spatial heterogeneity of dtpa-extractable zinc in cultivated soils induced by city pollution and...

82 Science in China Ser. C Life Sciences 2005 Vol.48 Supp I. 82—91

Copyright by Science in China Press 2005

Spatial heterogeneity of DTPA-extractable zinc in cultivated soils induced by city pollution and land use

JIANG Yong1,2, LIANG Wenju1,2, WEN Dazhong2, ZHANG Yuge3

& CHEN Wenbo2

1. Key Laboratory of Terrestrial Ecological Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China;

2. Shenyang Experimental Station of Ecology, Chinese Academy of Sciences, Shenyang 110016, China; 3. College of Land and Environment, Shenyang Agricultural University, Shenyang 110161, China Correspondence should be addressed to Jiang Yong (email: [email protected])

Received October 13, 2004

Abstract The spatial heterogeneity of DTPA-extractable zinc in the cultivated soils of Shen-yang suburbs in Liaoning Province of China was investigated, and its map was drawn by the methods of geostatistics combined with geographic information system. The data of soil DTPA-extractable zinc fitted normal distribution after logarithm transformation, and its semivariogram fitted a spherical model. The semivariogram indicated that the spatial depend-ence of soil DTPA-extractable zinc content was moderate, with the spatial dependence range of 1.69 km and the fractal dimension of 1.96. Stochastic factors contributed to 49.9% of the spatial variability, while structural factors contributed to 50.1% of it. The spatial heterogeneity of soil DTPA-extractable zinc shown by a kriged interpolation map was deeply influenced by stochastic factors such as city pollution, land use pattern and crop distributions. For example, the average content of Zn in vegetable garden soils was 2.5－4 times as much as in their originated soils, and was lower in paddy soils than in their originated soils. The areas with a higher content of soil DTPA-extractable zinc appeared in the near suburbs and the riverside along Hunhe River and the wastewater drainage of Xihe River, and the extremely high values in the near suburb of the city’s residential area were a striking feature, indicating the key role of city pollution in the spatial heterogeneity of soil DTPA-extractable zinc. When recorded in the form of a soil pollution map, the results of such a survey make it possible to identify the unusually polluted areas, and to pro-vide more information for precise agriculture and environmental control. Keywords: soil zinc, spatial heterogeneity, city pollution, land use, geostatisics.

DOI: 10.1360/04yc0146

Zinc is an essential element for all life, and soil is the primary source of Zn in plant, animal and human nutrition. Maps illustrating the spatial distribution of soil available Zn should contribute to the improvement and sustainability of agriculture and livestock produc-

tion, the improvement in diet quality, and a better un-derstanding of the nature and extent of plant, human and animal Zn deficiencies[1,2]. They will be useful in considering the consequences of applying Zn- contain-ing solid wastes from agriculture and sewage sludge.

Spatial heterogeneity of DTPA-extractable zinc in cultivated soils induced by city pollution and land use 83

Soil zinc is necessary for plant nutrition, but its high content may induce phytotoxicity and affect the bio-logical transformation of other nutrients such as N and P in soil[3]. Soil and plant factors influencing the die-tary supply of both essential and toxic Zn have been widely studied, including their speciation and bioavailability[4], while DTPA-extractable Zn from top soils represents the more bio-available fraction of the total Zn content in soils, it is always regarded as plant available Zn in soils of north China[3].

Soils are characterized by a high degree of spatial variability, even in intensively managed ecosystems[5]. The spatial distribution of soil properties should be monitored for the effective management of agricultural fields. The most effective way to analyze and interpret the existing soil survey data to extract useful informa-tion for further study is often of great concern[6]. Ra-tional soil management requires an understanding of how micronutrient status varies across cultivated land, and we can use soil sampling and soil analysis for micronutrients to develop maps to delineate the areas that would benefit for us, or to identify the areas of particular concern such as micronutrient deficiency or environmental pollution. The inherent thing in this process is the assumption that a soil property meas-ured at a given point represents the surrounding un-sampled neighborhood. The validity of this assump-tion depends on the spatial variability of the soil prop-erty. Geostastistics combined with geographic infor-mation system (GIS) have proven to be useful in pre-dicting the spatial distribution of soil properties that are very spatially dependent in fields having a limited number of soil samples. The interpolation technique usually used is kriging[7,8].

