**2. Sources of metallic trace elements**

#### **2.1 Natural sources**

Soil naturally contains metallic trace elements. They are generally in the natural pedogeochemical background (FPGN) resulting solely from the geological and pedological evolution of the soil. This natural pedogeochemical background varies greatly depending on the nature of the rock and the type of soil that has developed there [2, 3]. Other natural phenomena such as wind erosion and volcanic activities can transport metallic trace elements from one soil to another [4]. Indeed, according to Ilyinskaya and al [5], in 2018, emissions from a volcano in Hawaii had concentrations of cadmium, lead, and zinc respectively of 4.10−3 μg.m−3, 2.10−3 μg .m−3, 0.04 μg.m−3. In addition, Ma and al in 2019 [6] showed in volcanic ash, the presence of chromium, copper, lead, zinc and manganese respectively at concentrations of 0.824 mg.kg−1, 0.95 mg. kg−1, 16.16 mg.kg−1, 367 mg.kg−1, 518.6 mg.kg−1.

#### **2.2 Anthropogenic origins of trace metals in soils**

The main anthropogenic sources of increased fluxes of heavy metals in soils are urban and industrial discharges as well as agricultural activities [7, 8].

With regard to pollution linked to agricultural activities, inputs of trace metals come mainly from the spreading of NPK and phosphate fertilizers [9]. These types of fertilizers generally provide agricultural soils with cadmium, arsenic, chromium and lead. In addition, the application of certain pesticides, and the use of sewage sludge, untreated industrial wastewater, and landfill compost to amend agricultural plots can be an important source of chromium, molybdenum, lead, zinc, manganese, arsenic, copper, mercury, uranium, and vanadium, copper, nickel, and lead to the soil [1, 10, 11].

Industrial activities such as mineral processing, refining, galvanisation, manufacturing of electric batteries, pigments and plastics are releasing large amounts of trace metals into the environment [13].

In road traffic, the wear of car tyres and exhaust gases are also sources of nickel and zinc emissions, and lead [12–14].
