Direct and legacy effect of sewage sludge biochar on soil phosphorus pools: evidence from a ten-year assessment

The use of sewage sludge biochar (SSB) in agricultural systems is a promising strategy for nutrient recycling, as it can serve as a multi-nutrient fertilizer, particularly a source of phosphorus (P). However, P release from SSB is associated with biochar aging in soil, highlighting the need to understand its temporal effects. The effects of biochar on soil P pools may vary depending on pyrolysis temperature and evaluation period. The objective of this study was to assess the direct and legacy effects of SSB on soil P pools over a sequence of 10 corn growing seasons. The following treatments were evaluated: control (no biochar and no mineral fertilization), mineral fertilization, SSB pyrolyzed at 300 °C and 500 °C. Biochar was applied only during the first two years, with residual effects persisting over the remaining eight years. Soil total P and its fractions, including organic P, inorganic P, and available P, were determined. Overall, both biochars increased total P and produced higher values across the different P pools than NPK and the control. The positive effects of biochar on soil P pools were pronounced up to the fifth growing season. No significant differences in soil P fractions were observed between SSB produced at 300 °C and 500 °C. Despite differences in soil P contents, the average corn grain yield was not affected by the treatments. These results improve understanding of the direct and residual effects of SSB on soil P fraction dynamics and provide a basis for establishing agronomic recommendations for using SSB as an alternative P source.