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Journal of soil science and plant nutrition

versão On-line ISSN 0718-9516

J. Soil Sci. Plant Nutr. vol.13 no.3 Temuco set. 2013  Epub 27-Ago-2013

http://dx.doi.org/10.4067/S0718-95162013005000053 

Zinc desorption kinetics in wheat (Triticum Aestivum L.) rhizosphere in some sewage sludge amended soils

 

H.R. Motaghian1*, A.R. Hosseinpur1

1Department of Soil Science, College of Agriculture, Shahrekord University, Shahrekord, Iran.
*Corresponding author:
hrm_61@yahoo.com


Abstract

The objectives of this study were to determine Zn desorption characteristics and the correlation of these parameters with Zn extracted by DTPA-TEA method in bulk and rhizosphere amended soils with sewage sludge under greenhouse conditions by using a rhizobox. For amended soils 10 g of sewage sludge was added to 1 kg of 10 soil samples. The kinetics of Zn desorption in bulk and rhizosphere was determined by successive extraction with DTPA-TEA in a period of 1 to 504 h at 25±1 0C. The results showed that Zn extracted using successive extraction in the rhizosphere were significantly (p<0.01) higher than in the bulk amended soils. The best model for describing extraction data for bulk and rhizosphere soils was the power function equation. The results indicated that Zn desorption rate in the wheat rhizosphere soils more than bulk soils. The results showed that significant correlation (p<0.05) between Zn desorption characteristics and DTPA-TEA-Zn were found in the bulk and rhizosphere soils. The results of this research revealed that Zn desorption characteristics that are help to estimate the Zn supplying power of soils in the wheat rhizosphere, are quite different than bulk amended with sewage sludge.

Keywords: Calcareous soils, DTPA-TEA, power function, rhizosphere, successive extraction


1. Introduction

Chemical extractive schemes have been largely proposed for assessing the pools of metal such as zinc (Zn) in the soil solid phase (Young et al., 2006). Single extraction provides a simple approach for estimating the mobile pool (Mocko and Waclawek, 2004; Young et al., 2006; Manouchehri and Bermond, 2009). Two chelating agents, DTPA (Diethylene three aminepenta aceticacid) and EDTA (ethylene diaminetetra aceticacid), routinely used to estimate available-Zn in soil (Lindsay and Cox, 1985) provide only a static measure of Zn availability to plants. Since DTPA and EDTA chelate Zn, and maintain low Zn ion concentrations in soil solution, thus to an extent simulating Zn adsorption by roots of growing plants, the rate of Zn desorption from soil by DTPA or EDTA should provide a measure of a rate factor of Zn availability (Dang et al.,1994). This is especially important in calcareous soils, which generally contain quite large quantities of total Zn, but have only very small quantities of Zn in soil solution (Reyhanitabar and Gilkes, 2010), and thus the rate of Zn release in the soil largely determines Zn availability to plants. Moreover, in most of these methods, the metal concentration is measured at equilibrium whereas natural systems are generally subject to changing conditions and are practically never at the equilibrium. Indeed, for these dynamic systems, the metal availability may be believed to be controlled by kinetic factors (Errecalde et al., 1998; Ma et al., 1999; Slaveykova et al., 2003).

Reyhanitabar and Gilkes (2010) studded the kinetics of Zn extraction by DTPA from 12 calcareous soils of Iran for 0.25 to 192 h. They reported that the best model for describing extraction data for all soils was the exponential rate equation. However, other equations have been proposed for describing release kinetics and they may provide better descriptions of Zn extraction from soil (Sparks 1986).

