Environ Earth Sci (2013) 69:1571–1577 DOI 10.1007/s12665-012-1991-6

ORIGINAL ARTICLE

Levels and distribution of total nitrogen and total phosphorous in urban soils of Beijing, China Xinghui Xia • Xiuli Zhao • Yunjia Lai Haiyang Dong

•

Received: 28 September 2011 / Accepted: 11 September 2012 / Published online: 21 September 2012 Ó Springer-Verlag 2012

Abstract Urban soil nitrogen and phosphorus have significant implications for the soil and water quality in urban areas. The concentrations of total nitrogen (TN) and total phosphorus (TP) of soil samples collected from six types of land use, which included residential area (RA), business area (BA), classical garden (CG), culture and education area (CEA), public green space (PGS) and roadside area (RSA) of Beijing urban area, were investigated. Results showed that the geometric mean of TP (857 mg/kg) in urban soils was slightly higher than that (745 mg/kg) in rural soils of Beijing. The concentration of soil TP was higher in the center of the city, and showed an increasing trend with the age of the urban area. The TP concentrations in the six types of land use followed the sequence of CG [ BA [ RSA [ RA [ CEA [ PGS, which were affected by the use and disposal of phosphorus-containing materials in each type of land use. However, the geometric mean of TN (753.8 mg/kg) in urban soils was much lower than that (1,933.3 mg/kg) in rural soils. TN level in urban soils of Beijing had no correlation with the city’s urbanization history, and was influenced by the coverage of natural vegetation and human activities in each type of land use. This study suggested that the city’s urbanization history and land use were the main factors affecting the distribution of nitrogen and phosphorus in urban soils. Keywords Total nitrogen (TN)
Total phosphorus (TP)
Land use
Urban soil
Beijing

X. Xia (&)
X. Zhao
Y. Lai
H. Dong State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China e-mail: [email protected]

Introduction Urban soils are defined as the soils that distribute in city area. In contrast to natural soils, urban soils are substantially influenced by human activities (Norra and Stuben 2003; Scharenbroch et al. 2005) and have been dramatically altered during various stages of construction, destruction and reconstruction of buildings and infrastructure. Therefore, the characteristics of urban soils have inevitably undergone tremendous change (Vetterlein 1999; Lehmann and Stahr 2007; Rossiter 2007; Civeira and Lavado 2008). In addition, urban soils, as parts of urban environment, have great impacts on the way people live and work. Therefore, the study on urban soils is of critical significance. Nitrogen and phosphorus are the most important nutrients for organisms; however, excessive nitrogen and phosphorus will lead to the eutrophication of water bodies. Urban soil nitrogen and phosphorus can be desorbed from soils and run into the lakes and rivers in or around the city (Jarvie et al. 2006; Nyenje et al. 2010); and this transportation would enormously influence the urban water quality (Majumdar et al. 2008; Cheong et al. 2012). Furthermore, nitrogen and phosphorus are the major plant nutrients; therefore, the levels of total nitrogen (TN) and total phosphorus (TP) in urban soils would affect the urban greening directly. Therefore, a need exists to better understand the distribution and influencing factors of TN and TP in urban soils. Until now, only few researches have been conducted on TN and TP in urban soils, and they mainly focused on the levels and spatial distribution of nitrogen and phosphorous (Jim 1998; Scharenbroch et al. 2005; Liu and Liu 2008). For example, Liao et al. (2007) reported that phosphorus showed high relative enrichment in top-soils, with