City pollution and land use were considered as the two of the main contributors to trace element en-richments in cultivated soils of city suburbs[9—11]. Re-searches conducted by Manta et al.[12] in Italy and by Sharma et al.[13] in India demonstrated that Zn could be inferred to be the tracers of anthropogenic pollution, and hence, using spatial heterogeneity of DTPA- ex-tractable Zn by the methods of geostatistics combined with GIS should be helpful in assessing the influences of city pollution and land use on Zn distribution[1].

This study is aimed to obtain the factors affecting the spatial variability of DTPA-extractable soil Zn, and to develop a map illustrating the geographic distribution of DTPA-extractable Zn in the cultivated soil surface horizons of Shenyang suburbs.

1 Materials and methods

1.1 Description of the survey region

Shenyang is the capital city of Liaoning Province of China, which is located on the alluvial plains of the Liaohe and Hunhe Rivers, with 5.68 million of popu-lation living in the urban area. It is an industrial city, named as one of the most polluted cities in the world by the United Nations in early 1980s. It is in the con-tinental temperate monsoon zone, with dry and cold winters and warm and wet summers. The annual mean temperature is 7.0℃, and annual mean precipitation is 700 mm, of which, 70% occurs during May to Sep-tember. The length of the annual non-frost period is around 150 days. The survey was conducted in the four suburb districts of Shenyang, with Dongling Dis-trict in the east, Sujiatun District in the south, Yuhong District in the west, and Xinchengzhi District in the north (41°28′—42°10′ N; 123°1′—123°48′ E). The total area of the four suburbs is 3299 km2, of which, 1.63×105 hm2 is cultivated land. According to the Chinese Soil Taxonomy[14], the soils in the east of the survey region are classified as brown soils (Typic Hapli-Udic Argosols), in the west as paddy soils (Stagnic Anthrosols) and meadow soils (Typic Hapli-Udic Cambosols), and in the near suburbs with long term planting of vegetables as vegetable garden soils (Orthic Anthrosols). The main crops in the near suburb are vegetables, and in the far suburb are corn (Zea mays L.) and rice (Oryza sativa L.). The parent materials, rivers and soil types in the survey region are shown in fig. 1.

1.2 Soil sampling and laboratory analysis

1984 samples were collected during April—May of 2000, and the sampling sites are shown in fig. 2. All soil samples taken from 0－20 cm surface layer were georeferenced by latitude and longitude with the global positioning system (GPS). Each sample repre-

84 Science in China Ser. C Life Sciences

Fig. 1. Parent materials, rivers and soil types of the survey region.Ⅰ, Alluvial deposit of Liaohe River (meadow soils); Ⅱ, Mesa from slope deposit (brown soils and meadow soils); Ⅲ, slope deposit (brown soils); Ⅳ, alluvial deposit of Hunhe River (meadow soils and paddy soil); Ⅴ, alluvial deposit between Liaohe and Hunhe Rivers (meadow soils and paddy soil); Ⅵ, flat fields from alluvial deposit (paddy soil and meadow soils).

sented 70－90 hm2 of cultivated soils. The samples collected were air-dried, sieved through a 2-mm nylon screen, and split into two equal parts. One part was for DTPA-Zn determination, and the other part was stored as reference for later investigation.

Wastewater samples were collected from Xihe River in May and June of 2000. Each time, five sam-ples were taken along the coastwise of Xihe River.

