Effects of Ardeidae bird droppings on soil organic carbon and its driving factors across habitats in Hangzhou Bay

Ardeidae birds are important components of wetland ecosystems and play a significant role in regulating nutrient cycling and ecosystem functioning. Through long-term roosting activities, large quantities of bird droppings are deposited onto the forest floor, introducing considerable amounts of organic matter and nutrients into the soil. These inputs can substantially alter soil physicochemical properties, microbial communities, and biogeochemical processes, thereby affecting soil organic carbon accumulation and storage. However, the mechanisms by which bird-dropping inputs influence soil organic carbon (SOC) dynamics in different habitats remain poorly understood. Therefore, this study investigated the effects of bird-dropping inputs on SOC and its driving factors in Ardeidae bird-roosting forests located in the coastal mudflat and inland plain regions of Hangzhou Bay, China. Soil samples were collected from bird-roosting forests and adjacent control forests at depths of 0-10 cm and 10-20 cm. Soil physicochemical properties, microbial diversity, community composition, and SOC content were analyzed to evaluate the impacts of bird-dropping inputs on soil carbon dynamics and to identify the major factors controlling SOC variation. The results showed that bird-dropping inputs significantly enhanced soil nutrient availability in both habitats. Compared with the corresponding control forests, total nitrogen, total phosphorus, and available phosphorus contents in the 0-10 cm soil layer increased by 66.96%, 120.33%, and 178.14%, respectively, in the coastal mudflat forest, and by 115.90%, 688.57%, and 2 693.44%, respectively, in the inland plain forest. Similar increases were observed in the 10-20 cm soil layer. Meanwhile, bird-dropping inputs significantly decreased soil pH and increased electrical conductivity, suggesting substantial alterations in soil chemical properties. The magnitude of nutrient enrichment and soil acidification was generally greater in the inland plain forest than in the coastal mudflat forest. Bird-dropping inputs also markedly affected soil microbial communities. In the inland plain forest, bacterial Shannon-Wiener diversity decreased significantly compared with the control forest, whereas no significant change was observed in the coastal mudflat forest. In contrast, fungal diversity increased significantly in the surface soil (0-10 cm) of both habitats, with increases of 34.62% and 25.99% in the coastal mudflat and inland plain forests, respectively. Community composition analysis further revealed that the relative abundances of copiotrophic taxa, including Proteobacteria, Actinobacteriota, and Ascomycota, increased, whereas oligotrophic taxa such as Acidobacteriota, Chloroflexi, and Basidiomycota decreased. These results indicate that nutrient enrichment caused by bird-dropping inputs favored nutrient-demanding microorganisms while suppressing taxa adapted to nutrient-poor environments. Bird-dropping inputs significantly promoted SOC accumulation, particularly in the 0-10 cm soil layer. Correlation analysis, random forest modeling, and partial least squares path modeling indicated that (total nitrogen) TN, (total phosphorus) TP, soil pH, and microbial diversity were the major factors influencing SOC variation. SOC was positively correlated with nutrient availability and fungal diversity, but negatively correlated with soil pH, electrical conductivity, and bacterial diversity. These findings suggest that bird-dropping inputs enhance SOC accumulation not only through direct organic matter inputs but also through indirect regulation of soil nutrient conditions, microbial community structure, and associated carbon cycling processes. Overall, bird-dropping inputs substantially altered soil nutrient status and microbial communities, ultimately promoting SOC accumulation in both habitats. This study highlights the ecological importance of bird-dropping inputs in regulating soil carbon dynamics and provides new insights into the mechanisms linking bird activities with belowground carbon cycling in wetland ecosystems.