Urbanization-induced cropland intensification and fragmentation result in threshold responses in soil antibiotic...

Urbanization substantially alters the intensity of human activities and land-use patterns, potentially accelerating the accumulation of antibiotics in soil environments and thereby posing risks to human health and crop productivity. However, studies directly examining the impacts of urbanization on soil antibiotic contamination remain limited, and the response characteristics of soil antibiotic contamination to urbanization, as well as the mechanisms, remain poorly understood. In this study, an urbanization index (UI) was constructed using multi-source socioeconomic variables whose optimal weights were determined by a genetic algorithm (GA) combined with a random forest (RF) model. The UI was then used to identify the threshold responses of soil antibiotic contamination to urbanization levels, and a structural equation model (SEM) was employed to reveal the mechanisms. The results showed an inverted U-shaped relationship between soil antibiotic contamination and urbanization level within the study area. This threshold effect can be attributed to shifts in the mechanisms of soil antibiotic contamination across different stages of urbanization. Below the threshold, urbanization indirectly promoted the accumulation of antibiotics in soil by intensifying agricultural production activities, whereas above the threshold, urban spatial expansion increased farmland fragmentation, thereby reducing the input of agricultural pollution sources. This study reveals the response characteristics of soil antibiotic contamination to urbanization and the mechanisms of this response, providing new scientific evidence for the identification and management of soil antibiotic contamination risks in rapidly urbanizing regions.