Beneficial Effects of Fulvic Acid on Activating Phosphorus and Stimulating phoD Bacteria Were Greatly Weakened by Soil Drying in Calcareous Soils

Organic amendments are widely used to activate soil phosphorus (P) and stimulate phoD-harboring bacteria. However, their effects on P dynamics and the phoD community under soil drying remain unclear. Here, we investigated the effects of fulvic acid on P transformation and the phoD community under soil drying (100 → 60 → 20% water-holding capacity (WHC)). Soil drying accelerated the transformation of labile-P (CaCl₂-P, citrate-P, Ca₂-P) and moderately labile-P (Ca₈-P) to recalcitrant P (occluded-P, Ca₁₀-P), thereby decreasing the integrated P availability index (IPAI) by 49.4–54.1%. FA increased microbial biomass-carbon (MBC), -phosphorus (MBP), and phoD abundance. It also strengthened the phoD community’s network complexity. However, FA’s beneficial effects were attenuated along the soil drying gradient. Overall, the beneficial effect of FA in activating soil P was greatly counteracted by soil drying. Our findings highlight that maintaining optimal moisture (60%WHC) is essential to realize FA’s potential in activating soil P and recruiting phoD bacteria in calcareous soils.