From fertilizer form to microbial function: soil pH controls nitrogen cycling pathways under conventional and recovered phosphorus fertilization

The transition towards circular nutrient management requires fertilizers that enhance nitrogen (N) use efficiency while minimizing environmental losses. Struvite, a recovered magnesium ammonium phosphate, is increasingly proposed as an alternative to conventional mineral fertilizers such as monoammonium phosphate (MAP). However, the extent to which fertilizer chemistry interacts with soil properties to regulate microbial functioning and N cycling processes remains insufficiently understood.In this study, we investigated short-term N transformations, microbial activity, and greenhouse gas emissions in two agricultural soils differing in pH (acidic and alkaline) following fertilization with struvite and MAP. Soils were incubated under controlled conditions, and temporal changes in mineral N forms, soil chemical properties, and CO₂ and N₂O emissions were monitored. Microbial respiration and growth were quantified to assess microbial carbon use efficiency (CUE) and biomass turnover. To resolve underlying process rates beyond net fluxes, stable isotope techniques were applied to quantify gross ammonium and nitrate production and consumption, allowing the calculation of microbial nitrogen use efficiency (M-NUE).Fertilizer effects were strongly regulated by soil pH. In acidic soil, struvite promoted a more gradual and microbially efficient N turnover compared to MAP, characterized by distinct ammonium and nitrate transformation pathways and higher M-NUE. In alkaline soil, N cycling was dominated by rapid nitrification, which reduced functional differences between fertilizer types. Across both soils, fertilizer-specific shifts in microbial growth, CUE, and biomass turnover revealed changes in microbial resource allocation and N processing pathways.By integrating gas flux measurements, microbial efficiency indicators, and stable isotope–derived gross N transformation rates, this study highlights how soil chemical context governs the biogeochemical performance of recovered fertilizers. Our findings emphasize the need to account for soil pH and microbial functioning when optimizing the use of struvite and other circular fertilizers in sustainable agricultural systems.