Avian activity alters the speciation and coupling of inorganic sulfur and reactive iron in wetland sediments.

Avian activity introduces exogenous organic matter into sediments via excreta, profoundly influencing the cycling of key elements such as sulfur and iron. However, its impact on sulfur-iron coupling processes and associated environmental effects remains unclear. This study was conducted in the Bird Paradise National Wetland Park to analyze the vertical distribution and coupling relationship of sulfur and iron. Results showed notable differences in the vertical distribution of inorganic sulfur and reactive iron in sediment cores from the high-bird-density area (X1) and the low-bird-density area (X2). The pronounced enrichment of Acid-Volatile Sulfide (AVS) in core X1 may be associated with enhanced sulfate reduction driven by labile organic matter from avian guano. However, the rapid accumulation and aging of AVS may have inhibited the conversion of AVS into pyrite (chromium-reducible sulfur nearly stagnated at 25-40 cm), shifting the sulfur-iron cycle from "progressive pyritization" to "short-term AVS accumulation and long-term suppressed pyritization". This shift weakened the buffering capacity of reactive iron against sulfide (a 12.2% decrease relative to X2 core), reduced the sediment's resilience to sulfide impacts, and increased potential ecological risks. These findings provide an important reference for the ecological conservation and environmental management of bird habitats.