Impact of pyrolysis temperature on phosphorus plant availability in biochar—A pot experiment using ³³ P dilution

Separation and pyrolysis of the solid fractions of biogas digestate and animal slurry offer potential solutions to environmental and logistical challenges associated with direct slurry application as fertilizer. However, thermochemical transformations during pyrolysis typically reduce P availability. This study evaluated biochars produced at 400°C, 500°C, and 600°C from the solid fractions of biogas digestate (BDF) and pig manure (PMF) for their P-fertilization effects using a pot experiment with perennial ryegrass (Lolium perenne var. Soriento) and the ³³P dilution approach. The ryegrass biomass across two harvests remained similar for all biochar treatments but was significantly lower than for the mineral fertilizer (KH₂PO₄) treatment. Significant differences were evident in P contribution from biochars and raw feedstocks, as well as in total P uptake rates between treatments. The readily available P contents of biochar and P-recovery rates in plant shoots were negatively correlated with pyrolysis temperature, which was especially pronounced for digestate-derived biochars. All materials except high-temperature biochar (600°C) had mineral fertilizer replacement values exceeding 50%, indicating substantial P-recycling potential. Biochars produced at 400°C and 500°C had a similar fertilizer value as their original feedstocks. Therefore, low-temperature pyrolysis of separated solid fractions represents a promising approach that preserves the P fertilizer value while providing climate benefits through soil C sequestration and reduced energy requirements for transport.