Beneficial Effects of Fulvic Acid on Activating Phosphorus and Stimulating <i>pho</i>D Bacteria Were Greatly Weakened by Soil Drying in Calcareous Soils

Organic amendments are widely used to activate soil phosphorus (P) and stimulate <i>pho</i>D-harboring bacteria. However, their effects on P dynamics and the <i>phoD</i> community under soil drying remain unclear. Here, we investigated the effects of fulvic acid on P transformation and the <i>phoD</i> 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 <i>pho</i>D abundance. It also strengthened the <i>pho</i>D 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 <i>pho</i>D bacteria in calcareous soils.