Fangli Wangab,
Wei Ouyang*a,
Fanghua Haoa,
Andrea Crittob,
Xuchen Zhaoa and
Chunye Lina
aState Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, P. R. China. E-mail: wei@bnu.edu.cn; Fax: +86 10 5880 2078; Tel: +86 10 5880 2078
bDepartment of Environmental Sciences, Informatics and Statistics, University Ca' Foscari, Calle Larga S. Marta 2137, I-30123 Venice, Italy
First published on 30th April 2015
Multivariate interactions are far more complex between natural factors and pollutants resulting from anthropogenic practices than between pollutants themselves. But little attention has been focused on the complex interpretation of multivariate interactions. To bridge this research gap, this study aimed to identify the interactive effect of multiple affecting factors including freeze–thaw cycles (FT), soil water (SW) content, and chlorpyrifos (CP) on soil Cd behavior in arable soil, based on the analysis of changes in Cd fractionations and Cd availability. Moreover, the significant effect was computed via design of experiments. The content of Cd fractions and the evaluated index of Cd availability were obtained by employing the modified six-step sequential extraction method. The results showed that the main effect of FT, SW and CP on Cd fractionation and availability was significant. The binary interactions weakened the main effect of FT or SW, but enhanced the main effect of CP on Cd fractionation. The ternary interactions further weakened the binary interactions, whereas CP enhanced the interaction between SW and FT. The interaction between SW and CP had a negative effect on residual Cd, but positively affected water-soluble and organic matter-bound Cd. The binary interaction between CP and FT had a positive effect on residual Cd (21.0%), but negatively affected water-soluble and Fe–Mn-oxide-associated Cd (25.9% and 21.1%). These results covered more innovative information on the multivariate interactions between natural and anthropogenic factors on Cd behavior in arable soil. A possible new way to quantify the significant impact of multivariate factors also was provided.
Seasonal freeze–thaw (FT) is fairly common in mid-high latitude regions among these natural factors. FT and soil water content (SW) play an import role in controlling the mass transport and energy exchange in the soil-plant-atmosphere-climate system.6 Also, both FT and SW can alter soil physio-chemical and biological properties,5,7 such as soil pH, the dissolved organic matter (DOM) and the activity of soil organisms, which are easily affected by pesticides.8 At least two of these properties are usually observed to control HMs behavior.9 For instance, FT can destroy soil aggregates, and then increase the content of DOM, which will subsequently alter the capacities of soil to bind HMs.10,11 The increased SW can enhance the extent to which soil aggregates are damaged by FT, which will decrease when SW exceeds the saturation value.12 A significant increase in the total amount of free amino acids and sugars caused by freezing is combined with an increment in soil respiration and dehydrogenase activities,13 which can easily be affected by pesticides.14 The above phenomenon indicates that interactions between the two natural factors (FT and SW) and pesticides are far more complex in the mechanism, which may directly/indirectly further affect the behavior of pollutants themselves. Therefore, a scientific basis should also be provided to consider the impact of the natural environmental factors, especially on that how natural stressors interact with chemical stressors. Further researches are desired to conduct focusing on the elaborate experimental design and the complex interpretation of multivariate interactions.2
Design of experiments (DOE) has been reported to provide maximized information of experimental statistics and to get unambiguous results taking minimized efforts.15 2n full factorial design in DOE is a statistical technique for designing experiments where n affecting factors are controlled.16 The effects of various affecting factors are investigated at each of two levels, containing the minimal and the maximal input–output data pairs.17 On this basis, the effect of main effects and interactions is assessed by the outputs viz the values of estimated effect (E). A higher E reveals a stronger effect. The main effect reveals the impact of one changed controlling factor, while the interaction reveals the combined impact of the multivariate affecting factors.16 This method has been widely used in behavior sciences to analyse a random response of output variables to a set of various affecting factors.
