Impacts of (bio)acidification with citric acid or anaerobic fermentation on phosphorus recycling: solubility and diffusion of raw and pyrolyzed biowastes in soil

Recycling phosphorus (P) from biowaste can improve nutrient circularity. However, low P bioavailability often requires H2SO4-acidification, which is complex to handle and prohibited in organic agriculture under European Union regulations. We tested acidification with citric acid (CA, 0–15%), and bioacidification with either dairy (Arla Cultura®) or silage inoculant (BONSILAGE®FORTE), with and without wheat flour, to increase water-extractable P (WEP) of meat and bone meal (MBM), digestate solid fraction (DSF) from biogas production, and their biochars (BCs). Additionally, 10% citric acid- or Arla + wheat-treated biomaterials were incubated in a one-dimensional slicing device to assess pH and WEP in the fertilized and adjacent (0–8 mm) soil layers. (Bio)acidification decreased pH (up to − 3.28 for BONSILAGE + wheat DSF-BC) and increased WEP (up to + 20.8 g kg–1 for 15% CA DSF), being less efficient in BC variants. When incubated, bioacidified materials promoted comparable or higher soil WEP to citric acid, except for MBM, whose soil WEP was higher under citric acid (+ 134.4 ± 19.5 mg kg–1). While relevant to P recovery, further validation of formulations, agronomic performance and safety is needed to ensure compatibility with organic farming frameworks.