Changes in phosphorus due to pyrolysis and in the soil-plant system amended with sewage sludge biochar compared to conventional P fertilizers: A global meta-analysis

Abstract

Phosphorus (P) plays an essential role for plant growth, but conventional P sources used in agriculture are finite and non-renewable. As a result, there is a growing need to explore alternative P sources such as sewage sludge (SS) - a P-rich solid waste and valuable renewable resource that is often mismanaged globally. Pyrolysis is a promising technique for managing SS. In this meta-analysis, we evaluated the pyrolysis effect on P concentration and fractions in SS using 381 paired observations from 77 peer-reviewed articles. Moreover, we assessed the impacts of SS biochar amendments on soil P pools and crop productivity, as well as the experimental factors influencing its efficacy. Our results indicate that pyrolysis significantly affects P solubility in SS biochar (SSB), reducing water-extractable P and slowing its release, while increasing P availability in NaOH and HCl extracts. Pyrolysis also leads to significant increase in apatite P (AP) by 188%, inorganic P (IP) by 107%, total P (TP) by 69%, non-apatite P (NAIP) by 33.9%, while reducing organic P (OP) by 65.2% compared to the original SS. Higher pyrolysis temperatures enhance the conversion of NAIP to AP. Biochar application increased soil available P content by an average of 324%, with the most significant effects at higher application rates (>20 t ha^-1) and in medium to fine-textured soils. Crop productivity improved by 50.5% with biochar application compared to non-fertilized controls, especially for maize and wheat. Overall, SSB application results in crop productivity similar to fertilized controls. Thus, SSB shows promise as a slow-release P fertilizer, offering long-term benefits for both crop productivity and soil health.

Keywords: Chemical transformation of P; P pools; P speciation; Pyrolysis temperatures; Sewage sludge biochar; Slow-release P fertilizer.