Legacy effects of long-term bird droppings input promoted the activation of soil inorganic phosphorus and associated microorganisms in coastal wetlands

Coastal wetland reclamation alleviates land scarcity and provides wading bird habitats, yet the legacy effects of bird droppings on soil phosphorus (P) cycling remain poorly understood. Here, we conducted an in situ incubation experiment in Hangzhou Bay coastal wetlands to investigate the legacy effects of long-term (12 years), continuous, and short-term (3 years) bird droppings inputs on soil P fraction transformation. We observed that bird droppings promoted soil inorganic P activation, increasing exchangeable P (Exch-P) and iron‑aluminium bound P ((Fe + Al)-P) by 349.45% and 189.26%, respectively, while decreasing calcium bound P (Ca-P) by 50.00%. Bird droppings also enhanced soil nutrients (SOC, TN, and TP), microbial functions (alkaline phosphatase activity and bacterial richness), and their correlations with Exch-P, (Fe + Al)-P, and organic P. The near - neutral pH of droppings weakened the soil alkaline environment, provided a more suitable growth environment for P activating microorganisms, and increased the negative correlation between pH and Exch-P and the positive correlation between pH and Ca-P. Notably, legacy effects showed a greater impact on the soil than current short-term droppings inputs, and topsoil (0–10 cm) showed stronger responses than subsoil (10–20 cm). The reduction rate of P was faster than that of SOC and TN, which could increase the risk of P loss. This study provides mechanistic understanding of how bird droppings drive soil P dynamics, a public concern, which is vital for sustainable management of P deficient coastal reclaimed wetlands during the “grain-oriented” reclamation.