Evaluating plant-available P methods and quality characterization of biochar derived from manure and sewage sludge

Phosphorus(P) is essential yet limited, and Norway, like the rest of Europe, remains dependent on imported phosphate rock. Biochar from pyrolysis organic residues offers a potential alternative P source, although quantifying plant-available P is complicated by diverse P speciation. In this study, biochar produced at 400 °C from raw manure, manure digestate, and biologically treated sewage sludge digestate was evaluated using multiple P extraction methods (Olsen, Mehlich-3, ammonium lactate, citric acid, water, oxalate, dithionite, nitric acid). Pot experiments with wheat, spinach, and faba bean were conducted to assess the relationship between extracted P and plant uptake. Results showed that manure-derived biochars had higher agronomic value than sewage sludge biochar, which was limited by Fe/Al-bound P and heavy-metal contamination. Concentrated nitric acid digestion best predicted wheat and faba bean P uptake, while the more common extractants for agronomic evaluation (Mehlich-3, Ammonium Lactate, Olsen, DI-water extracts correlated most strongly with spinach uptake. Digested manure biochar met regulatory requirements for trace metals, but raw manure and sewage sludge biochar exceeded Ni and Zn thresholds, respectively. These findings highlight the need for a tiered approach to biochar P testing and for regulatory frameworks that balance environmental safety with the potential for nutrient recycling.