Scholarly article on topic 'Effects of the Combined Pollution of Lead and Cadmium on Soil Urease Activity and Nitrification'

Effects of the Combined Pollution of Lead and Cadmium on Soil Urease Activity and Nitrification Academic research paper on "Biological sciences"

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{"Soil urease" / nitrification / lead / cadmium / "combined pollution"}

Abstract of research paper on Biological sciences, author of scientific article — Jinlong Yan, Guixiang Quan, Cheng Ding

Abstract The effects of heavy metals contamination on soil are quite alarming and can cause huge disturbances in the ecological balance and health of living creatures on earth. Effects of the heavy metals combined pollution of lead (Pb2+) and cadmium (Cd2+) cations on soil urease activity and nitrification were studied by soil incubation method to evaluate whether there is a synergistic interaction on soil enzyme activities, nutrient cycling and pollutants. Results showed that activated effect on soil urease activity was found only in the low concentrations (such as 0.5 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined) than the control, and the inhibitory effect was existed in most of the higher concentrations (P < 0.05). With the increasing of Pb2+ concentration in soil from 0.5 mg/kg to 100.0 mg/kg combined with 0.5 mg/kg of Cd2+, the soil urease activity decreased and varied as the incubation proceeded. At the same time, soil nitrification was also inhibited in a certain degree with the threat of heavy metals contamination, and the nitrifying activity in contaminated soil samples were significantly lower than the control. A statistical analysis indicated that there were some correlations between the inhibition of soil urease activities and nitrification.

Academic research paper on topic "Effects of the Combined Pollution of Lead and Cadmium on Soil Urease Activity and Nitrification"

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Environmental Sciences

Procedia Environmental Sciences 18 (2013) 78-83

2013 International Symposium on Environmental Science and Technology (2013 ISEST)

Effects of the combined pollution of lead and cadmium on soil urease activity and nitrification

Jinlong Yan*, Guixiang Quan, Cheng Ding

School of Environmental Science and Engineering, Yancheng Institute of Technology, 9 Yingbin Avenue, Yancheng 224051, China

Abstract

The effects of heavy metals contamination on soil are quite alarming and can cause huge disturbances in the ecological balance and health of living creatures on earth. Effects of the heavy metals combined pollution of lead (Pb2+) and cadmium (Cd2+) cations on soil urease activity and nitrification were studied by soil incubation method to evaluate whether there is a synergistic interaction on soil enzyme activities, nutrient cycling and pollutants. Results showed that activated effect on soil urease activity was found only in the low concentrations (such as 0.5 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined) than the control, and the inhibitory effect was existed in most of the higher concentrations (P < 0.05). With the increasing of Pb2+ concentration in soil from 0.5 mg/kg to 100.0 mg/kg combined with 0.5 mg/kg of Cd2+, the soil urease activity decreased and varied as the incubation proceeded. At the same time, soil nitrification was also inhibited in a certain degree with the threat of heavy metals contamination, and the nitrifying activity in contaminated soil samples were significantly lower than the control. A statistical analysis indicated that there were some correlations between the inhibition of soil urease activities and nitrification.

© 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beiji^ Institute of Technology. Keywords: Soil urease; nitrification; lead; cadmium; combined pollution

1. Introduction

In recent years, soil pollution was becoming more and more severe due to the increased using in mining, manufacturing and of synthetic products, such as pesticides, paints, batteries, industrial waste and land application of industrial or domestic sludge [1]. Polluted soil area has reached 20 million hm2 in China, accounted for about 1/5 of the total cultivated area [2, 3], threatening the soil ecological environmental quality, food security and social sustainable social and economic development [4]. Much concern has been addressednover the problem of soil contamination with heavy metals due to rapid

* Corresponding author. Tel.: +86-515-88298803; fax: +86-515-88298806. E-mail address: yjlyt4788@yahoo.com.cn.

1878-0296 © 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology.

doi: 10.1016/j.proenv.2013.04.011

industrialization and urbanization. Moreover, the soil environment is interfacial and heterogeneous, and the basic characteristics of soil contamination are different from those of air and water, such as concealment and hysteresis, accumulation, and irreversibility [5, 6]. Thus, the effects of heavy metals contamination on soil are quite alarming and can cause huge disturbances in the ecological balance and health of living creatures on earth.

Nitrogen has long been recognized as one of the most limiting nutrients for ecosystem productivity [7]. Plant species, both aboveground and belowground, are in intense competition for available N in agricultural ecosystems [8], and soil potential nitrification rate (PNR) represents great ecological indicator of ecosystem productivity [9]. Researches has also show that nitrification was inhibited significantly in soil amended with sludge containing high level of heavy metals, such as

effects of copper ions (Cu2+) on PNR in 17 kinds of typical soils in China was found to be decreased in the form of ladder with the increase of Cu2+ [11].

