Strontium (Sr) and Molybdenum (Mo)
The contents of strontium in the surface and subsurface horizons are 125.9 mg·kg–1 (Table 3). The mean worldwide background concentration for Sr in soil is 147.9 mg·kg–1 (Kabata-Pendias 2011) and in Castilla-La Mancha it is 380.0 mg·kg–1, although in this Community values of up to 3384.9 mg·kg–1 have been found in soils on gypsum or gypsum marls (Conde et al. 2008). Alcubillas is an active agricultural area and therefore the levels determined cannot be attributed to the use of soil amendments (e.g., phosphorus fertilizers). Various anthropogenic activities, including the application of fertilizers, the generation of nuclear power, and the burning of coal to generate power, have led to increased levels of strontium (Sr) in soil (Burger and Lichtscheidl 2019). The great potential for the accumulation of Sr in plants and animals can pose serious environmental and human health hazards, which include cancers and disorders of the nervous system (Burger and Lichtscheidl 2019). The average Sr contents in some materials commonly used in agriculture include 80.9 mg·kg–1 in manure, 75.9 mg·kg–1 in sewage sludge, and 270.9 mg·kg–1 in industrial sludge (Munthali et al. 2015). Phosphate fertilizers can also act as a local source of Sr contamination in soil (Abdi et al. 2015). It is worth highlighting that all Sr concentrations for all soil types in this study were below the background value, thus indicating that there is no Sr pollution in the study area.
Molybdenum is involved as a micronutrient anion in various physiological and enzymatic processes in plants (Mishra et al. 2006) and humans (Schwarz and Belaidi 2013). Sr is able to affect the health of plants and animals because in high concentrations it is a potentially toxic metal (Evans and Barabash 2010). The average Mo content in the surface soils of Alcubillas was 0.9 mg·kg–1 and in the subsurface soils it was 1.0 mg·kg–1 (Table 3), with a maximum level of 1.3 mg·kg–1. In comparison to the pedogechemical references for Castilla-La Mancha these values are lower or similar and they are also lower than the mean background content of Mo in surface soils as a worldwide average (1.1 mg·kg–1, Kabata-Pendias 2011).