Bull Environ Contam Toxicol DOI 10.1007/s00128-015-1562-0

Impact of Long-Term Irrigation with Treated Sewage on Soil Magnetic Susceptibility and Organic Matter Content in North China P. G. Yang1 • M. Yang2 • R. Z. Mao3 • J. M. Byrne4

Received: 2 September 2014 / Accepted: 8 May 2015 Ó Springer Science+Business Media New York 2015

Abstract This study assessed the effect on magnetic susceptibility and organic matter content of arable soil by irrigation with either treated sewage or groundwater. Results indicated that organic matter and magnetic susceptibility values in the soil irrigated with sewage were increased by 7.1 % and 13.5 %, respectively, compared to agricultural soil that irrigated with groundwater. Both the sewage and groundwater irrigated soils contained a significant fraction of ultrafine superpara magnetic grains, as indicated by high frequency dependent susceptibility (vfd [ 6 %). The enhancement of soil magnetic properties was determined to be caused by anthropogenic sewage irrigation and agrochemical use by investigation of vertical soil profiles. Magnetic susceptibility parameters were shown to be significantly correlated with organic matter content (y = 0.0057x ? 1.3439, R2 = 0.09, p \ 0.05). This work indicates that measurements of magnetic susceptibility may offer a rapid first step for identifying the potential pollution in arable soils.

& P. G. Yang [email protected] R. Z. Mao [email protected] 1

College of Life Sciences, Shanxi Normal University, Linfen 041000, China

2

College of Arts and Sciences, Shanxi Agricultural University, Taigu 030801, China

3

Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, China

4

Center for Applied Geoscience, University of Tu¨bingen, Tu¨bingen 72076, Germany

Keywords Groundwater  Magnetic susceptibility  Sewage irrigation  Soil organic matter The use of sewage water for the irrigation of agricultural sites is a widespread practice in many developing countries (Yildirim and Topkaya 2012), as an economic alternative to disposal in surface waters and also contributes to nutrient cycling. Sewage irrigation provides water, nitrogen and phosphorus as well as organic matter to the soils. However, in the long term pollutants may accumulate in the soils and cause a potential risk to soil quality and productivity (Friedel et al. 2000). Soil organic matter is one of the most important indicators of soil quality and plays a major role in nutrient cycling. The presence of organic matter favors the enhancement of magnetic susceptibility by providing the conditions necessary for the reduction of iron (Mullins 1977) which can lead to the formation of magnetite (Fe3O4) or maghemite (c-Fe2O3). Maher (1988) previously reported a correlation between magnetic susceptibility and organic carbon or clay content in UK cambisol. Other studies have also found a correlation between magnetic susceptibility and organic matter content in urban street dusts (Xie et al. 2000; Shilton et al. 2005) and in river terraces (Torrent et al. 2010). Magnetic methods can be used as a fast and inexpensive approach to investigate the degree of pollution, determine its sources, and also identify the level of anthropogenic activity in soil (Petrovsky et al. 2000; Blundell et al. 2009). Magnetic susceptibility is a particularly useful, rapid and efficient technique which can be used to provide qualitative information of many magnetic parameters. It is sensitive enough to detect ferromagnetic materials with concentrations of less than 1 % (Thompson et al. 1980). Soil magnetic susceptibility is significantly influenced by the

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lithology, soil-forming processes and anthropogenic factors (Grimley and Vepraskas 2000; Lecoanet et al. 2003; Blaha et al. 2008; Yang et al. 2012), however the parent material has an important impact on the content of magnetic minerals that are present (Hana et al. 2005; Hu et al. 2007). Furthermore, anthropogenic activities such as the combustion of fossil fuels, road traffic, use of fertilizers and pesticides and waste material management can all influence the content of magnetic minerals. The main objectives of the study were to: (1) test magnetic susceptibility and organic matter content of soil under irrigation with water from different sources (i.e. groundwater and sewage sources), (2) determine the relation between magnetic susceptibility and organic matter content, and (3) assess the degree of soil contamination using magnetic susceptibility.