Soil microbes are important reflection for soil quality and enzyme activity involved in the biogeochemical cycling of carbon, nitrogen, phosphorus, sulphur and other nutrients [12].The major parameters of soil quality are the soil biological properties. Among these special emphasis is made of the enzyme activities. Soil enzyme activities have been generally accepted as one of the diagnostic indices of soil fertility quality and can quantify changes as a result of human disturbance [13]. Application of enzyme activity as a diagnostic index promotes high sensitivity to external effects, simplicity of definition and low errors [14]. The negative effects of heavy metals on soil enzyme activity have been recognized, but the mode of action of heavy metals varies from one enzyme to another, and the extent to which enzymatic activity is inhibited depends both on the concentration and oxidation state of the heavy metal and on the soil characteristics, such as pH, texture, organic matter content [15].

The combined effects of Pb2+ and Cd2+ on the soil urease activity and nitrification with different concentrations in the incubation proceeded periods were studied by indoor incubation method in this research, which aims to evaluate whether there is a synergistic interaction on soil enzyme activities, nutrient cycling and pollutants by statistic analysis.

2. Material and Methods

2.1. Soil sample and preparation

We used surface soil (0-20 cm) from east campus of Yancheng Institute of Technology, Jaingsu Province, China (33°38'22"N and 120°13'58"E). According to the World Reference Base for soil resources [16 and Chinese Soil Taxonomic Classification [17], this soil is a silty clay (Stagnic Anthrosol). Stones and obvious pieces of organic materials were removed. After that, samples were stored in plastic bags in a refrigerator at 4 °C for further experiments. Some physico-chemical characteristics of the soil are shown in Table 1. Soil organic C content was determined by the Walkley and Black dichromate oxidation method [18]. Soil pH was measured in a 1:2.5 soil-water slurry using a glass electrode. Saturation paste extracts were prepared, the electrical conductivity (EC) was measured and the K+ and Na+ concentrations were analysed by a PE-Zeemam-5100 atomic absorption spectrophotometry (AAS). Soil total nitrogen (TN) was estimated using the semi-micro Kjeldahl method. Available phosphorus was determined by the classical Olsen method [19]. Exchangeable cations were extracted with 1.0 mol/L ammonium acetate (1:4 w:v for 2 h in a rotary shaker) and Ca2+, Mg2+, K+ and Na+ in extracts were analyzed by AAS. Clay mineral compositions were identified by X-ray diffraction analysis.

2.2. Soil amendment and incubation

Moist soils were sieved (< 2 mm) and incubated at 45 % water holding capacity (WHC) and 25 °C for 7 days prior to use. The soils (100 g) were treated with 5 mL of CdCl2 or PbCl2 solutions to give the heavy metals concentration at six different levels, which are 0.5, 1.0, 5.0, 10.0, 50.0 and 100.0 mg/kg Pb2+ with the constant concentration of 0.5 mg/kg Cd2+. Untreated soils were served as controls. The soil moisture was adjusted to 60 % WHC with distilled water and then the soils were incubated at 25 °C and kept darkness for 45 days. All treatments were replicated three times.

2.3. Soil urease and nitrifying activity measurement

The urease (EC 3.5.1.5) activity in the soil was measured with the buffered method of Kandeler and Bremner [20]. In this procedure, 0.5 mL of a solution of urea (0.48 %) and 4 mL of borate buffer (pH 10) were added to 1 g of soil in hermetically sealed flasks, and then incubated for 2 h at 37 °C. The ammonium content of the centrifuged extracts was colorimetrically determined at 578 nm by a modified indophenol-blue reaction. Controls were prepared without substrate to determine the ammonium produced in the absence of added urea, and the activity was expressed as NH4+-N mg/(g h).

After the soil amendment and incubation, ten grams of soil with six different levels was thoroughly mixed with 90 mL distilled water, respectively. 1 mL of the obtained soil solution was further transferred into 100 mL conical flask containing 30 mL nitrifying bacteria culture, and incubated for another 15 days at 28 °C. Nitrifying activity of those soils was measured by the decrease of nitrite (%). Here, the nitrifying bacteria culture consisted of 1 g NaNO2, 0.03 g MgSO4 -7^0, 0.01 g MnSO^HO, 0.75 g K2HPO4, 1 g Na2C0s, 0.25 g NaHP04 and 1g CaCOs in 1 L distilled water with the natural pH value.

2.4. Statistic analysis

All determinations were performed in triplicate; all values reported are means with their standard error on the basis of the oven dry (105 °C) weight of soil. Excel 2007 and Origin 7.0 analysis software for windows were used for statistical tests.

Table 1. Physiochemical Properties of Soils Tested.

