ATTRIBUTES OF DYSTROPHIC LATOSOL IN SOUTHERN MINAS GERAIS AFTER APPLICATION OF SEWAGE SLUDGE

Endereço para correspondência: Ronaldo Luiz Mincato – Rua Gabriel Monteiro da Silva, 700 – Centro – 37130-000 – Alfenas (MG), Brasil – E-mail: ronaldo.mincato@unifal-mg.edu.br ABSTRACT In general, tropical soils present low natural fertility, which results in a great amount of degraded areas if it is associated with improper handling. Sewage sludge, due to its high levels of organic matter and nutrients, can improve soil properties. Thus, this study evaluated the conditioning and fertilizing effects of sewage sludge on the attributes of a typical dystrophic latosol in Southern Minas Gerais. The experiment evaluated the application of sludge from Santana wastewater treatment plant in Varginhas (Minas Gerais, Brazil) in pots, with concentrations equivalent to 60, 120, and 180 t ha-1, and a control treatment without sludge. The sewage sludge has increased the levels of organic matter, cation-exchange capacity, sum of the bases and nutrients, as well as increased porosity and decreased soil density, thus indicating the potential for conditioning and fertilizing agricultural soils.


INTRODUCTION
Solid residue management is one of the biggest challenges faced by Brazilian municipalities that fight environmental problems brought on from scarce investments on urban planning, public health, and environmental sanitation policies (RICCI et al., 2010).Sewage sludge, also called biosolid, is the residue obtained from wastewater treatment plants (WTP), and its final disposal may rise to 50% of the operational budget in a treatment system (PROSAB, 1999).
In Brazil, due to the increase in the number of WTP, the volume of sludge that demands a special final disposal has grown significantly.There has also been a significant increase in the number of WTP since the end of last century.It is in this scenery that sewage sludge disposal has become a relevant theme to the scientific community, considering the high potential of its application as a soil fertilizer in agriculture and in the recovery of downgraded soils (SUTHAR & SINGH, 2008).
Tropical soils are quite peculiar and characterized as an advanced stage of evolution, which show an adsorption complex made of low-activity clays (clays 1:1 and gibbsite), low in organic matter (OM) and with low cation exchange capacity (CEC).Therefore, they present low natural fertility (KLIEMANN et al., 2003).In addition, improper handling of the soils, adoption of improper agricultural and cattle-raising methods and techniques, among other actions, have resulted in fertility reduction, accelerated oxidation of the OM, and decrease of quantity and diversity of organisms in the soil (LEITE et al., 2010).To solve those problems, various studies have focused on the use of organic residues to improve soil properties (PIMENTEL et al., 2009;SANCHEZ-MO-NEDERO et al., 2004;SINGH & AGRAWAL, 2008).
Sewage sludge has high content of OM besides micro and macronutrients essential to plants, mainly N and P, improving the chemical, physical, and biological properties of the soils (CEZAR et al., 2012).Therefore, its use in agriculture is an appropriate alternative for final disposal, due to its low cost and for promoting the re-use of nutrients and reduction in the costs of chemical fertilizers.However, this option must follow sewage sludge characterization, because the application of sludge to the soil may imply risk of contamination of the natural ecosystems and of the trophic chain due to content of heavy metals (FJALLBORG et al., 2005), pathogens and toxic organic compounds, depending on wastewater properties of the sewage sludge produced in each WTP.
The tillage of agricultural soils has led to considerable increase in the content of heavy metals (BAIRD & CANN, 2011).Even so, most of the investigations carried out in the last few years show that the application of sewage sludge on soils provides benefits to agricultural production, with few environmental risks (KIDD et al., 2007).On the other hand, in order to use sludge safely, the Conselho Nacional do Meio Ambiente (CONAMA) 2006 guidelines numbers 375 and 380 established the criteria and procedures for using sewage sludge produced in WTPs in agriculture (CONAMA, 2006).
Therefore, the aim of this study was to characterize the effect of applying sewage sludge at different concentrations on physical and chemical attributes of dystrophic latosol in Southern Minas Gerais, considering its use as an agricultural fertilizer and soil conditioner.