Therefore, to bridge the research gap in multivariate interactions between natural and anthropogenic factors on HMs behavior, based on the analysis of changes in HMs fractionations and HMs availability, this study aims to: (1) evaluate the potential binary and ternary interactions between natural factors (FT and SW) and pesticides on HMs behavior in arable soil; (2) provide a possible way to quantify the significant effect of these affecting factors on the basis of DOE analysis. Cd and chlorpyrifos (CP) were selected as the representatives due to their intrinsic properties, such as the tendency of accumulation,18 the most toxic and mobile in arable soil19 and the special chemical structure to inhibit the activity of acetylcholinesterase.8,14 Cd fractionation and Cd availability were selected to investigate because they have been reported to provide valuable information on represent Cd behavior.9 Soil samples were collected from an arable soil in a typical seasonal frozen region of northeast China. A 23 full factorial design was carried out based on the hypothesis that Cd behavior was influenced by the interactive effects of FT, SW and CP.
Prior to Cd analysis, a HF–HNO3–HClO4 acid mixture was used for the digestion of the ground soil and the constant-volume acid mixture was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, IRIS Intrepid II XSP, Thermo Electron, USA). The concentration of total Cd in the soil was 0.009 mg kg−1.
Parts of soil samples were sieved with a 2 mm nylon sieve. 1 kg of soil was weighed into a plastic box. 10 mg L−1 of Cd2+ solution was added into the soil with a volume of 1 L. To homogenize Cd in soil, the mixture was incubated under a room temperature for approximately 2 months. The total content of Cd in soil thereby was adjusted to 10 mg kg−1. Then, the soil was air-dried and sieved by a 2 mm mesh sieve again and subsample specimens (1 g each) were weighed in a plastic bag.
The Cd availability in soils was usually determined by the potential bioavailability and mobility of Cd. The potential Cd bioavailability reflected the portion of the total Cd that could be taken up directly or indirectly by organisms in the soil. On this basis, the index of potential Cd bioavailability was obtained using the following equation: K (%) = (Wat-Cd + Exc-Cd + Car-Cd)/(Wat-Cd + Exc-Cd + Car-Cd + Oxi-Cd + Org-Cd + Res-Cd), where K was the relative content of the potential bioavailable fractions including the sum of Wat-Cd, Exc-Cd and Car-Cd.25 The potential Cd mobility in soil usually indicated the absolute and the relative content of fractions weakly bound to soil components.21 Therefore, the relative index of potential Cd mobility was calculated using the following equation: M (%) = (Wat-Cd + Exc-Cd)/(Wat-Cd + Exc-Cd + Car-Cd + Oxi-Cd + Org-Cd + Res-Cd), where M was the relative content of the potential mobile fractions including the sum of Wat-Cd and Exc-Cd.
Quality assurance and quality control were assessed using duplicates, method blanks and standard reference materials (GBW07401) from the Chinese Academy of Measurement Sciences for each batch of samples. All chemicals were of analytical grade or better, and all glassware and centrifuge tubes were previously soaked in acid (10% HNO3) and rinsed with deionised water.
Furthermore, a 23 full factorial design was carried out to identify the impact of affecting factors (FT, SW and CP) on Cd fractionation and Cd availability. Based on the output results, the contribution rate of various factors as well as their higher-order interactions was calculated using the following equation: Ri (%) = Ei/(E1 + E2 +…), where Ri was the relative percentage of the i th estimated effect (E) of the significantly positive/negative factor in the sum of the estimated effects of all the significantly positive/negative factors.
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Fig. 3 Percentage distribution of Cd in each fraction of the six-step sequential extraction for variously treated soils. |
The K values slightly decreased to a minimum at F1, then gradually increased as a result of the increased FT frequency. A notable decrease in the K and M values (approximately 10.3% and 14.7%, respectively) was observed as SW content increased. However, there was no significant variation once the saturation value was exceeded. It was five FT cycles earlier for the minimal K value emerging due to the addition of CP, which also caused the K and M values significantly (P < 0.05) to increase by circa 3.49% and 4.96%, respectively (Fig. 4, Table 1).
