Phosphorus Use Efficiency and Canopy Spectral Reflectance of Alfalfa Fertilized with Aquaculture-Derived Bio-Based Fertilizers in an Andisol

Aquaculture-derived bio-based fertilizers (BBFs) represent a promising alternative to inorganic P in Andisols for sustainable alfalfa (Medicago sativa L.) cultivation. However, their agronomic performance and physiological impacts on alfalfa remain poorly understood. This study evaluated three BBFs, consisting of composted fish sludge (CFS), dried fish sludge (DFS), and fish bone meal (FBM), in comparison with inorganic P (InoP) and a zero-P control (NoP). Forage yield (FY), P use efficiency (PUE), spectral canopy indices, and leaf gas-exchange parameters were assessed across five harvests in a Mediterranean environment. Results showed significant differences among fertilizer types driven by their distinct P release dynamics. DFS consistently maintained stable leaf P concentrations, enhanced PUE and P uptake, and lead to higher FY, improved photosynthesis, and water use efficiency (WUE). It performs statistically similarly to the inorganic P. In contrast, CFS released P too slowly, which in turn lowered leaf P concentration, P uptake, and PUE, resulting in the lowest photosynthesis and WUE. FBM produced intermediate responses but maintained WUE comparable to inorganic fertilizer. Gas-exchange measurements demonstrated that photosynthesis ranged from 9.01 to 16.7 μmol m−2 s−1, with no significant difference between DFS and inorganic P. Transpiration remained stable across BBF treatments (mean 3.2 mmol m−2 s−1). The canopy reflectance indices such as RARS, Gite2, and PSSR proved to be strong predictors of both P concentration and PUE in alfalfa. In conclusion, DFS emerged as the most efficient BBF that matched inorganic fertilizer to enhance P nutrition, plant physiology, and FY. These findings highlight the potential of aquaculture-derived BBFs, particularly DFS as sustainable P sources for improving alfalfa productivity while reducing reliance on synthetic fertilizers.