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The Mediterranean countries are the largest producers of olives with Spain taking the lead in olive oil production. A two-phase extraction system is used to produce oil and dry olive residue (DOR), a waste product. DOR biochar was tested as an amendment for contaminated soils to reduce the trace element (TE) contents in crops. A DOR sample was transformed into biochar at 350°C and 500°C, and a pot experiment was conducted, where spring wheat was grown. Moreover, the mutual effect of biochar application and arbuscular mycorrhizal fungi (AMF) inoculation was assessed. The results showed the decreasing extractable proportions of Cd in the treated soils, whereas an ambiguous effect of DOR biochar on the mobility of As, Pb, and Zn in soil was observed. The changes in TE in the treated soils were related to enhanced soil pH due to the biochar application. Stepwise increases in extractable soil potassium (K) proportions were determined because of the high content of K in DOR. The element contents in wheat plants were affected by an interaction of the soil element contents and pH, and biochar pyrolysis temperature. The AMF inoculation did not affect the biochar-induced changes in element fate in the soils. The results proved the ability of DOR-based biochar to serve as the source of nutrients, especially K. However, further research is necessary to test a wider range the application rates of biochar, as well as the long-term fate of biochar in the treated soils.

A model small-scale field experiment was set up to investigate selenium (Se) uptake by four different varieties of broccoli plants, as well as the effect of Se foliar application on the uptake of essential elements for plants calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), phosphorus (P), sulfur (S), and zinc (Zn). Foliar application of sodium selenate (Na2SeO4) was carried out at two rates (25 and 50 g Se/ha), and an untreated control variant was included. Analyses of individual parts of broccoli were performed, whereby it was found that Se in the plant accumulates mainly in the flower heads and slightly less in the leaves, stems, and roots, regardless of the Se rate and broccoli variety. In most cases, there was a statistically significant increase of Se content in all parts of the plant, while there was no confirmed systematic influence of the addition of Se on the changing intake of other monitored elements. Selenization of broccoli leads to an effective increase in the Se content at a rate of 25 g/ha, whereas the higher rate did not result in a substantial increase of Se content compared to the lower rate in all varieties. Therefore, the rate of 25 g/ha can be recommended as effective to produce broccoli with an increased Se content suitable for consumption. Moreover, Se application resulted in an adequate increase of the main organic compounds of Se, such as selenocystine (SeCys2), selenomethionine (SeMet), and Se-methylselenocysteine (Se-MeSeCys).

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