The extractable Zn in soil was extracted by DTPA (diethylenetriamine penta-acetic acid) at pH 7.3, while the total Zn both in soil and in wastewater was di-gested by HF-HNO3-HClO4. Both DTPA-extractable and total Zn were measured by atomic absorption spectrometry (AAS), with flame or graphite furnace when required[15].

1.3 Geostatistical analysis

The spatial variability of DTPA-extractable Zn is

characterized by the semivariogram which graphs the variances between spatially separate data points as a function of the distance or lag separating them. A modeled semivariogram can then be used in kriging, a weighted linear interpolation with weights determined by semivariogram and variance minimization.

The semivariogram analysis is based on the the-ory of regional variables, and relies on several as-sumptions. On one hand, the data must be a continu-ous distribution. For parameter estimation, this distri-bution should be normal. On the other hand, the in-trinsic hypothesis or weak stationary assumes that the semivariance exists, and that it is a unique function of the lag distance, independent of location[16].

Experimental semivariogram in this study was obtained from omnidirectional semivariances, γ(h), of a set of spatial observations, z(xi), which was calcu-


Fig. 2. Sampling sites in the survey region.

lated as

( ) ( )( )

2

1

1( ) ,2 ( )

N h

i ii

h z x z xN h

γ=

= −⎡⎣∑ h+ ⎤⎦ (1)

where N(h) is the number of observations separated by a lag distance h. Semivariance estimations may de-pend on the parameters such as lag interval, lag num-ber, and anisotropy. Experimental semivariograms are fitted by theoretical models that have well known pa-rameters nugget C0, sill (C0 + C1), and range of spatial dependence a.

Traditional statistical parameters and the distribu-tion frequency of DTPA-extractable Zn were calcu-lated by SPSS (Statistics Package for Social Sciences,

10.0, SPSS inc., 1999). In this study, the data exhibited positive skewness, and kurtosis statistics indicated that the data of DTPA-extractable Zn were basically fitted normal distribution after log transformation (table 1 and fig. 3). The geostatistical parameters and the Kriged map for DTPA-extractable Zn were obtained by use of ArcInfo and ArcView.

2 Results and discussion

2.1 Descriptive statistics of soil DTPA-extractable Zn

The frequency distribution histograms for the data of DTPA-extractable Zn in soil before and after logarithm transformation are shown in fig. 3. It can be seen that DTPA-extractable Zn fitted normal distribu-

Table 1 Descriptive statistics of DTPA-extractable Zn in soil

Number (n)

Mean /mg·kg−1

Minimum /mg·kg−1

Maximum /mg·kg−1

Median /mg·kg−1 S.D C.V(%) Skewness Kurtosis

1984 1.93 0.01 9.80 1.38 1.65 85.49 1.86 3.47


Fig. 3. Histogram (bars) and theoretical normal distribution (lines) of DTPA-extractable Zn before and after logarithm transformation. (a) DTPA extractable Zn content (mg·kg−1); (b) after logarithm transfor-mation.

tion after logarithm transformation. The main distribu-tion interval was from 0.5 to 3.0 mg·kg−1, and about one fifth of the data showed DTPA-extractable Zn content more than 3.0 mg·kg−1. The descriptive sta-tistics for the data of DTPA-extractable Zn is listed in table 1. The mean and median were used as the pri-mary estimation of the central tendency, and the stan-dard deviation (S.D.), coefficient of variance (C.V. %), and the maximum and minimum values were used as the estimation of variability. The mean value was much higher than the median value, indicating the central tendency dominated by the outliers in the dis-tributions. Parkin et al.[17] reported occasionally the difference in the mean and median values of natural

soil denitrification, and they attributed this difference to the presence of localized areas with extremely high rate of denitrification. The difference of mean and me-dian values and the high coefficient of variance in this study should be attributed to extremely high values of DTPA-extractable Zn in the near suburbs and point- polluted sites.

