The thermal conversion of sewage sludge to biochar offers a promising alternative use for a hazardous waste material with potential benefits to agricultural productivity and soil quality. Three short-term greenhouse experiments were conducted to evaluate the effect of corn cob (CC) and sewage sludge (SS) biochars, with their anaerobically treated counterparts, on soil properties and plant growth in an infertile Oxisol. The anaerobically treated SS biochar showed the greatest concentration of bioavailable essential nutrients, but treatment only resulted in increased yields for the SS biochar in the first crop in the absence of added fertilizer. Both CC and SS biochars in combination with fertilizer doubled plant growth compared to the control in the first crop cycle, produced no significant effect in the second cycle, and more than tripled plant growth for the SS biochars in the third cycle. High ash content with high nutrient contributions (especially P) and a persistent liming effect explain the benefits of the SS biochars to plant growth. The SS biochar showed promise in mitigating the negative effects of soil Mn toxicity. Sewage sludge biochars reduced Cd bioavailability and had no significant effect on the bioavailability of other potentially toxic metals compared to the control.
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