HEAVY METALS IN MAIZE CROP FERTILIZED WITH ROCK PHOSPHATE AND BIOSOLID

This study aimed to evaluate effects of natural phosphate and sewage sludge compound fertilization in level of contents of heavy metals and nutrients in two consecutive cultivation of corn grain. The project was conducted in a Haplic Cambisol between April 10, 2009 and September 15, 2010. The treatments in factorial scale 2 x 4 meters corresponds to application on the first corn cultivation of 2 doses of rock Gafsa phosphate (0 and 90 kg ha-1 of P2O5) and 4 compound doses of sewage sludge (0, 25, 50 and 75 t ha-1, in dry specimen). The experiment followed random blocks design with three treatment repetitions. The fertilization with rock phosphate increased the levels of N, P, K and Mg only in the first corn grain cultivation and did not had influence on the levels of Zn, Cu, Ni, Pb, Cr and Cd in the two successive cultivations. The fertilization with sewage sludge compound increased the levels of N, P, K, Mg and Zn in the first corn grain cultivation and P and Zn for the second. In addition, the fertilization with sewage sludge did not had influence on levels of Ca, S, Cu, Ni and Pb in any corn cultivation. Independent of the effect of the treatments, only Pb showed concentration above of acceptable limits for nourishment.


INTRODUCTION
Increasing implantation of Wastewater Treatment Plant, attending to legal environmental recommendations for remediation of lakes and fountains and reduction of heath public problems, has been creating residues like sewage sludge (VON SPERLING, 2003).This is a relevant fact if counting the final disposal of this material, which can generate serious environmental harm becoming global concern (BIONDI & NASCIMENTO, 2005 & LEMAINSKI;SILVA, 2006a).Its inadequate destination can represent risks to human health and to the ecosystem due to toxic organic compounds (USMAN, 2012) and also to groundwater due to contamination by heavy metals (NASON, 2014).
Therefore, a solution is necessary to deposit this residue, and agricultural recycling is a promising solution, considering all the relevant points of its formation.In order to follow the CONAMA 375/2006 resolution Martins et al. (2003), Silva et al. (2005), Gomes et al. (2006) and Barbosa et al. (2007) point out the economy and environmental benefits of applying this material on the productive systems.The usage of the sewage sludge in a variety of crops is a worldwide trend (MARQUES et al., 2007), and this procedure stands out in Brazil during experiments with coffee (BETTIOL & CAMARGO, 2000), sugarcane (CHIBA et al., 2008a), eucalyptus crops (ANDRADE & MATTIAZZO, 2000), soybean (VIEIRA et al., 2005;LEMAINSKI;SILVA, 2006b), and corn (NASCIMENTO et al., 2004;TRANNIN et al., 2005;NOGUEIRA et al., 2007).
The exploitation of sewage sludge on agriculture also contributes to reduction of chemical fertilizers, to save natural resources and to reduce production cost.Sewage sludge is an important source of organic matter and essential elements to plants (LEMAINSKI & SILVA, 2006a), promoting physic and chemical improvements in the soil (MARQUES et al., 2002;TSUTIYA et al., 2002;SILVA et al., 2005).Also, sewage sludge and can also be used for silviculture, floriculture, landscaping or recovering degraded areas.
The amount of sewage sludge to be applied in the soil can be measured based on nitrogen levels present in the sludge and according to the requirements of the plant to be cultivated (ZUBA JUNIO et al., 2011), however, attention should be paid on levels of contaminants, such as persistent organic matter, microorganisms and heavy metals.In the long-term, the increase of soil metal concentration, resulting from successive application without adequate control, can contaminate the environment (NASCIMENTO et al., 2004;GOMES et al., 2006).The soil and plants contamination by heavy metals can restrict the usage of sewage sludge as a fertilizer (GOMES et al., 2006;NOGUEIRA et al., 2007;OLIVEIRA et al., 2009).Increased levels of heavy metals such as Cu, Pb and Zn due to sludge utilization on soils cultivated with corn were found by Galdos et al. (2004), Silva et al. (2006) andZuba Junio et al. (2011).Marques et al. (2007) also found increasing levels of heavy metals in sugar cane crops fertilized with sewage sludge.Nevertheless, Chiba et al. (2008b) used this same plant and did not find increase in Cu and Zn levels available on the soil, but rather levels below permitted by environmental law (policy).Several projects show that although fertilization with sewage sludge on corn crop permitted growth even in presence heavy metals, these remain within the levels established by law (RANGEL et al., 2004;GOMES et al., 2006;NOGUEIRA et al., 2007;ZUBA JUNIO et al., 2011).