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enrichment factor greater than 1.5 in the city of Nanjing, China. Zhang et al. (2008) found that the concentration of soil available nitrogen was higher in the outer and lower in the middle of the city of Shenyang, China. However, all of these studies did not concern the TN and TP levels in different types of land use. As the TN and TP in soils with different types of land use may exert different impacts on urban environment, the study regarding the TN and TP levels in different types of land use as well as their influencing factors is desired for urban soils. As the political, economic and cultural center of China, Beijing has acquired rapid development and experienced complete urbanization during the past 3 decades. The population of Beijing has exceeded 19 million, and its urban area has expanded nearly 7 times during the past 30 years (Yang et al. 2010). The intensity of public activity on various types of land use is different in Beijing. Therefore, in this research, Beijing was selected as a representative city to study TN and TP levels in diverse types of land use and the impact factor for their distribution as well as the effect of speedy urbanization on nutrient concentrations in urban soils. The concentrations of TN and TP of soil samples collected from six types of land use, which included residential area (RA), business area (BA), classical garden (CG), culture and education area (CEA), public green space (PGS) and roadside area (RSA) of Beijing urban area, were studied. Comparison was made between TN and TP concentrations in Beijing urban soils and those in suburb areas as well as other cities in China. TN and TP levels in different types of land use were also compared and the influencing factors of their levels were analyzed. In addition, the spatial distribution of TN and TP levels in urban soils of Beijing as well as their correlations with the city’s urbanization history was also investigated.

Environ Earth Sci (2013) 69:1571–1577

Beijing has a mean annual precipitation of about 600 mm with its annual average temperature around 11.5° C (Chen et al. 2010). Sample collection

Materials and methods

All the urban area sampling points are located in the 5th Ring Road and well distributed in the urban area of Beijing. Each type of land use contained at least seven sampling sites, with each site having a land use type. A total of 127 topsoil samples (0–20 cm) were collected with a stainless steel shovel during April–May of 2008. A GPS was used to record the coordinates of the sample location. The sampling location was shown in Fig. 1. A total of 8 samples were collected in BA, 7 samples were collected in CG, 9 samples were collected in CEA, 12 samples were collected in PGS, and 12 samples were collected in RA. In RSA, 79 samples were collected at different distances from both sides of 10 roads, which were expressed by double lines in Fig. 1. Each road contained at least 4 sampling sites, which were within 30 m distance from the road. To make the samples representative for each type of land use, the number of sub-samples was determined based on the area of sampling sites. For RSA and BA, each sample was the mixture of five sub-samples taken from the four vertexes and the center of a square block. For RA, each sample was the mixture of eight sub-samples fairly well distributed on the site. For PGS, CG and CEA, each sample was the mixture of 10 sub-samples fairly well distributed on the site. All the samples collected were kept in sealed manila packages, respectively, to avoid contamination and transported to the laboratory immediately. In order to understand the effects of human activities on TP and TN levels, 40 samples were collected from rural soils in 12 administrative regions in Beijing at the same time (Fig. 1). The levels of TN and TP between urban soils and rural soils were compared.

Study area

Sample preparation and analysis

As the capital city of China, Beijing (39.9 N, 116.4 E) is located on the northern tip of roughly triangular North China Plain. The city is surrounded by mountains in its west, north and northeast directions. Beijing has an urban area of approximately 1,040 km2, which is situated in the south-central part of the municipality and consists of 6 districts within 5th Ring Road, namely: Dongcheng, Xicheng, Haidian, Chaoyang, Shijingshan and Fengtai. The urban population density of Beijing is about 15,752 person/km2. Due to a tropical monsoon-influenced climate, Beijing has a hot and humid summer as the result of the East Asian monsoon, as well as a cold, windy and dry winter because of the Siberian anticyclone. Moreover,

Soil samples were dried in a shady place at room temperature, the impurities such as stones and tree leaves were removed from them; the samples were sieved to \1 mm after drying by gently crushing aggregates without grinding sand-sized mineral fragments (HJ/T166 2004). Then a fraction of samples was ground to pass a 100 mesh nylon sieve for TN and TP determination. TN was determined by semi-micro Kjeldahl method (Bremner 1960), and TP was determined by NaOH melt-colorimetry method with molybdenum and antimony (GB 9837 1988). Average recoveries for TN and TP were between 95.0 and 105.0 % based on matrix spike experiment. Deviation of TN level among three parallel samples was within 0.003 % and that