MATERIALS AND METHODS
The experiment was conducted in the greenhouse at Universidade José do Rosário Vellano (UNIFENAS), in Alfenas (Minas Gerais state, Brazil), at an altitude of 880 m.Its climate is tropical mesothermic or tropical of altitude, based on Köppen's classification.Annual average temperature is around 21° to 23°C, and 1.500 mm of rainfall (ALFENAS, 2006).
For the experiment, a sample of clayey dystrophic latosol, dominant in Southern Minas Gerais, was collected from 0 to 20-cm depth at a point (21°25'S and 45°57'W) that had been manure-free for at least five years.Following collection, the soil was air-dried in the shade, broken, passed through a 4-mm-mash sieve, homogenized and, then, an aliquot was stored for routine chemical characterization following Raij et al. (2001).The results of analyses are shown in Table 1.
The sludge used in the experiment was produced at the Santana WTP in Varginha (Minas Gerais state), managed by Minas Gerais State Sanitation Company (COPASA).The samples representing the above WTP were collected, systematically, top to bottom, from six drying tanks full of sludge, in February 2013.Sludge samples were dried in an oven at 65°C for three days and then ground, quartered, and stored for chemical characterization.
In order to check if the sludge met Brazilian requirements, analyses of Cd, Pb, Ni, Cr, Cu, and Zn were carried out due to the toxic potential of these elements.Samples were dissolved in acqua regia (MELO & SILVA, 2008) and the metals were quantified by inductively coupled plasma optical emission spectrometry (ICP-OES).The validation of metal content was done by simultaneous analyses of the certified sample of domestic sewage sludge BCR 144-R from the Belgium Institute for Reference Materials and Measurements (IRMM).The values obtained were compared to the legal limits established by CONAMA Resolutions 375 and 380 (CONAMA, 2006).Besides metals, parameters of agricultural interest were analyzed and determined content of nutrients, OM and pH, in accordance with Raij et al. (2001).Wetness and volatile solids were quantified by mass loss at 105 and 500° C, respectively, and total solids and inorganic elements were obtained by difference.
The experiment was carried out at random with four treatments and four repetitions, using 6 dm³ polyethylene pots as experimental unities.Treatments with concentrations equivalent to 60, 120, and 180 t ha -¹ of sewage sludge and a control without sludge were studied.Starting from the initial soil saturation index by bases (V%), the limestone quantity was estimated to raise V% to 70%.The dry mixture of portions of soil, limestone, and sludge was done following the adopted treatments, then it was homogenized and transferred to the pots.Next, the treated soil was incubated for 30 days, with a wetness similar to the field capacity being kept.
After incubation, a sample of approximately 0.5 dm³ of soil was collected from each pot and a non-deformed 50 cm³ sample with volumetric rings.Chemical attributes analyzed were the OM content, CEC, Ca² + , Mg² + , K + exchangeable content, sum of bases (SB), P, and S (CAMARGO et al., 2009).In addition, the physical attributes "apparent density" by the volumetric ring method and "total porosity" were determined (EM-BRAPA, 1997).
The results obtained underwent variance analysis and first and second polynomial regressions by adopting the best-adjusted model.For the calculation, the statistical appliance SISVAR was used (FERREIRA, 2011), taking into consideration a 5% statistical confidence level of probability.

RESULTS AND DISCUSSION
One of the largest problems that may turn application of sewage sludge to the soil impracticable is the content of heavy metals in the residue composition.Some metals are essential elements to the plants, such as Cu, Zn, and N, called micronutrients.Others do not present a known function and are toxic, like Cd, Pb, and Cr.However, even micronutrients may be toxic to plants at high concentrations in the soil (BAIRD & CANN, 2011).
Comparison of values of the heavy metals analyzed with certified values (BCR 144-R) validated the accur-cy of the conducted studies and were from 3.6 to 150 times lower than the maximum limits allowed by specific legislation (CONAMA, 2006), as seen in Table 2.
Heavy metals found in the sewage sludge are the result of, mainly, clandestine disposal of industrial effluents in the domestic draining system.Cd comes from paints, fuels, fertilizers, and pesticides, whereas Ni is from batteries, hydrogenated greases, and welds.Cr is used in tanning and electroplating processes.Cu is applied in electric wire plants, metal surface treatment, and agrotoxic industries.

