Urbanization is widely assumed to degrade soil ecosystem services, but the changes in the 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 an 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 NaOHPo) 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, 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., PH2O, PKCl, NaOH-Pi, PHCl, and residual P), as well as higher contents of related soil P-retentive properties, especially soil pH and Mehlich 3-extractable Ca and Mg. Phosphorus enrichment in the urban soils may become a source of aquatic pollution because the soil labile P content (the sum of PH2O and PKCl) was positively related to total P, PHCl, 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 has further implications for urban environmental management.

No abstract

No abstract

Context The phosphorus (P) sorption capacity of biochar and the effects of biochar amendment on soil P sorption and availability remain uncertain. Aims This study determined the specific contributions of the biochar inorganic component (mineral ash) and its carbon skeleton to the P sorption capacity of biochar. Methods A woody biochar and its acid-washed counterpart were added to two types of paddy soils with two P-retention capacities: high P sorption (HPS) and low P sorption (LPS). Key results Washing biochar with HCl solution drastically decreased the concentrations of calcium, magnesium, iron (Fe), and aluminium (Al) in the biochar. However, Fourier transform infrared and X-ray photoelectron spectroscopy spectra indicated that the washing procedure did not decrease the biochar organic carbon content and its surface functional groups. Acid-washed biochar showed nearly zero P sorption after 24 h of equilibration with P solution, and its incorporation had little effect on P sorption of the two paddy soils. Amendment with original alkaline woody biochar tended to increase P sorption capacity of LPS soils, but decreased that of HPS soils. The variations in Langmuir P sorption maxima, an indicator of soil P sorption capacity, among the soils and biochar treatments could be primarily explained by the amorphous Fe and Al concentrations, which indicates that the effect of biochar addition on soil P sorption was highly dependent on intrinsic soil P-retentive properties. Conclusion These results demonstrate that the biochar carbon skeleton had a very limited contribution to biochar P-retention capacity. Implications Our results suggest that the application of aged biochar with lower mineral contents to humid acidic highly weathered soils will have limited P retention capacity.