Materials and Methods The study area was located at longitude 37° 470 –38°010 N, latitude114°290 –114°470 E, in the southeast suburbs of Shijiazhuang city, the capital of Hebei province. The study site encompassed an area of 400 km2. The site has a temperate semi-arid climate with a mean annual temperature of 13°C and average annual rainfall of 513 mm. It is a part of the crop and fruit production base of the North China Plain, where winter wheat, summer maize, apple, pear and some vegetables are cultivated. Soils have developed on alluvial sediment materials and are mostly of the soil type Mottlic Hapli-Ustic Argosols. About 25 % of the agricultural land has been irrigation with sewage for more than 40 years. Such a long period of irrigation with sewage of agricultural land in a temperate region is rare, makes this an ideal study site. The soil sampling was performed in 2011. Soil samples that were formed on the same parent material were subdivided into two groups according to different water sources, one irrigated with treated sewage effluent, and one irrigated solely with groundwater (Fig. 1). The sampling sites were controlled by GPS device. Soil samples of 1 kg were taken from a point of 20 9 20 cm2, and from a depth of 20 cm. A total of 62 and 116 soil samples were collected from sewage and groundwater irrigated farmland, respectively. The composite samples were air-dried and passed through a 2-mm sieve in the laboratory. Samples for magnetic susceptibility were prepared in standard nonmagnetic 10 cm3 cylindrical pots and measured at low frequency (vlf 0.47 kHz) and high frequency (vhf 4.7 kHz) using a Bartington Instruments dual frequency MS 2B on the 0.1 scale (Bartington instruments, Oxford, UK). Frequency dependent susceptibility (vfd) was calculated using

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the low- (vlf) and high-frequency (vhf) susceptibility, as vfd% = (vlf–vhf)/vlf 9 100 (Dearing et al. 1996). Organic matter (OM) content was measured by means of the Walkley–Black wet oxidation (K2Cr2O7) method (Nelson and Sommers 1982). The parameters of soil magnetic susceptibility (v) and OM for the two different water irrigation programs were analyzed by student’s t test using SPSS 19.0 (SPSS Inc., Illinois, USA). Significance was set at alpha equal to 0.05.

Results and Discussion The OM in the sewage irrigated soils content ranged from 1.35 % to 3.07 %, with an average value of 1.81 % ± 0.27 % (Table 1). These values were higher than that of groundwater irrigated soils, in which the organic matter varied from 1.03 % to 2.17 %, with an average value of 1.69 % ± 0.21 %. The coefficient of variation (C.V.) for OM in with sewage effluent irrigated soil (0.15) was only slightly higher than that with groundwater irrigated soil (0.12), suggesting a similar variability of organic matter for the two irrigation programs. The OM content was increased by 7.10 % as a result of long-term sewage irrigation (Table 1). Similarly, Yadav et al. (2002) reported higher organic carbon in soils irrigated with sewage water as compared to those irrigated with tubewell water. Rattan et al. (2005) also observed that the continuous use of sewage water for irrigation led to an increase in soil organic carbon. Magnetic susceptibility (v) is defined as the magnetization acquired per unit field (Evans and Heller 2003). The vlf is used to estimate the concentration of magnetic minerals in a sample as well as the types of magnetic grains present and their size (Thompson et al. 1980; Maier and Scholger 2004; Yang et al. 2009; Magiera et al. 2011), whereas the vhf measures the contribution of the paramagnetic and antiferromagnetic minerals that are present (Evans and Heller 2003). The vlf values were 73.8 ± 13.9 9 10-8m3 kg-1 and 65.0 ± 9.9 9 10-8m3 kg-1 for soils irrigated with sewage and groundwater, respectively. The vlf values ranged from 44.1 9 10-8m3 kg-1 to 108.0 9 10-8m3 kg-1 in the sewage irrigated areas and from 40.0 9 10-8m3 kg-1 to 87.4 9 10-8m3 kg-1 in groundwater irrigated region. The effluent irrigated soils were characterized by generally higher vlf values compared to the groundwater irrigated area. Our results were similar to those of Zhang et al. (2013), who studied farmland that was irrigated with polluted river water. The same observations also applied for a study of chernozem on loess, which was determined to have vlf of about 80 9 10-8m3 kg-1 in the topsoil and approximately 40 9 10-8m3 kg-1 in the subsoil within Bulgaria (Jordanova and

Bull Environ Contam Toxicol Fig. 1 Location map of soil sampling sites and the study area. The shaded area with oblique lines is the sewage irrigated region

Table 1 Descriptive statistics of soil magnetic susceptibility and organic matter

n

Min

Max

Mean

Increase (%)

SD

C.V.

