Abstract
Mineral fertilizers containing phosphorus (P) are widely used in agriculture to enhance soil fertility, yet their impact on soil microbial communities remains unclear. Most studies rely on a single-site approach and observe short-term effects, limiting our ability to extricate long-term P impacts from other drivers of microbial communities. This study aimed to explore the influence of long-term P fertilization on soil microbial communities along the depth gradient across four countries, including grasslands and arable sites. Microbiomes were strongly affected by location, land management, and soil depth. No changes in the composition of prokaryotic communities were detected in response to P fertilization, while fungal communities demonstrated a modest response, but only at 0–10 cm depth in grassland soil. The almost complete absence of P fertilization impacts on communities could be due to a lack of changes in soil properties and nutrient availability after P fertilization. This is likely because of the applied P rate being below the threshold needed to alter soil properties or as a consequence of a legacy from previous P fertilization events. The strongest effect of P fertilization was observed at 0–10 cm depth, where carbon (C) and nitrogen (N) availability was higher. Together, these findings suggest that soil microbial communities are largely resistant to long-term P fertilization, with responses strongly mediated by site-specific conditions. Our study highlights the importance of considering multiple long-term sites, land use types, and soil depths in research focused on P fertilization effects on soil microbial communities.