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Fig. 1 Sampling sites in urban area of Beijing

of TP level among three parallel samples was within 0.005 %. Mean of each sample’s three parallel samples was regarded as the final result. In addition, parameters such as pH, cation exchange capacity (CEC), particle size distribution, conductivity and total organic carbon (TOC) were determined for general characters of these urban soil samples. For TOC, the soil samples were first ground with agate mortar to pass through a 100 mesh stainless steel sieve. An elemental analyzer (Vario El, Elementar Analysensysteme GmbH, Germany) was used for the TOC analysis after the samples were treated with HCl (1:1, volume). The analysis of pH, CEC, and particle size distribution was conducted with samples sieved to \1 mm. Particle size distribution was analyzed by the hydrometer method (LY/T1225 1999); soil pH was measured by potentiometric method, with 2.5:1 ratio of water and soil and soil conductivity was measured by conductivity meter with 5:1 ratio of water and soil; CEC was determined by CH3COONH4 exchange method (LY/ T1243 1999). Data treatment Statistics of soil TN and TP levels and concentration in different types of land use, including minimum value, maximum value, median value, geometric mean, standard deviation and coefficient of variation (CV) were carried out with EXCEL 2007 (Microsoft Inc., Redmond, USA). Correlation analyses were carried out with Origin 8.0 (Microsoft Inc., Origin Lab, USA). Contour maps of distribution of TN and TP in urban soils of Beijing were drawn by SURFER v.8.0 (Golden Software Inc., Colorado, USA).

Results and discussion Properties of urban soil samples The properties of urban soil samples including pH, CEC, TOC, conductivity and particle size distribution were investigated and presented in Table 1. The pH of Beijing urban soil samples ranged from 7.79 to 8.80, with a mean of 8.34, suggesting a slightly alkaline condition. The mean pH values among six types of land use followed the sequence of RSA (8.37) [ CG (8.26) [ PGS (8.25) [ CEA (8.23) [ RA (8.16) [ BA (7.80). The CEC of soil samples varied from 4.23 to 20.29 cmol/kg with a mean value of 11.03 cmol/kg. The mean CEC values in six types of land use followed the sequence: CG (11.66 cmol/kg) [ CEA (11.31 cmol/kg) [ PGS (11.24 cmol/kg) [ RSA (10.88 cmol/kg) [ BA (10.73 cmol/kg) [ RA (9.70 cmol/kg). The TOC ranged from 0.22 to 6.94 %, with a mean value of 1.5 %, and the mean TOC values in six types of land use followed the sequence: CG (2.5 %) [ CEA (1.86 %) [ RSA (1.49 %) [ BA (1.40 %) [ RA (1.38 %) [ PGS (0.96 %). The mean conductivity values among six types of land use followed the sequence of RA (260.36 us/cm) [ BA (233 us/cm) [ RSA (232 us/cm) [ CEA (216 us/cm) [ CG (211 us/cm) [ PGS (192 us/cm).

Levels of TN and TP in urban soils of Beijing Figure 2 showed the histograms of log-transformed data of TN and TP in urban soils of Beijing, indicating that the distribution of TP and TN in urban soils of Beijing followed a lognormal distribution. The levels of TP and TN

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showed no significant correlation with particle size distribution, CEC, pH, conductivity and TOC values (P [ 0.05). The concentrations of TN in urban soils of Beijing varied from 424.2 to 1,764.0 mg/kg with a geometric mean of 753.8 mg/kg. In comparison with other cities in China (see Table 2), TN level in urban soils of Beijing was lower than that in Shanghai (1,120 mg/kg), Harbin (2,891 mg/kg) and Nanjing (1,316 mg/kg). In addition, the TN level in urban soils of Beijing was lower than that in Beijing rural soils (1,933.3 mg/kg). The reason can be analyzed as follows. Despite the nitrogen compound use and disposal, nitrogen fixation, etc. will import nitrogen into the urban soils (Zhang et al. 2010), the human activities, including the mechanical top-soil removal (Lorenz and Lal 2009), introductions of new soils and other natural substrates (Lehmann and Strhr 2007), the vegetation management practices such as the removal of grass clipping, tree leaves and other organic debris (Craul 1999), will lead to the nitrogen loss from the urban soils. For example, Raciti et al. (2012) found that, due to physical removal of topsoil, the nitrogen concentration of soils (0–15 cm) under impervious surfaces was 95 % lower than that in open area soils. Sims (1995) and Alvarez et al. (1999) reported that Table 1 The range and mean of selected soil properties of urban soils Compounds