The rhizosphere is defined as volume of soil affected by the presence of the roots of growing plants (Uren, 2007). The conditions at the rhizosphere are considerably different from those existing some distance from root system (Nye, 1981). Plants have the ability to transform metal fractions for easier uptake through root exudation or pH changes in the rhizosphere (Hinsinger, 2001; Wang et al., 2002). The soil properties, metal fractions, plant species and especially soil - plant interactions determine the bioavailability of metals in soils (Ehlken and Kirchner, 2002). Low-molecular-weight organic acids are exudates by plant roots and products of fungi and bacteria activity which desorption in the rhizosphere. Furthermore, there is increasing evidence that low-molecular-weight organic acids play an important role in supplying metals to roots, especially in metal deficient soils, and enhancing the uptake of metals by plants (Shan et al., 2002). Therefore, study of kinetics desorption Zn in soils especially in the rhizosphere that is important environmental zone with quite different chemical properties gives information about potential of soils to supplying Zn for plant uptake. Sewage sludge have been increasingly used in numerous countries around the world as fertilizers on farm lands because of their high content of organic matter and essential nutrients that favor crop growth (Renoux et al., 2007). Sewage sludge is an important source of nutrients such as Zn. When soil is amended with sewage sludge a new source of heavy metals has been introduced. Therefore, it is necessary to determine the kinetics of Zn desorption in amended soils with sewage sludge to understand how it may influence soil Zn availability.

Study of kinetics Zn desorption in the rhizosphere amended soils with sewage sludge hasn't been carried out. Therefore, the objectives of the present study were (i) to evaluate the Zn desorption from bulk and rhizosphere soils amended with sewage sludge (ii) to evaluate different mathematical models for describing Zn desorption and (iii) to examine the relationship of Zn desorption rate obtained in the kinetics studies with conventional soil test method.

 

2. Materials and Methods

Ten soil samples used in this study were collected from the surface soils (0-30 cm) of Chaharmahal-Va-Bakhtiari province, in Central Iran. The soil samples were air dried and ground to pass through a 2 mm sieve for laboratory analysis. Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1986). Electrical conductivity (EC) was determined in 1 to 2, soil to water ratio by conductivity meter (Rhoades, 1996) and pH was measured in a soil to water ratio of 1:2 (Thomas, 1996). Organic carbon was determined by wet oxidation (Nelson and Sommers, 1996), equivalent calcium carbonate was determined by titration (Loeppert and Suarez 1996) and cation exchange capacity of the soils were determined by the 1 M NaOAC, at pH 7 (Sumner and Miller, 1996). Total Zn was determined using 4 M HNO3 (Sposito et al., 1982).

Solid sewage sludge was used from the refinery of Shahrekord city, central Iran. Sewage sludge were air-dried and ground to pass through a 1 mm sieve.

Properties of sewage sludge such as pH (soil to water suspension ratio of 1:5) (Thomas, 1996) and electrical conductivity (1 to 5, soil to water by conductivity meter), (Rhoades, 1996) were determined. Total and available Zn were determined using 4 M HNO3 (Sposito et al., 1982) and DTPA-TEA (Lindsay and Norvell, 1978), respectively.

A homemade rhizobox (Figure 1) (Wang et al. 2002) was used to plant wheat. The dimension of the rhizobox was 200mm X 130mm X 200mm (length X width X height). The rhizobox was divided into three sections: a central zone or rhizosphere zone (30mm in length), which was surround by nylon cloth (300 mesh), and left and right nonrhizosphere zones (bulk soil) (50 mm in length).

Figure 1. Sketch of a rhizobox used in this study

For amended soils, 1% (w/w) of sewage sludge was added to soils, and then soil samples were incubated at field capacity, for 1 month. After incubation, soils were air-dried. Wheat (Triticum aestivum L.) seeds were thoroughly rinsed with water and surface-sterilized by soaking in a mixture of NaClO (3%), and germinated on a filter paper. After the wheat seeds were germinated for 24 h at 20 0C in the dark, uniformly germinated seeds with radical emerged were sown in the soil, which had been left in the rhizobox to equilibrate overnight. A total of 5 seeds were sown per rhizosphere zone and subsequently thinned to 3 plants. Plants were grown under greenhouse conditions. The temperature ranged from 20 to 25 0C during the day and 15 to 20 0C during the night. Plants were harvested 8 weeks after germination. The aboveground parts of the plants were first harvested as shoots. The rhizobox was then dismantled. The roots were found completely in the central zone. The rhizosphere soil was sieved gently to remove the roots by keeping the root mass intact as much as possible. Rootlets that passed through the sieve were subsequently removed with a pair of tweezers. The soils taken from two rhizosphere zone and a bulk soil (zones of the left and right) were mixed separately for further analysis.