2.2 Semivariogram analysis and interpolation via kriging

The best fitted semivariogram model of DTPA- extractable Zn was the spherical model, and the cor-responding parameters are shown in table 2 and fig. 4. The nugget semivariance expressed as the percentage of the total semivariance enables the comparison of the relative size of the nugget effect among soil prop-erties[18]. Cambardella et al.[8] used this percentage to drive distinct classes of spatial dependence as follows: if it was less than 25%, the variable was considered strongly spatially dependent; if it was between 25% and 75%, the variable was considered moderately spa-tially dependent; and if it was larger than 75%, the variable was considered weakly spatially dependent. The ratio of C0 to C0+C1 in this study was 49.9%, in-dicating that the DTPA-extractable Zn in soil was moderately spatially dependent.

Table 2 Parameters for best-fitted semivariogram model of DTPA-extractable Zn

Model Nugget C0

Sill C0+C1

Nugget/sill C0/(C0+C1), (%)

Range a/km

Spherical 0.532 0.534 49.9 1.69

Fig. 4. Experimental and model-fitted semivariograms for DTPA- extractable Zn.


Range value (a) is the most important parameter indicating the heterogeneity caused by spatial auto- dependence. In this study, it was only 1.69 km, indi-cating the short distance over which soil DTPA- ex-tractable Zn was spatially dependent, and some im-portant ecological processes having deeply affected the heterogeneity of soil DTPA-extractable Zn. The spatial dependence range was much shorter for DTPA-extractable Zn than for DTPA-extractable Fe (34.9 km)[19].

The kriged map of DTPA-extractable Zn in cultivated soils (fig. 5) indicated the lower average value of DTPA-extractable Zn in the northwest of Xincheng- zhi District, southeast of Sujiatun District, and west of Yuhong District, while the soils in the near suburbs around the city and in the riverside along the Hunhe River and the wastewater drainage of Xihe River had a higher average value. The extremely high values in the

near suburb in Yuhong District were a striking feature in the kriged map. Compared fig. 5 with fig. 1, we could see that the lowest value of DTPA-extractable Zn was in areasⅠand Ⅵ, the highest value was in areas Ⅳ and Ⅴ, while areasⅡand Ⅲ had the mid-dle value. The kriged map of DTPA-extractable Zn showed the strongly heterogeneous characteristics in the study area.

2.3 Effects of land use and soil types on the hetero-geneity of soil DTPA-extractable Zn

Soils are varied widely in their Zn content and in their ability to supply sufficient Zn for optimal crop growth. The soils with poor ability to supply Zn to crops are alarmingly widespread across the globe, and this problem is aggravated by the fact that many mod-ern cultivars of major crops are highly sensitive to low Zn levels[20]. Although the original geologic substrate and its subsequent geochemical and pedogenic regimes

Fig. 5. Kriged map of DTPA-extractable Zn in cultivated soils of Shenyang suburbs.


determined the total Zn levels in soils, they are rarely indicative of the Zn availability, because the Zn avail-ability depends on soil pH, organic matter content, adsorptive surface, and other physical, chemical, and biochemical conditions in the rhizosphere, and hence, land use practice and soil type may induce the hetero-geneity of soil DTPA-extractable Zn. Fig. 6 shows the average DTPA-extractable Zn content in different soil types of Shenyang suburbs, which indicated the effects of soil type and land use on Zn levels. Vegetable gar-den soils, either derived from brown soil or from meadow soil, had the extremely high values of DTPA-extractable Zn. The average content of Zn in vegetable garden soils was 2.5—4 times as much as in their originated soils. Carbonate meadow boggy soil had the lowest average content of DTPA-extractable Zn (0.685 mg·kg−1, n =9), only 12.08% of that in vegetable garden soil derived from brown soil (5.672 mg·kg−1, n=18). The average content of DTPA- ex-tractable Zn was lower in paddy soils than in their originated soils, e.g. paddy soil derived from meadow soil had an average content of 1.181 mg·kg−1 (n= 579), while the meadow soils with sandy, loamy, and clay texture had an average content of 1.803 mg·kg−1

(n=33), 1.887 mg·kg−1 (n=18), and 1.393 mg·kg−1 (n=399), respectively (fig. 6).