Range

pH

7.79–9.10

8.34 ± 0.19

2.2–6.9

1.5 ± 1.0

CEC (cmol/kg)

4.23–20.29

11.03 ± 2.74

Conductivity (us/cm)

110–951

229 ± 116

TOC (%)

Mean ± SD

Particle size distribution mm (%) 57.6–85.5

71.2 ± 5.45

0.01–0.001

1–0.01

4.9–18.6

12.4 ± 2.88

\0.001

9.6–24.3

16.4 ± 3.28

Fig. 2 Histogram of log-transformed data of TN and TP

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organic materials, which were used during various stages of urban construction, are subject to high mineralization and resulting in nitrate release. The slather of organic materials might lead to nitrate leaching from urban soils. Besides, the denitrification is another removal way of urban soil nitrogen; Kaye et al. (2004) reported that the conversion of grasslands to urban use can increase N2O emission. For the urban soils of Beijing, the import of nitrogen might be less than the export, leading to the lower level of nitrogen in urban soils compared to the rural soils. In the rural area, the frequency of the above human activities which led to nitrogen loss in soils is relatively lower than that in the urban area, and the nitrogen fixation activities are stronger than that in the urban area. In addition, the use of nitrogen fertilizers in the rural area especially in the cropland will result in a higher nitrogen concentration. The concentrations of TP in urban soils of Beijing varied from 452 to 2,263 mg/kg, with a geometric mean of 857 mg/kg. According to Table 2, the average TP level in Beijing was higher than that in Hong Kong (6 mg/kg) and Nanjing (709 mg/kg), but was lower than that in urban soils of Urumqi (1,243.19 mg/kg) and Harbin (1,747 mg/kg). In contrast to the level of TN, the level of TP in urban soils of Beijing was higher than that in rural area (745.00 mg/kg). Liu and Liu (2008) reported that phosphorous in urban soils (1,243.19 mg/kg) of Urumqi, China showed an accumulation level compared to its background (810 mg/ kg), and Liao et al. (2007) also found that phosphorous in urban soils of Nanjing, China showed an enrichment level with enrichment factor greater than 1.5. In comparison with TN levels, TP levels are also influenced by the abovementioned factors, except denitrification. The extensive usage of phosphorus-containing substances might be the main reason for phosphorus enrichment in Beijing urban soils. In urban areas, phosphorus-containing materials are widely used. For example, Ca(H2PO4)2
H2O is the major

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Table 2 Comparisons of TN and TP levels in urban soils of different cities in China City

TN (mg/kg) Range

TP (mg/kg) Mean ± SD

Range

References Mean ± SD

Nanjing

450–3,140

1,316 ± 330

210–33,300

709 ± 270

Liao et al. (2007)

Harbin

2,050–4,590

2,891 ± 780

1,010–24,300

1,747 ± 494

Meng and Zhou (2005)

Shanghai

370–2,260

1,120 ± 40.00

–

–

Shi et al. (2010)

Hong Kong

0–4,700

Urumqi

–

400 ± 700 –

0.58–34.29

6.00 ± 6.19

Jim (1998)

768.50–1,956.36

1,243.19

Liu and Liu (2008)

Beijing (urban area)

424.0–1,764.0

753.8 ± 254.4

452–2,263

857 ± 333

This study

Beijing (rural area)