For bulk soil and rhizosphere soil samples analyses were triplicated. The DTPA- TEA method (Lindsay and Norvell, 1978) was used for Zn extraction in the bulk and rhizosphere amended soils. Then concentrations of Zn were determined using an atomic absorption spectrophotometer (model G.B.C. 932). Dissolved organic carbon (DOC) was extracted from the fresh soil samples using double-deionized water with a soil to water ratio of 1:2 (Corre et al., 1999). The suspensions were shaken for 2 h at room temperature and then centrifuged at 2500g for 5 min. The extracts were filtered on nylon 0.45 µm filters. Finally, the DOC content was assayed using Walkley and Black method (Nelson and Sommers, 1996). Soil microbial biomass carbon was estimated by the chloroform-fumigation incubation method, adapted from Jenkinson and Powlson (1976).

Kinetics of Zn desorption was studied by successive extraction with DTPA-TEA solution. Two grams of the bulk and rhizosphere soils, in triplicate, suspended in 20 ml of DTPA-TEA solution were equilibrated at 25±10C for 1, 8, 24, 48, 72, 96, 120, 144, 168, 336 and 504 h by shaking for 15 min. before incubation and 15 min. before the suspensions were centrifuged. The supernatant liquid was measured for Zn by an atomic absorption spectrophotometer (model G.B.C 932). Zn desorption with time was fitted by using different equations (Table 1).

Table 1. Kinetic models used in this study
Zn0: amount of Zn (mg kg-1) desorption at 504 h., Znt: amount of Zn (mg kg-1) desorption at time t (h) and a and K0, K1, R, and 1/β are constants.

Five models were tested by the least-square regression analysis to determine which equation described better the Zn desorption from the soils. Standard error of the estimate (SE) was calculated by:

where Zn and Zn* represent the measured and calculated amounts of Zn in soil at time t, respectively, and n is the number of data points evaluated. The rate constant of Zn desorption from soils were calculated on the basis of these models.

The significant differences of dissolved organic carbon, microbial biomass carbon, extracted Zn and amount Zn desorption between rhizosphere and bulk soils were determined by using paired-samples t-test. The Pearson's correlation between concentrations of available-Zn and amount of Zn desorption and Zn desorption characteristics were determined. Statistical analysis of the data was performed using SPSS 17.0.

 

3. Results and Discussion

Properties of the soils are presented in Table 2. This results show that there was a wide variation in the selected chemical and physical properties of the soils. The mean of clay contents was 44 g kg-1. The soils were alkaline and low in EC and organic matter. The pH ranged from 7.6 to 8.1. The EC ranged from 0.12 to 0.25 dS m-1. The OC ranged from 3.0 to 11.9 g kg-1. The equivalent calcium carbonate contents varied from 113 to 410 g kg-1. The CEC ranged from 11.5 to 22.5 cmolc kg-1. The total Zn in all soils ranged from 34 to 58 mg kg-1. Amount of Zn extracted using DTPA-TEA ranged from 0.28-1.39 mg kg-1.

Table 2. Selected properties of soils
Value of pH of solid sewage sludge was 7.5. Electrical conductivity was 2.25 dS m-1. Also, the available and total concentrations of Zn were 558 and 1321 mg kg-1, respectively.

The DOC and MBC in the wheat rhizosphere compare to the bulk soils were increased significantly (p<0.05) with respect to the bulk in each soil (Table 2). The DOC ranged from 47-106 mg C L-1 in the wheat rhizosphere, while it ranged from 32-91 mg C L-1 in the bulk soils. In the wheat rhizosphere the MBC ranged from 333940 mg C kg-1, while it ranged from 235-735 mg C kg-1 in the bulk soils. The R/S ratio of DOC ranged from 6-67% and R/S ratio of MBC ranged from 13-104%. The results illustrated that properties of rhizosphere soils was different camper to bulk soils. Irrespective of soil type there was a significant (p< 0.01) increase in DOC in the rhizosphere. The concentration of DOC in the rhizosphere can be affected by both plant root exudates and microbial activities. The results showed that in the rhizosphere soils correlation between DOC and MBC was significant (r= 0.69*), its correlation wasn't significant in the bulk soils (r=0.59, p> 0.05).