Soil characteristics

Values ± Standard errors

EC (ds/m) CEC (meq/100 g) Clay (%) Silt (%) Sand (%) TOC (%) TN (%)

Available phosphorus (%) Available potassium (%) Available sodium (%) EC (ds/m)

2.43 ± 0.06

0.042 ± 0.011

0.073 ± 0.018

42.50 ± 1.46

2.34 ± 0.08

32.30 ± 1.28

25.20 ± 0.78

3.12 ± 0.16

3.12 ± 0.16

1.06 ± 0.12

19.40 ± 1.0

3. Results and dicussion

3.1. Effects of the combined pollution of Pb2+and Cd2+ on soil urease activity

Effects of the combined pollution of Pb2+ and Cd2+ on soil urease activity were shown in Table 2. Compare with the controls, the enzymes activities were decreased with the increasing concentrations of Cd2+ and the incubation periods except for treatments of 0.5 mg/kg Cd2+ only and 0.5 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined. Urease activities were found to be sensitive to the inhibition effect of heavy metals. After 45 days incubation under the concentrations of 100.0 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined, the inhibition rates of soil urease activity was determined at 73.1 % compared to the control.

Disagree with the results of Zhou et al. [21], soil enzyme activity was found to be a different degree of recovery after a long period of heavy metals stress in their research, which would be contributed to the different concentrations of heavy metals. The inhibition effect of heavy metals to soil enzyme activities was the results of the changes of chemical conformation mainly due to the coordination reaction. Based on Lewis's hard and soft acids and base theory (HSAB), the active sites in enzyme protein molecular, such as thiol or imidazolyl groups, were preferred coordinated with soft heavy metals.

Table 2. Effects of the heavy metals pollution on soil urease activity .

Treatments Incubation period

10 days 20 days 45 days

CK 4.12 ± 0.16 4.26 ± 0.14 4.20 ± 0.18

0.5 mg/kg Cd2+ 4.46 ± 0.23 4.37 ± 0.17 4.53 ± 0.21

0.5 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 4.18 ± 0.19 4.24 ± 0.15 4.27 ± 0.15

1.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 3.88 ± 0.14 3.74 ± 0.24 3.50 ± 0.16

5.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 3.52 ± 0.17 3.32 ± 0.09 3.21 ± 0.13

10.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 3.24 ± 0.13 3.10 ± 0.12 3.01 ± 0.18

50.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 2.12 ± 0.10 1.98 ± 0.14 2.04 ± 0.08

100.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 1.64 ± 0.15 1.28 ± 0.17 1.13 ± 0.12

3.2. Effects of the combined pollution of Pb2+and Cd2+ on nitrifying activity

The influences of combined pollution of Pb2+ and Cd2+ on soil nitrifying activity after 45 days incubation was listed in Table 3. Disagree with that on soil urease activity, the inhibition effect was appeared in all these amendments including the lower concentration, such as the 0.5 mg/kg Cd2+ only treatment and 0.5 mg/kg Pb2+ and 0.5 mg/kg Cd2+ combined. In comparison with the control, soil nitrifying activity in soil contaminated with 0.5 mg/kg Cd2+ was found to be 79.23 ± 4.20 %, lower than the control 83.12% ± 4.16 %. The relative inhibition was increased with the increasing of Pb2+ concentration. When the content of Pb2+ increased from 0.5 mg/kg to 100.0 mg/kg combined with the constant concentration of 0.5 mg/kg Cd2+, the relative inhibition increased from 4.7 % to 47.6 %.

Table 3. Effects of the heavy metals pollution on soil nitrifying activity after 45 days incubation.

Treatments Nitrifying activity Relative inhibition (%)

CK 83.12 ± 4.16 0

0.5 mg/kg Cd2+ 79.23 ± 4.20 4.7

0.5 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 74.20 ± 4.58 10.7

1.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 68.30 ± 3.28 17.8

5.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 62.50 ± 3.41 24.8

10.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 60.12 ± 3.12 28.3

50.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 52.07 ± 3.18 38.4

100.0 mg/kg Pb2+ + 0.5 mg/kg Cd2+ 43.52 ± 3.16 47.6

3.3. Statistic analysis of soil urease activity and nitrifying activity

Soil enzyme activities, soil microbial community structure and biochemical processes usually have complicated relationships among them. It was noted that numerous factors control their relative abundance, e.g., original contents of heavy metals, various processes of soil formation, and anthropogenic factors such as the contamination by human activities [22].

In order to evaluate whether there is a synergistic interaction on soil enzyme activities, nutrient cycling and pollutants, the correlation between the relative inhibition of soil urease activity and soil nitrifying activity were depicted in Fig. 1, and a significant positive correlation was found between them (P < 0.05). The correlation coefficient was found to be 0.942 (R2), which reflect that heavy metals had similar effect on soil nitrogen cycling and if s relative microbial activity.

Fig. 1. Correlative curve of the inhibition of soil urease activity and nitrifying activity.

4. Conclusions

Urease activities were found to be sensitive to combined pollution of heavy metals lead and cadmium, and the inhibition effect was not found to be recovery in the whole incubation period. There was a significantly positive correlation between soil urease activity and nitrifying activity, indicated that there was a synergistic interaction on them.

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