Comprehensive assessments of soil quality, plant diversity, and their interrelationships in urban green spaces: a case study in Hangzhou, China

Soil quality and plant diversity are essential for the health and functioning of urban green spaces (UGS). This study evaluated the soil quality of four UGS types in Hangzhou city using an enhanced Nemerow index based on four physical and seven fertility indicators, and quantified plant diversity in arbor, shrub, and herb layers using four diversity indices across UGS types. Relationships between soil conditions and plant diversity were also examined. Results indicated an acceptable overall soil quality (P N = 1.07) of the UGS types, but individual indicators were highly unbalanced. Soil electrical conductivity and clay content were relatively high (grade I–III), whereas pH and bulk density spanned grade I–VI. Except for soil organic matter, available P and available K, most fertility indicators (TN, TP, TK and AN) were severely deficient (average grade V), with TN and TP as the main limiting factors. Scenic recreational green spaces (SRGS) had the best soil quality, while transportation green spaces (TGS) ranked lowest, below residential (RGS) and park green spaces (PGS). Plant diversity was highest in the arbor layer (S = 7.18) and lowest in the shrub layer (S = 1.79), giving the order arbor layer > herb layer > shrub layer. Among USG types, SRGS had the highest plant diversity, followed by RGS, PGS and TGS. In PGS, 69.42% of soil–plant diversity correlations were positive, indicating that balanced nutrients and suitable bulk density favor higher diversity. In contrast, 46.28%, 52.07%, and 52.89% of correlations in RGS, TGS, and SRGS were negative, associated with excess available nutrients, high bulk density, and strong interspecific competition. Overall, UGS health depends on the balance between soil nutrients, physical structure, and plant diversity, with management intensity mediating these relationships.