Sewage sludge biochars effects on corn response and nutrition and on soil properties in a 5-yr field experiment

Abstract Sewage sludge biochar (SSB) is the solid product obtained after the pyrolysis of sludge. In the last ten years, hundreds of studies have evaluated the effects of SSB as a soil amendment over a short period of time. Thus, doubts remain about the temporal behavior of SSB under field conditions. The objective of this study was to evaluate, under field conditions, the effects of SSB on the fertility indicators (pH, potential acidity, Mehlich1-P and -K, CEC and KCl-exchangeable Ca, Mg and Al) and corn yield in an Oxisol over five years. For this purpose, soils receiving biochar produced at 300 and 500°C (SSB300 and SSB500, respectively) and biochar in combination with mineral fertilization (SSB300 + NPK and SSB500 + NPK) were evaluated. Grain yield, soil pH, soil H + Al, and macronutrient contents (N, P, K, Ca, Mg and S) in soil and corn leaves were determined. Over five years, SSB increased total N and Mehlich1-P in the soil. Nevertheless, biochars were unable to raise soil pH and provide satisfactory amounts of K. The combination of SSB with mineral NPK fertilizer provided all the macronutrients required for corn and increased the leaf contents, especially for P, leading to an average 16% increase in grain yield in soils that received the treatment SSB300 + NPK. In general, pyrolysis temperature did not alter the parameters evaluated. SSB has the potential to provide nutrients to the soil, with a residual effect of at least three years, which can be enhanced by combined application with mineral NPK fertilizer.