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At last, Zn is used in metallurgic industries (SILVA et al., 2001).Heavy metals can cause several problems to the food chain due to the phenomena of bioaccumulation and biomagnification (BAIRD & CANN, 2011).
Ni and Zn presented the highest content in the sludge samples, i.e. 188 and 185 mg dm³.Such values are, respectively, 3.6 and 15 times smaller than the maximum limits allowed for the elements by the legal standards (CONAMA, 2006).The content of Cd, Pb, Cr, and Cu in the sludge was also far short from regulations, not prohibiting therefore the use of sludge in agriculture.
The sludge was also analyzed as for characteristics of agricultural interest (Table 3).For agricultural purposes, sewage sludge is considered stable if the relation between volatile soluble substances and total solids is lower than 0.70 (CONAMA, 2006).In such case, sludge presented a rate of 0.68, which is suitable for agricultural use.
Nitrogen is the most important plant nutrient among organic fertilizers and the element that plants need in larger quantity.Sludge presented a 3.4% N content, which gives a great potential to supply this element to plant development.pH was near neutrality, which is ideal for soil use.It also presented a high content of OM, which makes it a choice for soil conditioning, improving its structure and the content of P, Ca, Mg, and S, despite low amounts of K, and it may favor soil fertility.
After incubation in the pots, sewage sludge provided an increased content of OM and CEC in the soil, which varied from 7.03 to 50.35 g dm -3 and 283.50 to 503.56 mmol c dm -3 , respectively (Figures 1A and B).
The increase in OM content is the most important benefit of the agricultural use of organic residues due to its contribution to the improvement of the chemical, physical, and biological properties of soils (BERTON & VALA-DARES, 1991).Due to the presence of negative charges on the surface, OM is able to retain cations such as Ca 2+ , Mg 2+ , and K + , as well as heavy metals.Thus, the application of sewage sludge, in addition to increasing OM content, also improves soil CEC (FERRER et al., 2011).In tropical and subtropical soils, negative charges of OM represent a large percentage of total CEC, as illustrated in Figure 1 and in the results obtained by Lima et al. (2011).

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Application of sludge also improves soil sorption complex in relation to K, Ca, Mg, and Na, thus increasing the SB of soils, especially when the residue is treated with limestone (FERRER et al., 2011).From the control to the treatment with the highest concentration of sludge, Ca increased linearly from 260.70 to 484.03 mmol c dm -3 (Figure 2A), and K from 1.41 to 4.42 mmol c dm -3 .Similarly, Mg improved from 10.75 to 17.5 mmol c dm -3 (Figure 2C) and the SB from 275.83 to 499.48 mmol c dm -3 .
Such results are in agreement with those obtained by Ricci et al. (2010), who observed the growth of K, Ca, and Mg content after application of up to 80 t ha -1 of the sewage sludge compound.The increase in content of those nutrients in the soil was continuous provided that the proportion of applied sludge was increased, which also resulted in the elevation of the SB.
As far as fertility is concerned, the evaluated sludge presented high P content (347.5 mmol c dm -3 ) (Figure 2E).
From the macronutrients required in larger quantities (N, P and K), P is the one that is requested in smallest quantities by plants, but it is the most used nutrient in fertilization in Brazil (RAIJ, 1991).This happens because plants do not absorb more than 10% of the applied P since acid tropical soils, rich in Fe and Al, retain P firmly in the soil (MALAVOLTA, 1989;SANTOS et al., 2008).Another point that must be considered is the scarcity of phosphate reserves to be feasibly explored in order to produce fertilizers.At current consump-tion rate, it is estimated that phosphate reserves are large enough for only 50 or 100 years of exploration (CORDELL et al., 2009).This fact makes the study of new sources of P for agriculture necessary.Variation in P content in response to the application of sludge was from 9.58 to 77.68 mg dm -3 , illustrating quadratic behavior (Figure 2E), which shows the high sludge potential in supplying this nutrient to plants.
The level of S in the soil also changed from 20.5 to 999 mg dm -3 by increasing sludge dosage (Figure 2F).This is due to the 713 mg dm -3 content of S in the residue (Table 3).According to Rheinheimer et al. (2005), soils with low OM content may present low S availability and limit plant development.Considering that the Brazilian soil, in general, presents low OM content, the addition of residues with high organic content as sewage sludge may also be important to supply S to plants.The origin of S in sewage sludge is attributed to the decomposition of proteins from human feces, to the presence of surfactants, and to S derived from the burning of fossil fuels (SÍGOLO & PINHEIRO, 2010).
Significant differences were observed among treatments and in the physical characteristics of apparent density and porosity of the soil.The results are in agreement with those of Aggelides & Londra (2000), for whom the application of sewage sludge supplies OM, which improves the particle aggregation condition, and, in turn, decreases density and increases soil porosity.In accordance with the regression equations, the decrease in density and the increase in porosity occurred linearly by making the dosage of applied sludge higher, which varied from 1.38 to 1.06 g cm-3 and 42.38 to 63.73%, respectively (Figures 3A and 3B).
The lowest density and highest porosity values provide the expectation of an increase in water infiltration in soil, and, consequently, less surface draining, which decreases erosion and offers more favorable conditions for the development of plants.) Total porosity (%)

Figure 1 -Figure 2 -
Figure 1 -Effect of the application of different sewage sludge dosages on soil CEC (A) and on organic matter (B).

Figure 3 -
Figure 3 -Effect of the application of different sewage sludge dosages on apparent density (A) and total porosity (B) of the soil.

Table 2 -Comparison of the content of heavy metals found in the sludge from Santana wastewater treatment plant in Varginha, and the limits established by the 2006 CONAMA Resolutions 375 and 380.
Average obtained from the six samples collected from the drying tanks. *