Effect of Dry Olive Residue–Based Biochar and Arbuscular Mycorrhizal Fungi Inoculation on the Nutrient Status and Trace Element Contents in Wheat Grown in the As-, Cd-, Pb-, and Zn-Contaminated Soils

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.