, carboxylic and total acidity. In addition cation holding capacity was determined. From the knowledge of this organic matter-metal interaction, the behavior and actual contribution of organic matter to soil may be inferred in relation to storage and availability of metallic elements, both nutrients and contaminants. The results of this study demonstrate the existence of electrondonor sites in the organic fractions of soil, which may be highly specific for heavy metals. They also confirm the possibility that metal-ion, especially micronutrient, bioavailability is affected by retention mechanisms involving the nature of SOM. Thus, the content of carboxylic groups in fulvic acid is higher than in humic acid. The number of phenolic groups is about the same in both. In addition, comparison of the maximum binding ability among the cations leads to the conclusion that the most frequent tendency is to retain cadmium (Cd) > zinc (Zn) > calcium (Ca) at the used concentrations. In wild forest, soils exhibit organic polymers with a higher content of phenolic groups and total acidity compared with the soils subjected to agronomic management and to some other use. Fulvic polymers exhibit a higher maximum cation binding ability in wild forest.