Direct and residual effect of biochar derived from biosolids on soil phosphorus pools: A four-year field assessment

Measures to improve the use of phosphorus (P), either by improved efficiency or reuse, are needed worldwide in order to preserve a finite resource and ensure that farmers have access to it. Currently, the rapidly growing global population has generated an increased demand for this mineral. Sustainably disposing for the massive amount of globally produced biosolids and alternative sources of P for agriculture are two major challenges to address. In this scenario, biosolids-derived biochar (BBC) has been presented as a win-win opportunity. However, the BBC-P dynamics in soil over consecutive cropping seasons remain unclear. Direct (first and second cropping season) and residual (third and fourth cropping season) effects of BBC on soil P fractions, P uptake and corn grain yield were assessed. Additionally, the relationships between soil P pools and grain yield were investigated by multivariate and multiple linear regression analysis. In a field experiment, BBC produced at 300 °C (BC 300) and 500 °C (BC 500) were applied to an Oxisol at a rate of 15 t ha^-1. Soil total P and its fractions (organic P, inorganic P, and available P) were determined. Phosphorus uptake and corn grain yield were also evaluated. BBC, regardless of pyrolysis temperature, increased soil total P and all P fractions. Moreover, BBC maintained high soil P contents for at least two years after stopping its application. These results suggest that BBC may act as a slow-release P fertilizer. Surprisingly, soil P fractions were unaffected by different pyrolysis temperatures, but BC 300 promoted higher grain yield than BC 500 in the third and fourth cropping seasons. Overall, the results confirmed that under direct application both biochars can replace mineral fertilization for corn production; and when considering the residual effect, BC 300 showed a higher potential to be utilized as a soil amendment for P supply.