Skew.

Kurt.

Sewage irrigated region vlf (10-8 m3 kg-1)

62

44.02

108.02

73.79

13.45

13.86

0.19

0.01

vfd (%)

62

3.83

13.17

6.91

0.88

1.69

0.24

0.71

2.09

Organic matter (%)

62

1.35

3.07

1.81

7.10

0.27

0.15

1.63

6.62

-0.27

Groundwater irrigated region vlf (10-8 m3 kg-1)

116

40.04

87.44

65.04

–

9.95

0.15

0.02

-0.43

vfd (%)

116

2.68

9.61

6.85

–

0.99

0.14

-0.24

2.55

Organic matter (%)

116

1.03

2.17

1.69

–

0.21

0.12

-0.08

0.28

Jordanova 1999), which is similar to values found in our study. The vfd can be used to indicate the presence of ultra-fine super paramagnetic ferromagnetic grains, and may be used to discriminate between grain size and domain state (Dearing et al. 1996). It is used as a proxy for the concentration of pedogenic maghaemite (Torrent et al. 2010). Anthropogenic magnetic particles have many different sources, but most research so far has suggested that they are generally dominated by multiple domain (MD) and stable single domain (SSD). Ferrimagnetic minerals produced in the pedogenic processes of this study were predominantly of super paramagnetic (SP) (\0.02 lm) and

stable single domain (SSD) (0.02–0.04 lm) grain sizes. Relatively higher values of vfd were observed in sewage irrigated soil 3.8 %–13.2 % compared to the groundwater irrigated area 2.7 %–9.6 %. The average vfd for both sewage and groundwater irrigation areas were measured to be higher than 6 %, which indicates that the samples were likely dominated by a significant proportion of SP grains. Soils in the groundwater irrigation areas have vfd values below 3 %, which means that coarser magnetic grains in the SD to MD range dominate while ultrafine superparamagnetic particles are not or little represented (Dearing et al. 1996). In comparison, previous magnetic pedological studies have shown mean vlf and vfd of topsoils to be

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73 9 10-8m3 kg-1 and 4.1 %, respectively (Dearing et al. 1996; Yang et al. 2012), which are to the values of 70 9 10-8m3 kg-1 and 6 % recorded in this study. The student’s t test confirmed significant difference in soil magnetic susceptibility and OM for the soils under sewage and groundwater irrigation. The parameters of vlf and OM had t test and p values of vlf (t = 4.85, p = 0) and OM (t = 3.13, p = 0), respectively. However, the p value for vfd was 0.79, indicating that there was not a significant difference in the content of frequency dependent susceptibility between soils irrigated with sewage or groundwater for this property. In soil, the total ferromagnetic component generally includes titanomagnetites, magnetite and maghemite (Thompson et al. 1980), whilst magnetite and maghemite are found in biologically active organic-rich topsoil horizons. The correlations between magnetic susceptibility and humus or clay content have previously been shown to reflect the main binding mechanisms of magnetic materials in the soil. Non-parametric Pearson’s correlation analyses between magnetic susceptibility and organic matter content are shown in Fig. 2. A significant positive linear correlation was obtained between vlf and OM (y = 0.0057x ? 1.3439, R2 = 0.09, p \ 0.05) in sewage and groundwater irrigated areas combined. A non-significant negative correlation was determined for vfd and OM in the study area. The trend of the vfd and OM relation is opposite to one presented in previous work. This is attributable to the fact that samples

3.5

3.5

y = 0.0057x + 1.3439

3

y = -0.0234x + 1.8948

3

2

R = 0.09

2.5

2

R = 0.0165

2.5

OM(%)

OM(%)

