At present, more than half of the world’s population resides in cities and towns. Migration from rural to urban areas is continuing at an alarming rate in developing countries, and the percent urban population is projected to increase to 66% by 2050 [1
]. The rapidly expanding urban areas have caused large areas of agricultural, pasture or forest soil to be changed to urban soil [2
]. As a significant component of urban ecosystems, urban soil has become increasingly important with regard to human health and wellbeing [5
]. Urban soil is more strongly perturbed by human activities (e.g., mixing, land filling, compaction, and soil sealing) than other soils, resulting in negative impacts on soil functions and the urban environment [3
]. Therefore, a growing body of literature focuses on environmental issues associated with urban soils, particularly contaminants such as heavy metals and organic pollutants (e.g., [8
]). However, the characterization of the forms of urban soil phosphorus (P), which is usually enriched in urban areas and may cause aquatic eutrophication, and the soil properties associated with P retention in urban areas require further investigation [11
In certain geographical regions, P build-up in the soil due to fertilization or other land management has become a concern, as it can represent a threat to water quality when transported into water bodies via surface runoff or leaching [12
]. Previous studies have suggested that strong enrichment of P occurs in urban soils [11
], which can originate from household ash and various anthropogenic waste deposits in urban areas [11
]. Phosphorus accumulation in urban soils may become more severe in the future because of the low mobility of soil P and the abundant external P input in urban areas. Human activities may also alter soil properties related to P accumulation such as soil pH [16
]. Thus, it is critical to investigate the P retention status (e.g., changes in soil P forms) in urban soils to gain a better understanding of P behavior in the urban environment. However, the specific P forms and associated properties in urban soils remain largely unknown.
In soils, P is present in several forms or pools, and different forms are often designated as either inorganic or organic and are commonly further distinguished as P dissolved in soil water, P sorbed to surfaces of clay or Fe and Al oxides, P in phosphate minerals, and P in organic substances and living organisms [17
]. These descriptions imply differences among P forms with respect to reactivity in the environment. Accounting for these various P pools, different approaches to extract P from the soil are available, and numerous soil P extraction methods have been developed (e.g., [17
]). Sequential fractionation schemes are widely used to identify different soil P forms; they extract P from the soil with selective solvents that isolate P pools of different solubility. Generally, the extractants are designed to solubilize groups of minerals usually defined as P associated with Al, Fe, Ca or residual forms [19
]. In this case, sequential fractionation schemes can also be employed to investigate the characteristics of P retention in urban soils.
Urban areas in China have been expanding rapidly in recent decades, and it has become urgent to obtain a comprehensive understanding of the effects of this land-use change, including P forms in urban soils. Determination of the P forms of surface urban soils will help to identify processes of P accumulation and the prediction of P mobility within such soils. The aim of our study was to investigate the specific P forms and associated P-retentive properties of urban soils in Nanchang, Southern China. We hypothesized that with abundant external P input and severe anthropogenic disturbance, urban soil would show different P forms and P-retentive properties compared with suburban or rural soils.
We found that urban soils had significant P accumulation compared with suburban and rural soils. Similarly, some previous studies have also reported that P was enriched in urban soils of a number of cities, such as Nanjing [11
], Hangzhou [15
], Beijing [31
], and Nanchang [20
] in China and London [32
] in the UK. Generally, P enrichment in urban soils increases with the urban development process, including the population size and level of urban infrastructure [33
]. The source and influx of P to urban soil were not investigated in our study. In general, solid waste and waste water are the main P-containing materials in urban ecosystems [34
]. Some urban soils historically functioned as waste water and household treatment sites unintentionally, especially those with long urban-use histories, resulting in P enrichment in these soils. In addition, management practices in urban parks selected in our study (e.g., fertilization and irrigation) can result in P in urban soil.
Similar to the suburban and rural soils, NaOH-extractable P was a major P form in the urban soils. In addition, NaOH-Pi was significantly correlated with Feox
contents for the three types of soils (r
= 0.38, p
< 0.01, data not shown). Since the natural soils in Southern China have a considerable amount of Fe and Al oxides [35
], the parent materials and original soils appeared to control the major P fractions of the urban soils. Soil PHCl
is usually present at a very low level or is absent in highly weathered acidic soils, as weathering mainly affects Ca or Mg P minerals. However, urban soils showed a significantly higher content of soil PHCl
(238 mg kg−1
) than the other two types of soils, and the increases in PHCl
contributed a major part of total P increases. Other studies also reported that urban soils had higher PHCl
contents than soils in suburban and rural areas [15
]. This is probably due to the inputs of materials with abundant Ca and/or Mg, as indicated by the higher content of CaM3
). In fact, CaM3
accounted for 52% of the variations in the PHCl
content across all soil samples and 72% of those of urban soils (Figure 5
). The dissolution of calcareous materials in cement and concrete that are ubiquitous in built environments may elevate the Ca and/or Mg contents of urban soils [36
] and further precipitate (or adsorb) P. Soil management in urban areas may also need to consider the enrichment of Ca, as some plant species that adapt to acidic soils may be sensitive to Ca enrichment.
Urban soils with relatively high total P contents may release more labile P, as labile P increased with the total P increase in urban soils. In addition, the strong relationships between labile P and NaOH-Pi, PHCl
, and residual P suggest that labile P that is lost from the soil via runoff, leaching or plant use may be replenished by these P pools. For example, soil PHCl
is probably not stable and may easily dissolve into soluble P due to soil acidification acceleration with N and acid deposition in urban ecosystems. Zhang et al. found groundwater P was significantly correlated with soil NaHCO3
-extractable P in Nanjing city, suggesting that enriched P in urban soil has the potential to be released from the soil into the water system in urban areas [11
]. Thus, P enrichment in urban soil may contribute partly to lake eutrophication in urban areas, which is usually caused by P enrichment of water [38
Soil pH can regulate P reactions in soils, i.e., P sorption by Fe or Al oxides mainly occurs in acidic soils, whereas in neutral and alkaline soils, Ca or Mg can precipitate with P in the form of Ca- or Mg-P minerals [39
]. The enrichment of Ca and Mg in urban soil along with increased soil pH suggested that P sorption characteristics may be changed in urban soils. Although Ca minerals can contribute to P sorption even in acidic soil [40
], the enrichment of Ca (and Mg) in urban soil along with increased soil pH will decrease P sorption in urban soil, which in turn will facilitate P loss because P is held with greater bonding strength in acidic soil than in neutral or calcareous soils [39
]. Further studies on the export flux of P from urban soils to adjacent ecosystems will advance our understanding of the consequences of P enrichment in urban soils.
A phosphorus fractionation scheme revealed that NaOH-extractable P (NaOH-Pi and NaOH-Po) and residual P were the dominant components of total P in urban soils, which is similar to rural and suburban soils. However, urbanization increased the contents of soil total P, PH2O, PKCl, NaOH-Pi, residual P, and especially PHCl. Soil pH and the CaM3 content were the crucial properties responsible for the variations in P fractions along the urban–rural gradient or among different soil depths. In other words, the enrichment of Ca and/or Mg materials in urban soils, thought to be impacted by human activities, changed the soil P forms in the urban areas. Our study highlights the urgent need to extend the evaluation of P accumulation in urban soils and the associated environmental consequences to other cities and countries, especially those with long habitation histories.