The high value of DTPA-extractable Zn in vege-table garden soils could be attributed to the high ap-plication amount of chemical fertilizers and pesticide inputs, and the use of manure and city sludge[9]. The zinc deficiency in paddy soils has been reported worldwide, particularly in calcareous soils[3]. In India, about half of the soils were low in Zn, with as much as 74% of the rice-growing soils of the Andra Pradesh (a southern state in India) being Zn deficient[20]. The re-dox condition in paddy soils when submerged might result in Zn deficiency[21,22], and the increase of soil pH in calcareous soils, the irrigation water containing high amount of HCO3

−, the soils containing high amount of free CaCO3, and the submerged condition might decrease the available Zn content in paddy soils as well[3]. In short, different land use practice may in-duce the spatial heterogeneity of DTPA-extractable zinc in cultivated soils of a certain region.

2.4 Effects of city pollution on the heterogeneity of DTPA-extractable Zn

Determinations on the total Zn contents in some vegetable fields in the near suburb of Shenyang, 10

Fig. 6. Average DTPA-extractable Zn content of different soil types. 1, Brown soil derived from acidic rocks (n, 27); 2, brown soil derived from alkaline rocks (n, 15); 3, brown soil derived from slope deposit (n, 40); 4, brown soil derived from loess deposit (n, 126); 5, meadow brown soil (n, 218); 6, vegetable garden soil derived from brown soil (n, 18); 7, sandy meadow soil (n, 33); 8, loamy meadow soil (n, 18); 9, clay meadow soil (n, 399); 10, vegetable garden soil derived from meadow soil (n, 134); 11, carbonate meadow soil (n, 90); 12, carbonate meadow boggy soil (n, 9); 13, paddy soil derived from meadow soil (n, 579); 14, paddy soil derived from carbonate meadow soil (n, 69); 15, paddy soil derived from brown soil (n, 68).


villages of 5 towns in Dongling and Yuhong districts, showed that (table 3) the total Zn contents in test vegetable garden soils had a striking contrast to the background value in soils of Shenyang[23]. The total Zn contents in test soils were 1.86－10.79 times as much as the background value, indicating the varia-tions of soil total Zn affected by anthropogenic pollu-tion. Although it was not significantly correlated to the total Zn in test soils, the DTPA-extractable Zn in test vegetable garden soils in the near suburb of Shenyang was much higher than its average value in the whole suburbs (tables 1 and 3).

City pollution such as atmospheric dust, vehicle exhaust and sewage sludge could increase the total Zn in soils both in urban and in the vicinity rural areas[3,24]. Heavy application of sewage sludge caused signify- cantly raised Zn concentrations in soils and in all plant parts over the whole growth season[25]. The Zn in both fresh chicken manure and the compost, according to Warman and Cooper[26], exceeded the maximum concentrations of ordinarily used compost. The average contents of total Zn in fresh pig and chicken ma-nure in Liaoning Province of China were 121.0 and 84.5 mg·kg−1, respectively[27], and hence, the heavy application of animal wastes was thought to be one of

the main contributors to the soil Zn enrichment in the near suburb of large cities[28]. Moreover, heavy appli-cation of chemical fertilizers and pesticides in vegeta-ble fields was also thought to be the contributors to soil Zn concentration[9,29]. According to Zhang et al.[27], the average contents of DTPA-extractable Zn in fresh pig and chicken manure in Liaoning Province of China were 38.9 and 31.4 mg·kg−1, respectively, suggesting that the use of manure, especially of chicken manure be the cause of high DTPA-extractable Zn content in soils of the near suburb. It could be concluded that city pollution was one of the main causes of the heteroge-neity of DTPA-extractable Zn in the vegetable field area of the near suburb.

