Potential short-term negative versus positive effects of olive mill-derived biochar on nutrient availability in a calcareous loamy sandy soil

In this study, a greenhouse pot experiment with maize (Zea mays L.) was conducted using treatments consisting of a control (CK), inorganic fertilizer of NPK (INF), and 1% and 3% (wt/wt) of olive mill solid waste (OMSW)-derived biochar (BC) at various pyrolytic temperatures (300–700 °C). The goal was to investigate potential negative versus positive effects of BC on pH, electrical conductivity (EC), and nutrient (P, K, Na, Ca, Mg, Fe, Mn, Zn, and Cu) availability in a calcareous loamy sandy soil. The results showed that application of OMSW-derived BC, especially with increasing pyrolysis temperature and/or application rate, significantly increased soil pH, EC, NH4OAc-extractable K, Na, Ca, and Mg, and ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA)-extractable Fe and Zn, while AB-DTPA-extractable Mn decreased. The application of 1% and 3% BC, respectively, increased the NH4OAc-extractable K by 2.5 and 5.2-fold for BC300, by 3.2 and 8.0-fold for BC500, and by 3.3 and 8.9-fold for BC700 compared with that of untreated soil. The results also showed significant increases in shoot content of K, Na, and Zn, while there were significant decreases in shoot content of P, Ca, Mg, and Mn. Furthermore, no significant effects were observed for growth of maize plants as a result of biochar BC addition. In conclusion, OMSW-derived BC can potentially have positive effects on the enhancement of soil K availability and its plant content but it reduced shoot nutrients, specifically P, Ca, Mg, and Mn; therefore, application of OMSW-derived BC to calcareous soil might be restricted.