The overall effect of plant-microbe interaction is an increase in microbial biomass in the rhizosphere, owing to the high supply of organic carbon by roots and soils (Lynch and Whipps, 1990). Kim et al. (2010) reported that bacterial populations and DOC were higher in the rhizosphere soils than that in bulk soils. They stated that DOC increase in the rhizosphere could be due to both plant root exudates and microbial populations. Also, they reported that correlation between number of bacterial colony and concentration of DOC in the rhizosphere after plant culture was significant (p< 0.01). Plant roots release a considerable amount of organic materials, including water soluble exudates (sugars, organic acids, and amino acids) and water insoluble materials such as cell walls, sloughed material and mucilage into soil. Many of these materials are readily degraded by soil microorganisms, but their continuous release maintains the organic matter concentration at higher levels than in bulk soil (Lombi et al., 2001). It is well known that the root exudates serve as carbon source to heterotrophic microbial populations. Results of table 3 showed that Zn extracted using DTPA-TEA in the rhizosphere were significantly (p<0.05) higher than in the bulk soils. The Zn extracted using DTPA-TEA in the rhizosphere and bulk soils ranged from 3.29-5.51 and 3.23-4.84 mg kg-1 respectively.

Table 3. DOC, MBC and extracted Zn by DTPA-TEA determined in the rhizosphere and bulk soils
R/S (%): (amount of property in the rhizosphere to the bulk)-1 x 100 Means in each row and final row for different properties followed by the different letters are significantly different (p<0.05).

The pattern of successive extraction of Zn from the bulk and rhizosphere of soil no. 2 and 9 in DTPA-TEA solution is presented in Figure 2. Also, other soils show same trend similar to soil no. 2 and 9. Amounts of Zn desorption from the bulk and rhizosphere soils are given in Table 4. The results of this table showed that Zn extracted using successive extraction in the wheat rhizosphere were significantly (p<0.01) higher than bulk soils. The amounts of Zn desorption after 504 h of extraction period ranged from 14.29 (no. 3) to 18.66 (no. 9) mg kg-1 in the bulk soils, and ranged from 14.59 (no. 3) to 22.11 (no. 9) mg kg-1 in the rhizosphere soils. The soil no. 9 has CaCO3 equivalent of 11.3% that may be accounted for the increase in Zn desorption compared to other studied soils. A possible explanation for the increase of extracted Zn in the rhizosphere could be the complex Zn with soluble exudates in the rhizosphere soils (Bernal and McGrath, 1994). Moreover, microbial growth in the rhizosphere might be stimulated by the continual input of readily assimilable organic substrates from the root which account, at least partly, for the increase of extractable Zn.

Figure 2. Cumulative amount of Zn desorption with time in soil No. 2 and 9 by successive extractions with DTPA-TEA.

Table 4. Concentration of Zn desorption (mg/kg) the bulk and the rhizosphere soils
Means in final row followed by the different letters are significantly different (p<0.01).