619.0–7,571.0

1,933.0 ± 1,544.2

220–1,363

754 ± 246

This study

component of food leavening agent; CaHPO4
2H2O is the main additive of pet food and toothpaste; PCl3 is used to make plastics; P4S10 and PCl3 are the raw materials of pesticide; polyphosphate and phosphate are adopted to keep the structures from corrosion; Na5P3O10 is one of the major components of detergent and H3PO4 is used to manufacture phosphate fertilizers. All the substances mentioned above are the necessities for urban sanitation protection and urban resident living. The use and disposal of these phosphorus-containing materials might be the main reason for the higher phosphorus level in urban area compared to the rural area. Comparisons of TN and TP levels in six types of land use As shown in Table 3, TN concentrations in Beijing urban soils of different types of land use followed the sequence: CEA (972.7 mg/kg) [ CG (898.3 mg/kg) [ PGS (716.3 mg/kg) [ RSA (713.7 mg/kg) [ RA (681.8 mg/kg) [ BA (652.6 mg/kg). The concentration of TN in CEA was nearly 1.5 times than that in the BA. Two independent sample test (Mann–Whitney U) showed that TN concentration in CEA was significantly higher than that in PGS, RSA, RA and BA (p \ 0.05). There were no significant differences between CEA and CG, and no significant differences among PGS, RSA, RA and BA. Culture and education area is the place for study and education, and there is enough greenery space area. CG is the classic garden where citizens often go for sightseeing and enjoy the beauty of nature. Similarly to CEA, CG maintains a larger green area and an ecological cycle relatively far from human activities. There exists a lot of natural vegetation keeping litter thick enough to prevent the nitrogen from loss (Harmon et al. 1990) in CEA and CG. Furthermore, in comparison with the other four types of land use, CEA and CG own relatively few artificial buildings and much lower reconstruction frequency; therefore, the factors mentioned in ‘‘Levels of TN and TP in urban soils of Beijing’’ have relative slight impacts on

the TN levels in CG and CEA. Different from CEA and CG, PGS, RSA, RA and BA have suffered more from human activities. The four types of land use own quite a few artificial buildings with high reconstruction frequency and low green plant coverage, leading to the lower TN level. This also inferred that the TN level in different types of land use was influenced by the coverage of natural vegetation as well as human activities. As presented in Table 3, TP concentrations among different types of land use followed the sequence: CG (1,326 mg/kg) [ BA (854 mg/kg) [ RSA (827 mg/kg) [ RA (813 mg/kg) [ CEA (807 mg/kg) [ PGS (755 mg/kg). The concentration of TP in CG was nearly twice than that in other types of land use. Two independent samples test (Mann–Whitney U) showed that TP concentrations in CG were statistically higher than those in the other five types of land use (p \ 0.05), while there were no significant differences among BA, RSA, RA, CEA and PGS. Although CG has maintained a natural ecological cycle, the use of phosphorus containing materials for the maintenance of historic buildings and the growth of plants, including pesticide, detergent, corrosion inhibitor, fertilizer, etc. might be the factors accounting for the high level of phosphorus in CG. In addition, CG is the oldest place among the six types of land use in Beijing. It can be inferred that the levels of phosphorous might have correlation with the age of the study area. The relationship between phosphorous levels and the age of the area will be analyzed in ‘‘Spatial distribution of TN and TP and their correlation with urbanization history’’. Spatial distribution of TN and TP and their correlation with urbanization history As shown in Fig. 3, great spatial heterogeneity of TN levels was observed. The high values mainly existed in the northwest, southeast and south parts of the study area, where most of the CG and CEA sampling sites located. There was no obvious trend for TN concentration from the inner to the outer city. However, as shown in Fig. 4, the TP

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Table 3 TN and TP in soil of six types of land use in Beijing Soils

N

TN (mg/kg)

TP (mg/kg)

Range

Geomean ± GSD

CV (%)

Range

Geomean ± GSD

CV (%)