Treatments | N | Wat-Cd | Exc-Cd | Car-Cd | Oxi-Cd | Org-Cd | Res-Cd | K | M |
---|---|---|---|---|---|---|---|---|---|
a Different small and capital letters in the same column indicate significant differences between the freeze–thaw and water content treatments, respectively (P < 0.05). Asterisk indicates that there is a significant difference relative to the data before adding CP (P < 0.05). | |||||||||
F0 | 32 | 0.47 ± 0.14c | 25.2 ± 2.40c | 20.8 ± 1.98a | 19.2 ± 2.48a | 4.63 ± 1.08b | 29.6 ± 4.45c | 46.5 ± 3.60a | 25.7 ± 2.48c |
F1 | 32 | 0.76 ± 0.42a | 25.7 ± 2.07bc | 19.6 ± 1.87b | 16.9 ± 2.84c | 4.23 ± 1.12c | 32.8 ± 4.46a | 46.1 ± 3.38a | 26.5 ± 2.15bc |
F3 | 32 | 0.52 ± 0.22b | 26.6 ± 3.25ab | 19.5 ± 1.54b | 16.9 ± 3.09c | 3.37 ± 0.67d | 33.0 ± 5.30a | 46.6 ± 3.86a | 27.1 ± 3.38ab |
F6 | 32 | 0.44 ± 0.12d | 26.7 ± 2.76ab | 19.4 ± 1.39b | 16.5 ± 3.36c | 4.82 ± 1.00a | 32.0 ± 4.76ab | 46.6 ± 3.35a | 27.2 ± 2.83ab |
F9 | 32 | 0.51 ± 0.09b | 27.2 ± 2.81a | 20.0 ± 1.48ab | 18.0 ± 3.30b | 3.42 ± 0.74d | 30.8 ± 4.15bc | 47.7 ± 3.33a | 27.7 ± 2.85a |
SW1 | 40 | 0.67 ± 0.17A | 29.2 ± 2.77A | 20.5 ± 1.76A | 17.7 ± 1.73B | 3.54 ± 0.64D | 28.4 ± 3.04B | 50.4 ± 3.19A | 29.9 ± 2.75A |
SW2 | 40 | 0.58 ± 0.31B | 25.6 ± 2.41B | 19.7 ± 1.73AB | 20.5 ± 3.98A | 5.05 ± 1.30A | 28.7 ± 4.18B | 45.8 ± 3.05B | 26.1 ± 2.46B |
SW3 | 40 | 0.49 ± 0.28C | 25.1 ± 1.79B | 19.9 ± 1.57AB | 16.3 ± 1.96C | 4.07 ± 0.92B | 34.2 ± 3.70A | 45.4 ± 2.48B | 25.5 ± 1.88B |
SW4 | 40 | 0.41 ± 0.15D | 25.4 ± 1.61B | 19.5 ± 1.73B | 15.6 ± 2.00C | 3.73 ± 0.82C | 35.4 ± 3.40A | 45.2 ± 2.48B | 25.8 ± 1.61B |
CP0 | 80 | 0.50 ± 0.21 | 25.7 ± 2.83 | 19.7 ± 1.73 | 18.5 ± 3.71 | 3.92 ± 1.32 | 31.7 ± 5.41 | 45.9 ± 3.37 | 26.2 ± 2.94 |
CP1 | 80 | 0.58 ± 0.29* | 26.9 ± 2.56* | 20.1 ± 1.71 | 16.5 ± 2.05* | 4.27 ± 0.82* | 31.7 ± 4.04 | 47.5 ± 3.47* | 27.5 ± 2.56* |
Variations in Cd fractions in soil were observed due to the different SW contents. Interestingly, an increase in the SW content caused a notable decrease in the Wat-Cd content. The maximal Exc-Cd content at SW1 was significantly higher than that under other SW content conditions. The Car-Cd content was markedly higher at SW1 than at SW4. The Oxi-Cd content at SW2 was the highest among the four-level of SW contents. A significant difference was observed in the Org-Cd content under various SW treatments. In detail, the minimum and the maximum was at SW1 and SW2, respectively. The Res-Cd content increased with the increase of the SW content. The K and M values were higher at SW1 than at other SW contents. The addition of CP caused a significant increase in the content of Wat-Cd, Exc-Cd, and Org-Cd, but a notable decrease in the Oxi-Cd content. In particular, the K and M values strongly increased due to the addition of CP, while no visible changes in the content of Car-Cd or Res-Cd was detected.
