Effects of novel P fertilizers on microbial abundance related to N and P cycling in two on-farm systems

Phosphorus (P) is an essential macronutrient element for plant growth and development. Its limited availability makes alternative P sources crucial for fertilizer production. This study investigated the effects of three recycling-derived fertilizers with varying P solubility on microbial nutrient turnover at two fields in central Germany, Kiebitzbreite and Schmatzfelder Breite, which differ in management practices and soil characteristics. Samples were collected during the stem elongation stage of winter wheat from bulk soil and rhizosphere. Fertilization treatments included traditional triple superphosphate (TSP) and a no-P control (P0) for comparison. The abundance of microorganisms involved in P and Nitrogen (N) turnover was assessed by quantitative real-time PCR. Potential acid and alkaline phosphatase activity, mycorrhizal colonization rate, Carbon (C) to P, N to P ratios in the soil and the plant, and water-extractable P were measured. Although all treatments received the same amount of P, the differing solubilities of the fertilizers significantly affected water-extractable P levels, while nutrient ratios in the plant biomass remained comparable among sites and fertilizer treatments. However, the microbial strategies for maintaining P levels varied significantly across the sites. At the Kiebitzbreite, the site with silty loam texture and deep plowing, high ratios of available C and N to P in the soil were accompanied by high alkaline phosphatase activity and a larger abundance of arbuscular mycorrhizal fungi in the rhizosphere. Conversely, P solubilization was more pronounced at Schmatzfelder Breite, a site with finer soil texture managed by deep chiseling. Notably, the fertilization treatments influenced not only the abundance of bacteria catalyzing P turnover but also those catalyzing major steps of the N cycle, especially at Schmatzfelder Breite, where higher P solubility led to increased bacteria involved in N mineralization. This non-targeted effect on N cycling underscores the importance of fertilizer type, beyond just P supply, in influencing broader nutrient turnover dynamics. Our findings suggest that recycling-derived P fertilizers are promising alternatives to conventional P sources, though their on-farm impacts on microbial nutrient turnover vary significantly with site conditions and management.