Phosphorus Forms and Associated Properties along an Urban-Rural Gradient in Southern China

Urbanization is widely assumed to degrade soil ecosystem services, but the changes in urban soil phosphorus (P) status due to urbanization, and the associated environmental implications have rarely been studied. The objective of this study was to investigate the P forms and associated soil properties in urban soils. Thirty sites were selected along urban-rural gradient in Nanchang, China, to examine the effects of urbanization on soil P fractions. Residual P and NaOH-extractable P (NaOH-Pi and NaOH-Po) were the major P forms in the 0–30 cm of urban soils, comprising on average 37 and 43% of the total P pool, respectively, which was similar to the suburban and rural soils. Compared with non-urban soils, urban soil had higher contents of total P and P fractions (i.e., P_H2O, P_KCl, NaOH-Pi, P_HCl, and residual P), as well as greater contents of related soil P-retentive properties, especially soil pH, Mehlich 3-extractable Ca and Mg. Phosphorus enrichment in the urban soils may become a source of aquatic pollution, because soil labile P content (sum of P_H2O and P_KCl) was positively related with total P, P_HCl, NaOH-Pi, and residual P, which implied that the labile P can be replenished by these P pools. This study increased the understanding of P stabilization characteristics (e.g., the specific P forms) of urban soils and had further implications for urban environmental management.