Greener but not more balanced: Cross-scale divergence between carbon sequestration and nitrogen retention in urban green spaces

Expanding urban green space is assumed to enhance carbon sequestration service (CSS) and nitrogen retention service (NRS) simultaneously, yet their responses to urbanization and spatial form may diverge. This study examined carbon–nitrogen functional imbalance in the Yangtze River Delta (YRD), China, by integrating long-term ecosystem-service assessment, city-scale morphology analysis, interpretable machine learning, and constrained NSGA-II optimization. CSS and NRS were estimated from 1990 to 2020 using CASA and InVEST-NDR, respectively. At the city scale, twelve green-space and building morphology indicators representing both two- and three-dimensional characteristics were linked to CSS, NRS, and CN_gap across four selected cities. Regionally, CSS first declined from approximately 700 to below 600 gC·m −2 ·yr −1 and then increased after 2000, whereas NRS peaked at 7.1–7.3 gN·m −2 ·yr −1 in the early 2000s before declining after the mid-2000s. This divergence produced a rising C/N ratio and persistent spatial mismatch, with high CSS concentrated in the southern and southwestern YRD and high NRS more evident in northern, central, and eastern plains. City-scale models showed moderate to strong performance ( R 2 = 0.61–0.83). SHAP analysis revealed both shared and distinct morphology responses among CSS, NRS, and CN_gap. ED consistently showed the strongest positive association with CSS, whereas NRS and CN_gap were more strongly influenced by city-specific combinations of green-space configuration and built-form intensity. NSGA-II candidate scenarios increased CSS by 16.28–17.38% and NRS by 5.15–12.31%, while reducing CN_gap by 43.59–77.06%. These findings suggest that increasing urban greenness alone does not necessarily improve carbon–nitrogen coordination, highlighting the need for morphology strategies tailored to local conditions rather than uniform greening targets.