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American Journal of Environmental Sciences

Year: 2009  |  Volume: 5  |  Issue: 1  |  Page No.: 94 - 98

Microbial Response to the Application of Amendments in a Contaminated Soil with Trace Elements

A. Branzini, M.S. Zubillaga and M.M. Zubillaga

Abstract

Problem Statement: The anthropogenic activities can cause adverse effects in soils, increasing in some situations trace elements contents, impacting negatively both the microbial biomass and activity. Among the practices used for the recovery of soil quality we can find the application of organic amendments or the product of their composting. These can adsorb trace elements decreasing their availability and increasing the soil microbial biomass. The microorganisms of the soil use to be considered as sensitive biological indicators of the changes produced in the soil quality. Approach: One processes to quantify soil biological activity is the respiration. The aim was to evaluate the effects of two organic amendments application on soil microbial activity, in a soil contaminated with copper (Cu), zinc (Zn) and chromium (Cr). To prove the raised aim we quantified CO2-C release. Results: The results showed that at the end of the incubation period, as much in contaminated soils as in soils without contamination, the total activity of microorganisms was significantly increased by the application of organic amendments (p = 0.0062 and p = 0.0005, respectively). The application of both composts to slightly acid soils increased the initial and final values of pH. There was no evidence of modification in Electrical Conductivity (EC) because of compost application. At the end of the incubation period a negative relationship was observed between EC and CO2-C (R2 = 0.74, p = 0.0028). Conclusions: The obtained results in this study suggested that it was possible to increase the total activity of soil microorganisms and to reduce the bioavailability of Cu, Cr and Zn in a contaminated soil. As a result, CO2-C release is a sensitive index of the soil quality, at least in the experimental conditions of this essay.

Table 1) in equivalent doses to 100 Mg ha-1. These amendments were mixed with the contaminated and no contaminated soils and later were made 4 cycles of wetting and drying for their stabilization. The resulting treatments were: in soils without contamination: control soil (CS), CS + biosolid compost (CS+BC), CS + equine compost (CS+EC); and in contaminated soils: contaminated soil control (CSC), CSC + biosolid compost (CSC+BC) and CSC+ equine compost (CSC+EC). All treatments were replicated three times in a complete randomized design.

Soil incubation test: The incubation test was proposed by[7] with few minor modifications. In flasks of 360 mL were incorporated 100 g dry sample from each treatment. Each flask constituted an experimental unit. A container with 5 mL of NaOH 1M was placed into flask, which captured CO2 produced by the microbial activity.

Table 1: General characteristics of used compost. Limits of trace elements into different origin compost, from European Union
Holland = Cu: 60 mg kg-1, Zn: 200 mg kg-1 y, Cr: 50 mg kg-1, DM = Dry matter, ND = Not detectable

Table 2: Chemical characteristics of treatments at initial and final time of incubation test. Same letters indicated were not significant differences at the 95% level of probability, according to LSD test. (Soil pH and EC in extract was measured using 1: 2.5 soil: distillated water ratio)

The incubation was kept at constant temperature of 28±2°C and 60% of equivalent humidity. The flasks stayed closed, opening it daily half an hour to maintain the aerobic conditions. The CO2-C released was measured through titulation with HCl 0.25 M, using phenolphthalein as indicator. The measurements were determinate on days 1, 2, 3, 6, 9, 13, 17, then weekly and later, every two weeks until the stabilization of the microbial activity.

The soil chemical characteristics were determined at two moments of the incubation (initial and final), using standard methodologies[22] (Table 2). The bioavailables concentrations of Cu, Cr and Zn were extracted by CaCl2 and quantified by Atomic Absorption Spectrometry (AAS).

Results were statistically Analyzed by Analysis Of Variance (ANOVA) and differences among means were obtained by the Least Significant Differences (LSD) test. Trace elements were analyzed by factorial adjustment of 3x2x2 (levels of compost, levels of contamination and moment of sampling); the means comparison was done with LSD test. The variances were stabilized when it was necessary using a logarithmic transformation of the data.