Different mathematical models (Table 1) were tested for their suitability to describe the Zn desorption from different soils including bulk and rhizosphere soils. Comparisons of R2 and SE values (Table 5) indicated that the power function, parabolic diffusion, simplified Elovich and first order equations were described the reaction rates fairy well, as evidenced by the high coefficients of determination values and low standard error of the estimate. The regression relationships between Zn desorption and the time for the bulk and rhizosphere soil no. 9 as described by the simplified Elovich, parabolic diffusion, power function and first order equations are shown in Figure 3. In the bulk amended soils power function (R2=0.994, SE=0.288 mg kg-1) had the highest determination coefficients and the lowest standard error of the estimate and could describe Zn desorption. Also, parabolic diffusion (R2=0.950, SE=0.661 mg kg-1), simplified Elovich (R2=0.950, SE=0.731 mg kg-1) and first order (R2=0.960, SE=1.005 mg kg-1) models could well describe Zn desorption. In the rhizosphere amended soils power function (R2=0.994, SE=0.291 mg kg-1) had the highest determination coefficients and the lowest standard error of the estimate and could describe Zn desorption. Also, parabolic diffusion (R2=0.956, SE=0.723 mg kg-1), simplified Elovich (R2=0.948, SE=0.886 mg kg-1) models could well describe Zn desorption. Therefore, the best model to describe the Zn desorption of bulk and rhizosphere soils was the power function equation. Also, Reyhanitabar and Gilkes (2010) reported that the best model for describing release Zn using DTPA-TEA was the exponential rate equation. Dang et al. (1994) tested different kinetics models to describe Zn desorption from Vertisols. Among the used models power function, parabolic diffusion and Elovich were chosen as the suitable models while the first, second, and the third order kinetics showed weaker fitness for the data. Pavlatou and Polyzopoulos (1988) proposed that the Elovich equation applies to desorption reactions, which are kinetically controlled by diffusion. This is evidenced by the close relationship between overall diffusion constants. Some of researcher reported that a parabolic diffusion model well described the interactions of metals with soils and soils constituents, the weathering of feldspars and the interactions of pesticides in soil (Garcia-Rodja and Gil-stores 1997).

Table 5. Coefficient of determination (R2) and standard error of the estimate (SE) of various kinetic models for Zn desorption in studied soils
SE, mg kg-1. All coefficients of determination (R2) are significant at the 0.01 level.

Figure 3. Cumulative Zn desorption (mg k-1) described by the four mathematical models for soil 9.

There was a wide variation in the Zn desorption characteristics between bulk and rhizosphere soils. Desorption rate constants and intercept of Zn for four models are given in Table 6. In the bulk soils the fit of the data to power function yielded a straight line. The constant b ranged from 0.192 (soil 7) to 0.236 (soil 2) (mg kg-1)-1. The constant a ranged from 3.35 (soil 2) to 5.00 (soil 7) mg kg-1 h-1. Cumulative Zn desorption was fitted to parabolic diffusion equation. The Zn desorption rates (R) range from 0.519 (soil 3) to 0.728 (soil 9) mg kg-1 h-1/2 in DTPA-TEA solution. Cumulative Zn desorption was fitted to the simplified Elovich equation. The constant 1/β ranged from 1.41 (soil 1) to 2.12 (soil 9) mg kg-1 h-1. In bulk amended soils fit of the data to first order equation yielded a straight line. The constant K1 ranged from 53.31x10-4 (soil 3) to 61.96x10-4 (soil 9) (mgkg-1)-1.

Table 6. Parameters of models used to describe desorption kinetics of Zn in bulk and rhizosphere soils

In the rhizosphere soils the fit of the data to power function yielded a straight line. The constant b ranged from 0.184 (soil 7) to 0.234 (soil 2) (mg kg-1)-1. The constant a can ranged from 3.69 (soil 3) to 5.54 (soil 7) mg kg-1 h-1. Cumulative Zn desorption was fitted to parabolic diffusion equation. The Zn desorption rates range from 0.522 (soil 3) to 0.863 (soil 9) mg kg-1 h-1/2 in DTPA-TEA solution. Cumulative Zn desorption was fitted to the simplified Elovich equation. The constant b ranged from 1.56 (soil 1) to 2.50 (soil 9) mg kg-1 h-1.

The constant K1 ranged from 53.14x10-4 (soil 3) to 62.39x10-4 (soil 9) (mg kg-1)-1. The difference between Desorption rate constants indicates that Zn supplying power of the amended soils is different in the rhizosphere and bulk soils. The values of constants a and b vary widely for different soil types. The constant b in simplified Elovich, parabolic diffusion and first order equations has been introduced an index of Zn desorption rates in soils. These results showed that Zn desorption rate in the wheat rhizosphere soils more than bulk soils.

