**2. Trace elements in soil**

*Trace Metals in the Environment - New Approaches and Recent Advances*

elemental toxicity or deficiency in humans and plants.

body and classified them into five groups:

molybdenum, and selenium;

ments in the WHO classification;

in the WHO classification.

calcium, phosphorus, magnesium, and sulfur;

organism [3].

quantifying the contamination level, but also to help solve problems associated with

Trace elements (TEs) are dietary minerals present in living tissues in small amounts; some of them are known to be nutritionally essential, playing a vital role in the normal metabolism and physiological functions of animals and humans [2]. The TEs' essentiality for the human body has been a matter of discussion throughout time and the term "trace elements" has never been clearly defined, being used both in geochemistry and biological sciences for chemical elements that occur in the Earth's crust in amounts less than 0.1% (1000 mg/kg) [1]. Despite their "low" content in the human body, TEs are components of a complex physiological system involved in the regulation of vital functions at all stages of development of the living

Throughout time, limited attempts have been made for classifying trace elements. In 1973, WHO [4] classified 19 trace elements into three groups: (i) essential elements: zinc, copper, selenium, chromium, cobalt, iodine, manganese, and molybdenum; (ii) probably essential elements; and (iii) potentially toxic elements. Shortly after, Frieden [5] considered 29 types of elements present in the human

i.Group I: basic components of macromolecules such as carbohydrates, proteins, and lipids. Examples include carbon, hydrogen, oxygen, and nitrogen;

ii.Group II: nutritionally important minerals also referred to as principal or macroelements. The daily requirement of these macroelements for an adult person is above 100mg/day. Examples include sodium, potassium, chloride,

iii.Group III: essential trace elements. The trace elements are also called minor elements. An element is considered a trace element when its requirement per day is below 100mg/day. The deficiency of these elements is rare but may prove fatal. Examples include copper, iron, zinc, chromium, cobalt, iodine,

iv.Group IV: additional trace elements. Their role is yet unclear and they may be essential. Examples include cadmium, nickel, silica, tin, vanadium, and aluminum. This group may be equivalent to probably essential trace ele-

v.Group V: these metals are not essential and their functions are not known. They may produce toxicity in excess amounts. Examples include gold, mercury, and lead. This group is equivalent to potentially toxic elements defined

More recently, Frieden [6] proposed a biological classification of trace elements based on their amount in tissues: (i) essential trace elements: boron, cobalt, copper, iodine, iron, manganese, molybdenum, and zinc; (ii) probably essential trace elements: chromium, fluorine, nickel, selenium, and vanadium; and (iii) physically

As TEs play a significant role in the regulation of many important adaptive mechanisms, including the functioning of all vital systems of the organism, the balance of each element in an optimum range of concentrations is fundamental. The chronic deficiency of essential TEs can, therefore, result in metabolic disturbances and distinct clinical and morphological changes; on the other hand, we must

promoter trace elements: bromine, lithium, silicon, tin, and titanium.

**102**

The main advances in trace element research have been made in soil sciences since soils are considered the most important environmental compartment functioning as a sink for TEs [7–9]. Trace elements are usually distributed over different soil compartments and their retention will depend on several soil characteristics, as well as the parent rock material.

The main soil characteristics include pH, cation exchange capacity (CEC), particle size distribution, electrical conductivity, and organic matter content [10, 11]. These soil properties can promote the accumulation of TEs in soils or their depletion. The most adequate pH for the maximum TE availability is within 6.0–8.0; however, some TEs such as manganese, iron, boron, copper, and zinc are more available to plants when the soil is acidic (pH between 4.5 and 6.5), which contributes to manganese and boron toxicities in plants growing on acidifying soils [12].

The CEC is also a very important soil property as it can influence the soil structure stability, nutrient availability, soil pH, and the soil reaction to fertilizers and other ameliorants [13]. For example, negatively charged sites increase the CEC, holding H+ , Ca2+, Mg2+, Na+ , and NH4+, while the positively charged sites increase the retention of OH<sup>−</sup>, SO4 <sup>−</sup>, NO3 <sup>−</sup>, and PO4 <sup>−</sup> [14]. All these soil properties, either combined or isolated, can promote the accumulation or the leakage of TEs in soils.

The parent rock material also assumes high importance in TE availability; when parent materials have high trace element concentrations, the resulting soils also have high or even higher TE concentrations, particularly when the former also result from anthropogenic activities, such as agriculture [15].

Considering that specific soil characteristics can affect the TE availability, the use of universal background concentrations is inadequate, as it may not reflect the "normal" values for specific regions. In this way, each country should determine the background levels for each region with different geological substrates and establish normative values for environmental legislation based on these studies, avoiding misinterpretation of abnormally low or high TE contents [16].
