Ecological Stoichiometry Characteristics and Influencing Factors of Soil Carbon, Nitrogen, and Phosphorus in Green Spaces Along the Urban-to-Rural Gradient of Nanchang, China

Analyzing the soil carbon, nitrogen, and phosphorus content, along with their stoichiometric ratios across different urban-rural gradients, can offer essential insights into enhancing soil quality and the sustainable management of urban green space ecosystems. This study focused on Nanchang, China, examining two typical urban forest types (Pinus massoniana forests and Camphora officinarum forests), two typical urban wetlands types (river wetlands and pond wetlands), as well as urban natural and artificial grasslands. It analyzed the distribution characteristics of organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and their stoichiometric ratios along the “urban-suburban-rural” gradients in surface (0–20 cm) and deep (20–40 cm) soil. The results indicated that in the deep soil of Pinus massoniana forests, rural areas exhibited significantly higher SOC content compared to suburban areas. In the surface soil of Camphora officinarum forests, the TN content and N:P were significantly greater in urban areas compared to rural areas (p < 0.05). Both soil layers in river wetlands showed significantly higher soil TN levels in urban areas compared to rural areas. Additionally, in the deep soil of pond wetlands, urban areas showed significantly greater TN content, C:P, and N:P, compared to rural areas (p < 0.05). For natural grasslands, soil C:N was significantly more in suburban and rural areas than in urban areas for both soil layers. In artificial grasslands, the SOC content in deep soil was significantly greater in rural areas compared to urban areas (p < 0.05). In the deep soil of suburban areas, soil TP content in Camphora officinarum forests was highly significantly greater than that in Pinus massoniana forests (p < 0.01). The SOC, TN content, and C:P were considerably higher in pond wetlands compared to river wetlands (p < 0.05). The SOC content of natural grasslands was significantly higher compared to artificial grasslands (p < 0.05). Nitrate nitrogen was highly significantly and positively correlated with soil N:P in the deep soil of Pinus massoniana forests (p < 0.01), and soil pH was highly significantly and negatively correlated with soil N:P in the surface soil of pond wetlands (p < 0.01). The urbanization process has altered the SOC, TN, and TP nutrient status to some extent, exacerbating the imbalance of nutrient elements in green space soils along the “urban-suburban-rural” gradients.