Source | Wat-Cd | Exc-Cd | Car-Cd | Oxi-Cd | Org-Cd | Res-Cd | K | M | |
---|---|---|---|---|---|---|---|---|---|
a The partial eta squared values were computed using alpha = 0.05, when P < 0.05, there was a significant difference, P = 0 implied P was below 0.001. | |||||||||
SW | F | 314 | 42.0 | 2.65 | 77.5 | 130 | 64.0 | 31.4 | 45.9 |
P | 0 | 0 | 0.05 | 0 | 0 | 0 | 0 | 0 | |
etaa | 0.89 | 0.51 | 0.06 | 0.66 | 0.77 | 0.62 | 0.44 | 0.54 | |
CP | F | 181 | 16.3 | 1.74 | 71.2 | 34.5 | 0 | 14.4 | 18.6 |
P | 0 | 0 | 0.19 | 0 | 0 | 0.97 | 0 | 0 | |
eta | 0.60 | 0.12 | 0.01 | 0.37 | 0.22 | 0 | 0.11 | 0.13 | |
FT | F | 314 | 5.95 | 3.44 | 16.9 | 105 | 8.03 | 1.56 | 5.47 |
P | 0 | 0 | 0.01 | 0 | 0 | 0 | 0.19 | 0 | |
eta | 0.91 | 0.17 | 0.10 | 0.36 | 0.78 | 0.21 | 0.05 | 0.15 | |
SW × CPe | F | 304 | 0.68 | 0.18 | 43.7 | 85.6 | 19.8 | 0.19 | 0.77 |
P | 0 | 0.57 | 0.91 | 0 | 0 | 0 | 0.90 | 0.51 | |
eta | 0.88 | 0.02 | 0 | 0.52 | 0.68 | 0.33 | 0.01 | 0.02 | |
SW × FT | F | 38.7 | 2.12 | 0.33 | 1.45 | 12.9 | 1.02 | 1.02 | 2.36 |
P | 0 | 0.02 | 0.98 | 0.15 | 0 | 0.44 | 0.44 | 0.01 | |
eta | 0.80 | 0.18 | 0.03 | 0.13 | 0.56 | 0.09 | 0.09 | 0.19 | |
CP × FT | F | 432 | 1.52 | 0.33 | 6.43 | 5.60 | 4.29 | 1.09 | 2.13 |
P | 0 | 0.20 | 0.86 | 0 | 0 | 0 | 0.37 | 0.08 | |
eta | 0.94 | 0.05 | 0.01 | 0.18 | 0.16 | 0.13 | 0.04 | 0.07 | |
SW × CP × FT | F | 60.8 | 1.32 | 0.55 | 1.12 | 0.96 | 0.51 | 0.35 | 1.13 |
P | 0 | 0.22 | 0.88 | 0.35 | 0.49 | 0.90 | 0.98 | 0.34 | |
Eta | 0.86 | 0.12 | 0.05 | 0.10 | 0.09 | 0.05 | 0.03 | 0.10 |
The binary interactions weakened the effects of FT and SW on the soil Cd fractionation and Cd availability. However, they enhanced the effects of CP on Cd fractionation except for Exc-Cd. The ternary interactions further weakened the binary interactions, while CP enhanced the binary interaction between SW and FT (SW × FT). The distribution showed a significant variation in the content of Wat-Cd, Exc-Cd and Org-Cd in soil as a result of SW × FT. The fractions of Cd except for Exc-Cd and Car-Cd in soil were remarkably affected by the binary interaction between CP and SW/FT (SW × CP and CP × FT). The ternary interaction of SW × CP × FT weakened the binary interactions on the Wat-Cd content. The M values were significantly affected only by SW × FT (Fig. 5).