According to CONAMA Resolution 375, corn culture is considered adequate for sewage sludge fertilization due to its characteristics and the fact that Brazil is one of the biggest global producer of corn, alongside China and the United States (RANUM et al., 2014).Sewage sludge usage in corn cultivation can serve as a satisfactory alternative in environmental and economic aspects.
The intense use of phosphate fertilizers in agriculture is justified by low availability of phosphorus in the soil, especially in Brazil, frequently limiting productivity of some crops (FREITAS et al., 2009).Furthermore, there is low efficiency on phosphorus absorption by crops and high fixation rate of this element in the majority of soils, increasing necessity of compost application containing this element in agriculture soils.These facts could lead to contamination since these fertilizers consist of a gateway for heavy metal introduction in the soil (CAMPOS et al., 2005;FREITAS et al., 2009).
As stated, this study has as main objective evaluate nutrient and heavy metals presence in corn grains fertilized with sewage sludge and reactive natural phosphate in two successive cultivation.

MATERIALS AND METHODS
The experiment was conducted in an experimental area of the Federal University of Minas Gerais in Montes Claros -MG; with latitude of 16º51'38" S and longitude 44º55'00" W. Haplic Cambisol composes the area with chemical and physical characteristics of layer 0-20 cm listed in Table 1, according to Embrapa's methodology (EMBRAPA, 1997).Corn (Zea mays) variety BR 106 was grown in two successive cultivations.
The treatments arranged in factorial 2 x 4 corresponded to 2 distinct doses of phosphate rock (0 and 90 ha -1 of P 2 O 5 ) combined with 4 different doses of sewage sludge (0, 25, 50 and 75 t ha -1 , in dry basis) with three repetitions following randomized block design.The Gafsa reactive phosphate rock used has the following chemical characteristics: P 2 O 5 total = 29.00%; P 2 O 5 soluble in citric acid at 2 % relation 1:100 = 10.00 %; P 2 O 5 soluble in formic acid at 2 %, relation 1:100 = 21.00 %; SO 3 = 3.20 %; SiO 2 = 3.6 %; Ca = 32.00%; MgO = 0.80 %; K 2 O = 0.11 % and heavy metals level presented on Table 2. Calculations to applied dose were based on level of available phosphorus on the soil and on recommendations made by Noce (2004) for corn BR 106 variety.Doses of sewage sludge were based on nitrogen concentration in this fertilizer and recommendations made by Noce (2004) for corn BR 106 (80 kg ha -1 of N).
The dehydrated sewage sludge was collected at a wastewater treatment plant located in Juramento -MG.The wastewater treatment plant is operated by COPASA-MG and has the capacity to treat 217 m³ of sludge per day.The treatment line is composed by a preliminary treatment and a UASB reactor, which is connected in series to a pond as an optional treatment.The sludge generated in a UASB reactor was dehydrated in a drying bed and subsequently disposed in a controlled landfill, which was implanted at the station area.The sewage sludge chemical characteristics are described in Table 2.
The composting was made by mixing of sewage sludge and bean straw in which presented characteristics described on Table 2. Mixing was made in order to achieve the C/N relation of 30/1 using three parts of bean straw for one of sludge and conducted by pallets approximately 1.5m high.Temperature and humidity were monitored daily.The systematic manual mixing of pallets using shovel and hoe is used to control participating factors on the process.
The fertilizer was applied using Gafsa phosphate rock and sewage sludge compound on furrows corresponding to each treatment.Row spacing for corn crop was 80 cm with crop of 5 seeds by linear meter (expected performance of fifty thousand of plants by hectare).The parcel size is equivalent to 6 x 4.8 m.The four central rows of 4 m in length were harvested whereas the two peripheral rows and 1 m at the end of the the borders were eliminated.
The second cultivation was made 30 days after the first harvest, manual weeding, and stubble deposition on the respective parcel and furrowing areas with hoes.Grooves were made on the planting lines for seed distribution during the first cultivation.The culture had been maintained cline throughout the cycle.The irrigation method used was by sprinkler irrigation system.After harvest, the grains were analyzed on levels of N, P, K, Ca, Mg, S, Cu, Zn, Ni, Cd, Cr, and Pb as methodology proposed by Tedesco et al. (1995).
The data collected were subjected to variance analysis, whereas the average for phosphate doses was tested by Tukey test with 5% probability.Sewage sludge doses average, adjusted to regression model, was tested by t-test with 10% probability.

