Particle size determines the short-term phosphorus availability in biochar produced from digestate solids.

Biochar pellets produced from the solid fraction of manure-based digestates are rich in phosphorus (P) and may represent a P source that is easy to handle and suitable for transport to P-deficient regions. However, the effect of feedstock composition and particle size on P availability in this type of biochar remains unexplored. To evaluate the effect of particle size on the short-term P availability in biochars derived from manure digestate solids, an incubation experiment was carried out, in which four biochars produced from digestate solids in powder and pellet form were incubated with three soils of low P content. The recovery of P in bicarbonate and water extracts was measured after 7 and 70 days of incubation. A subsequent pot experiment with barley on two of the soils was aimed at examining early crop recovery of P, comparing the effects of coarse and fine biochar particles. Biochars from digestate solids had total P contents ranging from 12 to 63 g kg^-1. In all three soils, the recovery of P in water and bicarbonate extracts was lower after incubation with biochar pellets compared to powder, and P recovery remained constant or decreased slightly over time. Early shoot biomass and P recovery in barley were also higher when fine biochar particles were applied. The results suggest that particle size reduction improves the immediate availability of P in biochars produced from manure-based digestate solids.