Table 3 showed the estimated effects and percent distributions of each term. The percent contribution of the significant effects of SW, CP and FT was calculated based on the data presented in Table 3, which was displayed in Fig. 6. The main effect of SW significantly and positively (P < 0.05) influenced the Res-Cd content and its contribution was 79.0%, while it had a significant negative effect (P < 0.05) on the content of Wat-Cd, Exc-Cd, Car-Cd, Oxi-Cd, the K and the M values, with a contribution of 51.3%, 100%, 100%, 44.9%, 100% and 100%, respectively. The main effect of CP negatively influenced the Oxi-Cd content (P < 0.05), accounting for 34.0% of the total negative effect on the Oxi-Cd content. Nevertheless, it positively influenced (P < 0.05) the content of Exc-Cd, Org-Cd, the K and the M values, which accounted for 36.8%, 39.7%, 54.2% and 39.7% of the total positive effect on the corresponding variables, respectively.
Source | Wat-Cd | Exc-Cd | Car-Cd | Oxi-Cd | Org-Cd | Res-Cd | K | M | |
---|---|---|---|---|---|---|---|---|---|
a E, computed using alpha = 0.05 via DOE, when P < 0.05, there was a significant difference; P = 0 implied P was below 0.001. T was the statistical value for evaluating the probability distribution, according to which P was obtained. | |||||||||
SW | E | −0.26 | −3.69 | −0.80 | −2.98 | −0.15 | 7.87 | −4.75 | −3.94 |
T | −5.4 | −7.65 | −2.18 | −5.06 | −0.66 | 9.87 | −7.52 | −8.08 | |
P | 0 | 0 | 0.03 | 0 | 0.51 | 0 | 0 | 0 | |
CP | E | 0.07 | 1.08 | 0.35 | −2.26 | 0.40 | 0.34 | 1.49 | 1.15 |
T | 1.86 | 3.01 | 1.26 | −5.13 | 2.36 | 0.58 | 3.18 | 3.15 | |
P | 0.06 | 0 | 0.21 | 0 | 0.02 | 0.56 | 0 | 0 | |
FT | E | −0.11 | 1.86 | −0.48 | −0.59 | −0.61 | −0.06 | 1.26 | 1.74 |
T | −2.43 | 3.89 | −1.31 | −1 | −2.69 | −0.07 | 2.02 | 3.6 | |
P | 0.02 | 0 | 0.19 | 0.32 | 0.01 | 0.94 | 0.04 | 0 | |
SW × CP | E | 0.14 | −0.33 | 0.20 | 1.06 | 0.61 | −1.70 | 0.01 | −0.19 |
T | 2.97 | −0.69 | 0.55 | 1.81 | 2.68 | −2.13 | 0.02 | −0.39 | |
P | 0 | 0.49 | 0.58 | 0.07 | 0.01 | 0.03 | 0.98 | 0.70 | |
SW × FT | E | 0.03 | −0.64 | 0.49 | 0.39 | −0.16 | −0.12 | −0.11 | −0.60 |
T | 0.54 | −0.99 | 1 | 0.49 | −0.52 | −0.11 | −0.13 | −0.93 | |
P | 0.59 | 0.32 | 0.32 | 0.62 | 0.60 | 0.91 | 0.90 | 0.35 | |
CP × FT | E | −0.13 | −0.82 | −0.11 | −1.40 | 0.38 | 2.09 | −1.06 | −0.95 |
T | −2.77 | −1.72 | −0.30 | −2.4 | 1.65 | 2.64 | −1.69 | −1.97 | |
P | 0.01 | 0.09 | 0.76 | 0.02 | 0.10 | 0.01 | 0.09 | 0.05 | |
SW × CP × FT | E | −0.08 | 0.98 | −0.36 | −0.70 | 0.04 | 0.11 | 0.54 | 0.90 |
T | −1.26 | 1.53 | −0.73 | −0.89 | 0.14 | 0.11 | 0.65 | 1.39 | |
P | 0.21 | 0.13 | 0.47 | 0.37 | 0.89 | 0.91 | 0.52 | 0.17 |
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Fig. 6 Contribution of main effects and interactions of FT, SW and CP on Cd fractionation and Cd availability in soil. |
Furthermore, the contribution generated from the binary interaction between SW and CP to the negative effect (P < 0.05) on the Res-Cd content was 100%, while its contribution to the positive effect (P < 0.05) on the content of Wat-Cd and Org-Cd was 100% and 60.3%, respectively. The binary interaction between CP and FT accounted for 21.0% of the positive effect (P < 0.05) on the Res-Cd content, and it caused a negative (P < 0.05) effect on the contents of Wat-Cd and Oxi-Cd, with a contribution of 25.9% and 21.1%, respectively. No other significant effects (P > 0.05) of the higher-order interaction on Cd fractionation and Cd availability were observed.