RESULTS AND DISCUSSION

Generally, the organic amendments addition increased significantly the initial and final pH, in contaminated soils and in soils without contamination. Although these results were in good agreement with the obtained by other authors with the compost application[23, 24], pH levels (between 5.19 and 5.89) were maintained within slightly acid soils (Table 2). Due to the little pH modification, it was not found correlation with trace elements bioavailability. In both soils mentioned above, amendments application did not modify significantly the EC (Table 2). However other authors have found increases in EC with the application of organic amendments[25]. On the other hand, when the incubation period was finished, it was observed that treatments with greater EC released minor amount of CO2-C (R2 = 0.74, p = 0.0028).

The CO2-C release pattern was generally similar in all treatments (Fig. 1). The final production of CO2-C accumulated in contaminated soils was lower than soils without contamination (p = 0.0001). The application of both compost to no contaminated soil, increased the CO2-C released since the second day of the incubation (p = 0.034). In general, it was observed that the biosolid compost application produced greater statistical significant released of CO2-C, respect to equine compost. In contaminated soils, amendments application began to increase CO2-C production since the day 17 of incubation (p = 0.0125). We did not observed statistically significant differences among both composts. These results indicated that at the end of incubation period, as much in the contaminated and no contaminated soil, the organic amendments application significant increased the CO2-C production (p = 0,0062 and p= 0,0005, respectively) and consequently the total activity of the microorganisms (Fig. 1). It has been reported that in contaminated soils with trace elements, microorganism biomass and activity increased with organic amendments application[24].

The used amendments showed lower levels of trace elements to the considered ones as toxics by different standards[19,20]. Their incorporation to soils did not modify initially their bioavailable levels neither in soils without contaminating nor contaminated. In fact, we did not find a significant relationship between each trace elements availability and CO2-C production. It is possible that these elements had addition effects.

In soils without contamination, Cu, Cr and Zn concentrations did not differ significantly with time (Fig. 2).

Fig. 1: Accumulated average CO2-C production (mg C g-1 soil) with incubation time. Vertical lines show the standard errors of the means

Fig. 2: Trace elements concentration in different treatments from the beginning (i) to the end (f) of the incubation test. Trace elements of treatments CS, CS+BC and CS+CE were multiplicity for 10. Vertical lines represent the standard errors of the means

Table 3: Contamination effects, compost application and incubation time. Here are presented the p values of the ANOVA and the contrast result

In general, the soil contaminated incubation diminished the trace elements bioavailability (Table 3). In the contaminated soil without amendment application this diminution could be produced by adsorption phenomena to the soil colloidal fraction, due to the greater electronegativity, or to the greater ionic strength of the trace elements[26].

Tordoff et al.[27] observed that mature compost with high proportion of humified organic matter and the increase of pH obtained with its incorporation to soils, diminished the bioavailability of trace elements in them[28]. The humified organic matter proportion from used compost, varied between 86.7% for the biosolid compost and 88.5% for the equine one, which made possible the mentioned process. Generally, at the end of the incubation, the amendments application in contaminated soils significantly increased the trace elements bioavailability with respect to soil without amendment (Table 3, Fig. 2). This was observed by Van Herwijnen et al.[29], attributing these results to the degradation in the time of organic matter contained into compost; reason why trace elements adsorbed in it was released to bioavailability forms. In our study we could realize the same reasoning due to the increase in microbial activity.

CONCLUSION

The total activity of microorganisms from contaminated soil increased by added amendments but it was always lower than no-contaminated soils. For this reason, we considered that the microorganism respiration is a sensible index of the soil quality. With incubation time, was observed diminution in the studied elements bioavailability. However with compost application it was not possible to demonstrate the same effect. We propose to continue with different long term studies, validating this methodology under field conditions; and complementing with microbial community studies.

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