Fig. 2 Relationships between magnetic susceptibility properties (vlf and vfd) and organic matter (OM) in soil samples from the entire study region

were collected from typical farmland topsoil and not from street dust. The magnetic susceptibility and organic matter content in agricultural soils is also observed to be lower than that of urban street dust (Shilton et al. 2005). Similar studies have indicated strong relationships between v and organic matter, which is usually considered to be caused by anthropogenic enhancement of v in the uppermost organic layer. In the A horizon of chernozems in Germany, magnetic susceptibility and humus content were found to be correlated (Neumeister and Peschel 1968). Hanesch and Scholger (2005) obtained the correlations between magnetic susceptibility and humus or clay content. These relationships were expected since the presence of organic matter favors the enhancement of susceptibility by providing the conditions necessary for iron reduction (Mullins 1977). It is also possible that another reason for the variability of soil susceptibility is the retention of magnetic particles in the soil by different mechanisms, e.g. adsorption on clay particles or on organic matter. A series of different processes are known to result in an enhancement of the magnetic signal in soils/sediments (Zhang et al. 2013). However, it must be clearly stated that for successful magnetic susceptibility mapping of polluted areas, a low geogenic/lithogenic background signal is required, as well as an isolated or dominating source of contamination. Therefore, the anthropogenic magnetic signal must be discriminated from the background. Two soil profiles were measured in order to qualitatively determine the magnetic susceptibility background value (Fig. 3). A stable

2 1.5 1

2 1.5 1

0.5

0.5

0 30

80

0

130

-8

3

-1

-8

3

-1

2

7

χlf (10 m kg )

Fig. 3 Magnetic susceptibility properties (vlf and vfd) for soil profile

χfd (%)

χlf (10 m kg ) 0

20

40

60

20

80

0

Depth (cm)

Depth(cm)

0

17

χfd (%)

40 60 80 100

0

2

4

6

8

20 40 60 80 100

Groundwater

123

12

Sewage

Groundwater

Sewage

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Bull Environ Contam Toxicol

background level (10–20 9 10-8m3 kg-1) was reached at about 100 cm depth. Sewage irrigated area exhibited larger enhancement of the susceptibility in the topsoil, which was about five times higher than the background level with corresponding values for groundwater irrigated areas about three times higher (Fig. 3). The vfd value can be used as an index to reflect the formation of pedogenic particles only when its value is larger than 5 %, but less than 10 % (Dearing et al. 1996). Soil profiles with vfd [ 6 % show samples that contain a significant proportion of SP grains (Fig. 3). It is interesting to note that groundwater and sewage irrigated soil profile magnetic susceptibility values exhibited similar changes to each other. Treated sewage is the only significant source of pollution in the sewage irrigated area, and no fertilizer was used for agricultural activities. In contrast, the groundwater irrigated region was subjected to the agricultural use of pesticides and fertilizers (fertilizer 527.8 kg hm-2 and pesticide 12.4 kg hm-2) and mixing by being plowed, which combined with biogenic processes, might have led to an enhancement of magnetic susceptibility in the topsoil. This is a complex process which needs further investigation. Soil irrigation using treated sewage contributed to a slight increase in the magnetic susceptibility and organic matter content compared with groundwater irrigation. A correlation between OM content and magnetic susceptibility in arable soil was demonstrated. At a regional scale, variations in soil magnetic susceptibility are linked to geology, soil processes, and intense human activities. The topsoil in the study area contained a significant fraction of SP grains, as indicated by high vfd values (vfd [ 6 %).The soil profiles showed an enhancement of the magnetic susceptibility in the topsoil, which suggests that the topsoil is the most suitable zone for the detection and monitoring soil quality with different water irrigation treatments. Farmland soils that were irrigated with sewage showed some elevated degree of anthropogenic pollution over soils irrigated with groundwater. Furthermore, it should be pointed out that health hazards to soil quality could potentially emerge from the use of sewage for irrigation purposes, although that has not been assessed here. Acknowledgments The authors are grateful to the editor Erin Bennett, anonymous reviewers for their useful suggestions and Pro. Erwin Appel for constructive discussions. This work was supported by the National Natural Science Foundation of China (NSFC No.31272258).

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