One-time application of struvites, ashes and superphosphate had no major impact on the microbial phosphorus mobilization capabilities over 15-months in a grassland 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. Here, a grassland field trial was established with 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 (replicates n = 5). The impact of these RDFs on the soil microbial P cycling community was compared to conventional mineral P-fertilizer and a P-free control. Topsoil samples were taken directly after Lolium perenne grass cuts at months 3, 5 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 taxa were affected by the treatments. The ash treatments resulted in significantly higher relative abundance of Bacillota and Rokubacteria and lower relative abundance of Actinomycetota. Sewage-sludge ash had significantly lowest abundances of genera Bacillus and Bradyrhizobium that are well known for their P cycling abilities. The struvite RDFs either positively influenced the P cycling microbial community as demonstrated through higher tri‑calcium phosphate solubilizing capabilities (month 3), or were similar to the superphosphate and P-free treatment. From a soil-microbial health perspective, the presented findings indicate that struvites are a suitable substitute for superphosphate fertilizers.