American Soil Taxonomy Compared to World Reference Base for Expressing Environmental Pollution, a Case Study: Lenjanat Region of Isfahan

Increasing demand for an international classification system as a unique language in soil science has caused development of different classification systems. Soil classification is a useful tool for understanding and managing soils. In recent decades, the role of human in soil formation has become a matter of great concern among soil scientists. Human is now considered as a soil-forming factor and anthrosolization is recognized as a soil-forming process that consists of a collection of geomorphic and pedological processes resulting from human activities. Industrial developments, mines and their activities and intensive agriculture led to soil changes in urban areas. One of the important missions of soils classification is to identify important properties which have effect on management purposes. In recent years, the importance of human impact on soil properties considered in soil classification systems like American Soil Taxonomy (2014) and World Reference Base (2015) and some revisions and changes have been made in this regard. In this study, the efficiency of American Soil Taxonomy and WRB soil classification systems soils were compared to describe the pollution of soils to heavy metals in Lenjanat region of Isfahan, Iran.
Agricultural lands located in Lenjanat region of Isfahan province were selected as the study area. Lenjanat is an industrial region in which intensive agriculture surrounded by different industries like steel and cement making factories and lead mining. Agricultural lands which consisted of five soil map units (Khomeini Shahr, Nekooabad, Isfahan, Lenjan and Zayandehroud) were selected and 400 topsoil samples were randomly collected. Six soil profiles were excavated in each map unit (totally 30 soil profiles) and after describing soil, the classification of soils was determined in the field. Then, representative pedons were chosen for each unit and routine soil morphological, physical and chemical properties were determined using common methods. Finally, the soil profiles were classified according to criteria of Soil Taxonomy up to family level and (WRB) at the second level. The amount of heavy metals was studied in some agricultural crops of the region and livestock muscles in the region. Total Cd and Pb were extracted from the soil samples using concentrated HNO3. Cadmium and lead of plant samples were prepared according to the procedure of Dry-ashing. Heavy metals were extracted by 3 N HCl. The metal contents of soil and plant samples were determined by flame atomic absorption spectrometry (FAAS). Descriptive statistics including mean, variance, maximum, minimum, and coefficient of variation (CV) were calculated using STATISTICA 6.0 software.
According to WRB (2015) classification, the soils were classified as 3 reference groups: Cambisols, Gleysols and Calcisols. The soils were also categorized as Aridisols and Inceptisols in Soil Taxonomy system. In this study, the environmental standards based on Swiss Federal Office of Environmental, Forest and Landscape were used for the threshold values of heavy metals pollution in the soils (VBBo). The results also indicate that the amount of cadmium in most of the soil samples was higher than the threshold limit. The amount of lead in soils was below the threshold limit. The results also indicated that all the crops had a lead average higher than the maximum of tolerance. The average of lead in cow and sheep livestock was also above Iran and Europe Union’s permissible limit. Despite American soil taxonomy classification system in the last version has a class (Anthraltic, Anthraquic, Anthrodensic, Anthropic) to show human impacts on soils at family level, it could not show the contamination of soils to heavy metals. However, WRB soil classification system defined qualifier “toxic” (Anthrotoxic, Ecotoxic, Phytotoxic, Zootoxic) which can be used in these conditions. Both systems had serious shortcomings to show poor drained soils in this area. Defining the Aquids suborder for Aridiosols in American Soil Taxonomy and revision of the definition of Gleysols, Anthrosols and also aquic conditions in WRB soil classification system are highly recommended.
The results indicated that WRB soil classification system could explain the soils pollution and also their effects on human health for the studied soils. Definition of some quantitative sub qualifiers for toxic can be useful to improve the efficiency of WRB for classifying polluted soils. Incorporating some criteria for pollution hazards in American Soil Taxonomy should be considered in early future.