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Background: While the mental health benefits of urban green spaces (UGS) are increasingly recognized, less is known about how these relationships vary for socially marginalized groups. This study investigates the association between UGS and mental health among rural-to-urban migrants in Wuhan, China, examining the roles of the quality and quantity of UGS and the intermediary function of perceived everyday discrimination. Methods: We used Multilevel Structural Equation Modeling to analyze data from a social survey, integrating with park-related social media ratings, street view imagery, and geospatial datasets to characterize UGS features and contextual factors, therefore verifying our hypotheses. Results: Both the quality and quantity of UGS significantly influence migrants' mental health, with quantity demonstrating a stronger overall correlation, challenging common assumptions. Notably, social media scores of parks, reflecting positive user experiences, were found to improve mental health. However, the relationship with UGS quantity was nuanced: higher park density and green view index were positively associated with mental health, while increased park area proportion demonstrated the opposite effect. Furthermore, perceived discrimination emerged as a critical socio-psychological factor and operated spatial heterogeneity. In inner-city areas, neighborhoods characterized by plaza-type parks and high park density were associated with reduced perceived discrimination among migrants, showing active social functions of UGS. However, larger park areas are paradoxically correlated with increased discrimination experiences and poorer mental health. Interestingly, this mediatory effect of perceived discrimination was less pronounced in inner-suburban areas. These findings suggest a nuanced role of UGS in the lives of migrants. While certain aspects of UGS quantity, such as plentiful smaller parks, can facilitate social inclusion and improve mental health, others, like overlarge parks, may unintentionally contribute to feelings of marginalization and negatively impact mental health. Conclusion: Our findings highlight the crucial need for context-sensitive green space planning that balances quality and quantity while mitigating discriminatory experiences to improve the mental health of rural-to-urban migrants. [ABSTRACT FROM AUTHOR]

Though much evidence demonstrates the benefits of urban green spaces (UGS) for mental health, it remains uncertain if a greener living environment necessarily leads to better mental health. This study makes up this gap by exploring the potential non-linear effects of UGS provision (availability, accessibility, visibility, quality) and utilization (frequency, duration) on mental health, focusing on their nonlinear patterns and thresholds. Using geospatial and social survey data from Wuhan, China, and controlling for socioeconomic, built, and social environmental factors, we find that: (1) Both UGS provision and utilization have significant nonlinear effects on mental health. (2) Inverted-U-shaped relationships exist between mental health and both UGS availability (measured by Normalized Difference Vegetation Index, NDVI) and accessibility (distance to nearest park), with peak benefits observed at an NDVI of 0.25 or a distance of 0.24 km. A similar L-shaped relationship is observed for UGS visibility, suggesting that higher visibility does not necessarily translate to improved mental health. The positive health effects of UGS quality exist, yet offer marginal benefits. (3) UGS utilization plays a crucial mediating role, explaining up to 47 % of the relationship between visibility and mental health. This highlights the importance of active engagement with UGS for realizing mental health benefits, supporting the 'environmental provision→individual behavior→mental health' pathway. These findings provide urban planners with valuable dosage references for UGS allocation, emphasizing the need to consider both provision and factors promoting utilization to maximize mental health benefits within urban environments. • We explore the nonlinear pathway: environmental provision → individual behavior → mental health by focusing on UGS provision and utilization. • Urban green space provision shows non-linear links to mental health, with thresholds for optimal benefits. • KHB results reveal a nonlinear mediating role of UGS utilization. • Study offers practical dosage guidance for urban planners to design green spaces that improve mental health. [ABSTRACT FROM AUTHOR]