The parameter b in power function was less than 1 in all bulk and rhizosphere soils, indicating the Zn desorption rates were decreasing with time. Reyhanitabar and Gilkes (2010) stated that the constant a in the power function may indicate the number of surface sites available for desorption of Zn. Also, the constant b is a measure of the affinity of Zn for these sites with increasing affinity of sites being indicated by the value of 1/b. On the other word, an increase in the value of a decrease in the value of b probably indicates an increase in the rate of Zn desorption from the soils with DTPA (Dang et al. 1994). In the wheat rhizosphere soils the constant a increased and b constant decreased than bulk soils (Table 6). Therefore the initial Zn desorption rate in the wheat rhizosphere soils was more than bulk soils. Dang et al. (1994) reported that the b values < 0.25 indicate more than a single diffusion rate of Zn, as suggested by the applicability of the parabolic double diffusion equation. Therefore in the bulk and rhizosphere soils with sewage sludge one diffusion mechanism were involved. Dang et al. (1994) concluded that more than one diffusion mechanism such as diffusion from macroaggregates and microaggregates can be involved, or that the energies of Zn desorption increase exponentially or as a power function as Zn is desorbed by DTPA from soil solids and diffuses into the soil solution.

The different chemical and biological conditions of the rhizosphere with respect to the bulk soil can change metal fractions and therefore bioavailability of metals (Dessureault-Rompre et al, 2008; Tao et al, 2003). Moreover, low-molecular-weight organic acids produced by plant roots exudation and bacterial metabolism are involved in many processes operating in the rhizosphere, including nutrient acquisition, metal detoxification, and alleviation ofanaerobic stress in roots (Jones, 1998). Through their acidification, chelating and oxidation-reduction reaction in the rhizosphere, they can affect the speciation and transportation of Zn in the rhizosphere soil. Qin et al. (2004) demonstrated that various low-molecular-weight organic acids were able to influence the rate of heavy metals from different soils and increase the solubility of heavy metals through the formation of metal - organic complexes. Due to the effect of low-molecular-weight organic acids in the rhizosphere, the bioavailability of Zn in the rhizosphere is different from that in the bulk soil. Moreover citric acid is very common in natural environments, especially in the rhizosphere soils, and usually ranges from 10-5 to 10-3 M (Robert and Berthelin, 1986). The presence of citrate ligands during the formation of Fe oxides can significantly modify the structural and surface properties of the Fe oxides formed. These surface properties include surface area, surface porosity, surface charge, and surface geometry (Liu and Huang, 2003). The surface of Fe oxides is the region of their interaction with the soil solution and with other solid phases, plant roots, and the soil biota (Schwertmann and Taylor, 1989). Correlations between the Zn desorption characteristics with Zn extracted using DTPA-TEA are shown in Table 7. The results of this Table showed that in the bulk and rhizosphere soils correlation between the rate constant values of parabolic diffusion, simplified Elovich and first order equations and axb in power function with Zn extracted using DTPA-TEA were significantly. Also, significant correlation between the a value in power function and first order equations with extracted Zn using DTPA-TEA were found. Probably the best supporting evidence for any equation used to describe the kinetics of Zn desorption is the investigation of relation between the constants of any equation with extracted Zn by routine extractant. Therefore, Zn desorption characteristics including constants of equations can be used to evaluate the Zn supplying power of amended soils.

Table 7. Correlation between the Zn desorption characteristics with Zn extracted using DTPA-TEA
** Significant at the 0.01 level, * Significant at the 0.05 level, and ns Not significant.

 

4. Conclusion

In this study Zn desorption characteristics and the correlation of these parameters with Zn extracted by DTPA-TEA in the bulk and rhizosphere amended soils with sewage sludge in greenhouse conditions were determined. The results showed that Zn extracted using DTPA-TEA in the rhizosphere were significantly higher than Zn extracted using this extractant in the bulk soils. The results showed that Zn extracted using successive extraction in the wheat rhizosphere were significantly higher than bulk soils. The best models for describing extraction data for all soils were the power function, parabolic diffusion, simplified Elovich equations in bulk and rhizosphere amended soils. The results indicated Zn desorption rate in the wheat rhizosphere soils more than the bulk soils. In bulk and rhizosphere soils significant correlation between determine Zn desorption characteristics and Zn desorption with extracted Zn using DTPA-TEA were found. Result indicated that after plant growth the soil which related to root is changed. So, this part of soil is different from others and must be considered in soil fertility studies.

 

Acknowledgements

This study supported by funds allocated by the Vice President for research of Shahrekord University.

 

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