The total Zn content in test wastewater of Xihe River during irrigation season ranged from 18.2 to 30.4 mg·L−1, with the average value of 25.4 mg·L−1, which was 12.7 times as much as China’s national irrigation limit (table 4). Xihe River is a principal wastewater drainage in the western suburb of Shen-yang, which collects the industrial wastewater from dozens of factories in western Shenyang’s Tiexi Dis-trict and the living sewage from northern Shenyang’s Huanggu District, with an average daily output of 5×105 t. Meanwhile, Xihe River is also an irrigation

Table 3 Total Zn content of vegetable garden soils in near suburb of Shenyang and its background value

Sample site DTPA-Zn/mg·kg−1 Total Zn/mg·kg−1 Total Zn/background value a)

Changqing, Wushan Town, Dongling District 4.665 275.8 4.30

Hunhe, Wushan Town, Dongling District 3.426 146.2 2.28

Qianzhai, Hunhe Town, Dongling District 6.852 119.0 1.86

Huoyu, Hunhe Town, Dongling District 7.281 128.0 1.99

Xiyao, Lingdong Town, Yuhong District 8.998 131.0 2.04

Fangxi, Lingdong Town, Yuhong District 7.631 162.2 2.52

Dahan, Beiling Town, Yuhong District 4.449 241.5 3.77

Liutiaohu, Beiling Town, Yuhong District 7.337 692.4 10.79

Fugan, Yangshi Town, Yuhong District 5.232 190.9 2.98

Dapu, Yangshi Town, Yuhong District 6.222 461.7 7.20

a) The background value of Zn in soils of Shenyang was 64.13 mg·kg−1[23].

Table 4 Total Zn content in wastewater of Xihe River (n=10)

Total Zn content/mg·L−1

range mean Variance coefficient (%) National irrigation standard /mg·L−1 Pollution index

18.2—30.4 25.4 15.7 ≤2.0 12.7


drainage which has irrigated the fields of the riverside villages and towns for about forty years[30]. It could be concluded that the Zn enrichment in soils of this area was due to the long-term irrigation by wastewater, and hence, the DTPA-extractable Zn was heterogeneous in this area compared to the nearby areas. The Zn incre-ment in cultivated fields irrigated by wastewater was reported in some large cities of China, and most re-sults were in accordance with ours. For example, the total Zn in wastewater irrigation area in western sub-urb of Beijing was 1.45 times as much as in its fresh-water irrigation area, and 2.33 times as much as the background value[31].

3 Conclusion

The spatial heterogeneity of DTPA-extractable Zn in cultivated soils of Shenyang suburbs was deeply influenced by the stochastic factors such as city pollu-tion, land use patterns and crop distribution. The high values of soil DTPA-extractable Zn in the near suburb and in the wastewater irrigation area of Shenyang were basically induced by the city pollution from an-thropogenic activities. Currently, chemical Zn fertiliz-ers are so ordinarily used in cultivated soils in the suburbs of Shenyang and of other large cities of northern China that the soil Zn map and related infor-mation obtained in this study could be useful in guid-ing Zn fertilization practice. Wastewater irrigation was considered as a main source of Zn and other heavy metal enrichments in soils, which should be forbidden in city suburbs. When recorded in the form of a soil map, the results of such a survey make it possible to identify the unusually polluted areas, and also, to pro-vide more information for precise agriculture and en-vironmental control.

Acknowledgements The authors wish to express their appreciation to Prof. Likai Zhou for his revision and valuable comments on the earlier manuscript of this paper. This work was supported by the High-tech Research and Development Program of China (Grant No. 2004AA246020), and by the Fund of Shenyang Experimental Station of Ecology, Chinese Academy of Sciences (Grant No. SYZ0204).

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