The results indicated that the binary interactions were further weakened by the ternary interactions. For example, SW × CP × FT further affected the binary interactions on the Wat-Cd content. It also suggested that natural environmental factors not only modify effects of chemicals directly but also influence them through interactions among them indirectly. This conclusion was also demonstrated in the previous study.2 One possible mechanism is the fact that CP inhibits the activity of soil enzymes or reduces the buffering capacity of soil,14 and thereby reduces the sensitivity of the potential Cd bioavailability in soil to FT and SW.
Moreover, the increased SW can enhance the extent to which soil aggregates are damaged by FT and thereby can promote the effects of FT on Cd fractionation and availability, which will decrease once SW exceeds the saturation value.12 Labile Cd fractions and Cd availability will decrease because the cationic Cd is more immobilized in the saturated soils with higher pH, lower Eh and potentially contain sulphides, compared to unsaturated soils.33 Cationic Cd dissolved in the soil solution can also migrate with soil water. As a function of FT, the unfrozen water with the dissolved Cd will migrate to the freeze front, beside the in situ water that is frozen during the freezing process.29 Moreover, an increase in temperature during the thawing process can be beneficial for ion exchange sorption but detrimental to specific sorption, which results in a process that is faster than the ion exchange sorption.34 Based on this information, FT can substantially affect the contents of Cd fractions. This is mainly due to the formation of chelates and precipitates on soil surface by combining with aluminosilicate and Fe–Mn oxides or by bonding with the oxygen or hydroxyl molecules within soil particles.35
Furthermore, FT can also affect Cd behavior via the alteration of the microbial structure and functions of soil, which readily respond to the soil humidity level and temperature.3,9 Simultaneously, the freezing process can cause a significant increase in the total amount of free amino acids and sugars, in combination with an increase in soil respiration and dehydrogenase activity.13 Nevertheless, either the microbial functions or the enzymatic activity can be easily affected by CP via ecological effects, organic effects and the coordination reaction.14 The complexation ability of the organic ligands that originate from CP can control the hysteresis quality of the Cd desorption process in soil, together with the soil colloid content.36,37 This is the reason that CP can alter the emerging time of visible changes in Cd fractions resulting from FT. The ability of CP that increased the potential Cd mobility coincided with the findings of other previous studies,32,38 where the presence of chloride and SOM promoted this process by forming soluble complexes.
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