Long-term phosphorus fertilization alters soil organic matter molecular composition via lowers microbial carbon and energy use efficiency in a temperate cropland

Underlying mechanisms via phosphorus (P) fertilization driving soil organic matter (SOM) formation and stabilization remain largely unclear. In this study, we employed a suite of biomarkers (i.e., free and bound lipids, lignin phenols, PLFAs, neutral and amino sugars), ¹³C NMR techniques, and soil extracellular enzyme activities to investigate SOM characteristics in response to an 18-year P fertilizer gradient (i.e., 0, 50, 190 kg P ha⁻¹ yr⁻¹, defined as P₀, P_L, and P_H) down to 60 cm depth in Northeast China. Despite limited changes in soil organic C, P fertilization distinctly modified the SOM signatures (e.g., molecular composition, degradation, and source) across soil profile (particularly within 20 cm of topsoil). On average, P additions increased plant-derived free lipids by 10.5–48.6% and microbial-derived free lipids by 39.8–49.0% in this topsoil compared to P₀. P enrichment increased cutin compounds by 21.9–44.7% while decreased suberin compounds by 21.3–35.1% as compared to control in the topsoil. P_L enhanced lignin phenols by 53.1% relative to P₀ in the topsoil due to high plant C input. Compared to control, P fertilization reduced microbial-derived neutral sugars by 20.0–30.6% and the plant-derived neutral sugars by 36.6-37.0% in the topsoil relative to P₀. Moreover, bacterial necromass carbon decreased by 10.8–23.8% and fungal necromass carbon by 7.9–27.9% under P fertilization, driven by enhanced microbial residue decomposition via elevated residue-decomposing enzyme activities. Using stoichiometric theory, we estimated that P fertilization reduced microbial carbon use efficiency by 4.3–8.6% and energy use efficiency by 4.7–9.5%. Overall, P fertilization marked by more reduced compounds (e.g., lipids, lignin phenols) and fewer oxidized ones (e.g., microbial necromass, neutral sugars), reduced the nominal oxidation state of C for SOM by 5.6–15.6%, which corresponded to lower microbial energy and carbon use efficiency.  These findings suggest that P fertilization alters SOC composition by modulating plant- and microbial- derived C contributions and their turnover. Such findings are critical for advancing our understanding of SOM stabilization and microbial-driven SOC dynamics in P-fertilized agroecosystems.