The impact of struvite and ash recycling-derived fertilizers on microbial phosphorus mobilization capabilities and community structure in a Lolium perenne field trial

Rock phosphate is a non-renewable primary source for mineral phosphorus (P) fertilizers that intensive agriculture is highly dependent on. To avoid P fertilizer shortages and limit negative environmental impacts, circular economy approaches are needed with recycling-derived fertilizer (RDF) applications such as struvites and ashese. Hence, a grassland field trial was conducted with four RDFs, two struvites (potato wastewater, municipal wastewater) and two ashes (poultry-litter ash, sewage-sludge ash) at a P application rate of 40 kg P ha-1 (n=5). The impact of the RDFs on the soil microbial P cycling community was compared to conventional mineral P-fertiliser and a P-free control. Topsoil samples were taken directly after Lolium perenne grass cuts at months three, fife and 15. Cultivable phosphonate and phytate utilizing bacteria, potential acid and alkaline phosphomonoesterase activity, and phoC and phoD copy numbers responded stronger to seasonal effects than treatment effects. No significant overall effect of the fertilizer application was detected in the beta diversity of the bacterial and fungal communities after 15 months, but individual phylogenetic groups were affected by the treatments. The ash treatments demonstrated some distinguishing results across the bacterial and fungal community, with significantly higher relative abundance of Firmicutes and Rokubacteria and lower relative abundance of Actinobacteriota. Sewage-sludge ash had significantly lowest abundances of genera Bacillus and Bradyrhizobium (month 15s) that are well known for their P cycling abilities. The struvite RDFs either positively influenced the P cycling microbial community or did not affect it at all, while demonstrating better tri-calcium phosphate solubilizing capabilities after the 3 months harvest. These findings indicate that struvites could be a suitable replacement for conventional P fertilizers in the future.