Sewage sludge biochar: Nutrient composition and its effect on the leaching of soil nutrients

•Higher-temperature biochar contained less N but more P and K.•Higher-temperature biochar contained less water-soluble N, and more water-soluble P and K.•Adding biochar increased soil nutrients.•Adding biochar could retain soil nutrients in a long term.

Biochar was prepared by the pyrolysis of sewage sludge at temperatures ranging between 300 and 700 °C. Extraction and column leaching experiments were then conducted to investigate the amount of nitrogen (N), phosphorus (P), and potassium (K) as well as the water-soluble contents of the nutrients in the sewage sludge biochar. In addition, the retention effect of these sewage sludge biochar samples on the leaching of ammonium (NH4+), nitrate (NO3−), phosphate (PO43 −), and potassium (K+) ions from Typic Plinthudult soil was determined. The higher-temperature sewage sludge biochar contained less N but more P and K, as well as less water-soluble N, and more water-soluble P and K. As nutrients were released from this sewage sludge biochar, so the nutrient value of the Typic Plinthudult soil was enhanced. Although the sewage sludge biochar produced at 300 °C had little effect on the retention of NO3−, the sewage sludge biochars produced at 500 and 700 °C reduced the Typic Plinthudult soil leaching of NH4+, NO3−, PO43 −, and K+. However, this retention effect varied little when the cumulative water volume exceeded 1000 mL. Thus, adding the sewage sludge biochar to the Typic Plinthudult soil might be advantageous in retaining nutrients in the long term.