Active park use, nature connectedness, and multidimensional long-term mental health: dose-response relationships and mechanistic pathways in a high-density Chinese metropolis.

Rapid urbanization poses significant mental health challenges to the working-age population in high-density metropolises. Although urban parks offer mental health benefits, non-linear dose-response relationships, optimal exposure thresholds, and the mediating or moderation roles of nature connectedness (NC) remain inadequately understood. This study examined 620 working-age adults in Wuhan, China, using generalized additive models (GAMs) to trace dose-response trajectories and determine park use duration (PUD) thresholds across stress, depression, anxiety, and well-being. A counterfactual mediation framework decomposed these associations to clarify the mediating and moderating roles of NC, while a leave-one-covariate-out (LOCO) analysis evaluated the relative importance of active versus passive exposure. Results demonstrated that higher PUD was consistently associated with favorable long-term mental health, while exhibiting distinct, outcome-specific dose-response trajectories: a curvilinear pattern for stress, contrasted with essentially linear patterns for anxiety, depression, and well-being. Estimated minimum monthly PUD thresholds associated with favorable mental health outcomes were relatively lower for stress (3.84-4.70 h) and well-being (4.54-4.92 h), whereas depression (5.65-6.69 h) and anxiety (5.63-8.05 h) required higher thresholds across multi-scale buffers. Counterfactual analysis revealed that the effects on stress and anxiety were primarily direct. In contrast, the effects on depression and well-being were partially mediated by NC, operating alongside direct pathways. Sociodemographic subgroup and moderating analyses showed remarkable homogeneity, suggesting broad equigenic potential. LOCO analysis demonstrated that PUD substantially outperformed traditional passive greenness metrics. These findings underscore the importance of active behavioral engagement over mere passive proximity, offering critical benchmarks for time-conscious, behaviorally oriented urban green space planning and public health interventions.