PGS

12

424.2–1,251.0

716.3 ± 261.2

34.5

452–1,404

755 ± 235

31.1

RSA

10

444.3–957.3

713.7 ± 174.4

23.7

629–1,347

827 ± 216

29.3

CG

7

655.0–1,764.0

898.3 ± 361.9

38.0

980–1,833

1,326 ± 271

28.5

BA

8

483.0–1,175.0

652.6 ± 208.4

30.7

593–1,247

854 ± 206

30.3

RA

12

455.0–1,021.0

681.8 ± 182.2

25.9

540–2,263

813 ± 475

67.4

9

771.0–1,209.0

972.7 ± 154.1

15.6

727–959

807 ± 77

7.8

CEA

40.05

40.05

450-550mg/kg

N

550-750mg/kg

450-600mg/kg

N

600-700mg/kg

5th Ring Road

5th Ring Road

750-950mg/kg

40

700-850mg/kg

40

950-1100mg/kg

1100-1300mg/kg

39.95

850-1000mg/kg

1000-1300mg/kg

1300-1800mg/kg

39.95

2nd Ring Road

1300-2500mg/kg

2nd Ring Road

Tian An Men

Tian An Men

39.9

39.9

39.85

39.85

3rd Ring Road

0

TN 116.2

2.5

5km

4th Ring Road

116.25

116.3

116.35

116.4

116.45

116.5

116.55

Fig. 3 Map of TN distribution in urban soils of Beijing

concentration in Beijing showed a decreasing trend from the city center to the suburb. The high values existed in the centre of Beijing, within the 3rd Ring Road in particular. The TP value of hotspots was 2,263 mg/kg, which was three times the TP level in rural area. Outside the 5th Ring Road, the TP values were much lower than the center of Beijing, but still slightly higher than those in rural area. The urban area of Beijing has substantially expanded from the inner city to the outside areas, showing a decreasing urbanization history with increasing distance to the centre of the city. The inner city defined as the primary urban district of Beijing has a history of more than 500 hundred years. Confined in the 2nd Ring Road, the inner city had stayed almost unchanged until the middle of the 20th century. In 1978, the urban area of Beijing was demarcated within 3rd Ring Road. Vast expansion of the city has started since the 1980s and recent urban areas have now expanded to the 5th Ring Road and some place even outside the 5th Ring Road (Liu et al. 2010). Therefore, the spatial distribution of TP concentration and the urbanization of Beijing inferred that TP concentration had a correlation with the urbanization history of Beijing and increased with the age of the urban area. As shown in Fig. 5, the TP concentration decreased gradually from the center of the city to the outside of 5th Ring Road. Additional analyses on TP concentrations in different types of

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3rd Ring Road

0

TP 116.2

2.5

5km

4th Ring Road

116.25

116.3

116.35

116.4

116.45

116.5

116.55

Fig. 4 Map of TP distribution in urban soils of Beijing

Fig. 5 TP concentration from the inner city to the outside areas of Beijing

land use from the area inside the 2nd Ring Road to the area out the 5th Ring Road have obtained the same results. Taking the RA for example, the geometric mean of TP within the 2nd Ring Road was 1,524.43 mg/kg; it decreased to 924.64 mg/kg between the 2nd and 3rd Ring Road, 811.31 mg/kg between the 3rd and 4th Ring Road, and 740 mg/kg between the 4th and 5th Ring Road, respectively. This analysis can also be an explanation for the reason why the phosphorous concentration in urban areas was much higher than that in rural areas.

Environ Earth Sci (2013) 69:1571–1577

Conclusions In the present research, the levels, spatial distribution of TN and TP and their correlations with types of land use as well as the city’s urbanization history were studied. The main conclusions are as follows: 1.

2.

3.

TN in urban soils of Beijing (753.8 mg/kg) was much lower than that (1,933.3 mg/kg) in rural soils of Beijing. TP in urban soils of Beijing (857 mg/kg) was higher than that (745 mg/kg) in rural soils of Beijing. Both TN and TP levels were different in the six types of land use; the concentrations of TN were mainly influenced by natural vegetation coverage and human activities, and the concentrations of TP were mainly affected by the use and disposal of phosphoruscontaining materials in each type of land use. The TP concentration in Beijing showed a decreasing trend from the center to the outside of the city; TP levels in urban soils of Beijing showed a correlation with the city’s urbanization history and increased with the age of the urban area. However, there was no obvious trend for TN concentration from the inner to the outer city.

Acknowledgments This study was supported by the Major State Basic Research Development Program (2010CB951104) and the Program for New Century Excellent Talents in University (NCET-090233).

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