RESULTS AND DISCUSSION
Analysis of variance revealed that there was no interaction between reactive phosphate rock and sewage sludge compound in relation to nutrient levels and heavy metals in corn grains as shown in Table 3.
According to Table 4, the application of phosphate rock increased the N levels in corn grains for the first cultivation.On the other hand, the same effect was not found for the second cultivation.Considering nitrogen absorption process by plants depends on energy from ATP (SOUZA & FERNANDES, 2006), the major P availability by application of phosphate rock could have contributed for bigger absorption of N, which caused increase on levels of this element in grains.Araújo and Machado (2006) state the existent synergism between N and P, and, thus, emphasize the importance of P on photosynthetic reactions and on C metabolism processes, which are fundamentals for N assimilation.--------------------------g kg -1 ----------------------------------------------------mg dm -3 ------------------------  In the same way as N, the P levels in corn grains were positively affected by phosphate rock application for the first cultivation (Table 4).However, phosphate absorption did not have effect on this element for the second cultivation.Machado et al. (2001) also reported increase on levels of P in corn grains in response to phosphate fertilizer.Thus, this highlights corn capacity to store P in grains given the bigger availability of the element in the soil.
The levels of K and Mg in corn grain also increased with phosphate rock application on the soil during the first cultivation (Table 4).
This increase of K and Mg in corn grains can be assigned to phosphate composition that present such elements.Although present on phosphate rock differences in levels of Ca and S on corn grain were not diagnosed for the first cultivation as well as K, Mg and S for the second cultivation.
In addition were not detected levels of Ca on the second cultivation.
In relation to heavy metals, the presence of Cd and Cr was not detected in corn grains.As for the other elements that had not been observed, there was an increase in Cu, Zn, Ni, and Pb levels in corn grains due to phosphate application in the first and second cultivation (Table 5).Although these elements have been detected in phosphate rock, literature also mentions, systematically, that levels of heavy metals increase when phosphate fertilizers are applied (CAMPOS et al., 2005;FREITAS et al., 2009).
Reduction of nutrient levels in corn grain was observed when comparing the first and second crop (Table 4 and 5), mainly on Ca and Zn minerals.Such fact may have occurred due to nutrient exportation in the first crop besides nutrient loss by leaching.Concerning Ni, there was an increase in concentration for both the first and second crop.As for Pb, the values were kept approximately at the same levels on both crops, highlighting significant residual effect of these elements on soil.
On Figure 1, N levels were observed to have increased in corn grains for the first crop with the increment sewage sludge, reaching the maximum value of 1,56 dag kg -1 with the biggest applied dose.The obtained values in this experiment were approximately in accordance to results found by Vyn & Tollenaar (1998).They found levels of N varying from 1.41 to 1.76 dag kg -1 using corn grains from 6 different varieties in 2 crop densities.Nitrogen is one of the major elements in sewage sludge (GOMES et al., 2006;RANGEL et al., 2006) and, since it is linked mainly with organic compound, its liberation is gradual.However, in this experiment, after one year of sewage sludge application on the soil, effects of the increase in doses on N levels could not be found in corn grain, being the average values 1.37 dag kg-1.
The application of rising doses of sewage sludge promoted increment on phosphorous levels in corn grains for the first crop (Figure 1), reaching maximum of 0.66 dag kg -1 with a dose of 75 t ha -1 and on second crop, reaching the maximum of 0.47 dag kg -1 with the same dose of sludge compound.One notices that because of nutrient exportation in the first crop, there was a reduction on the maximum concentration of P in the second crop.Even so, the values were below from those found by Vyn & Tollenaar (1998), which fluctuated between 0.25 and 0.30 dag kg -1 in grains for 6 different varieties of corn.
There was an increase of K and Mg levels, for the first crop, with the increment of the sewage sludge doses applied, reaching maximum of 0.66 dag kg -1 and 0,24 dag kg -1 , respectively, with the application of 75 t ha -1 sewage sludge.In the second cultivation, there was no influence found between the sewage sludge and K and Mg levels, being that K level was 0.44 dag kg -1 and Mg 0.16 dag kg -1 .In other words, these concentrations lower than the ones found in the first crop.These values of K and Mg are in accordance to USDA for corn grain, which are 0.29 dag kg -1 for potassium and 0.13 dag kg -1 for magnesium.To each variable, average followed by same vertical lower case did not differ statistically, up to 5% probability, by Tukey test.
The average values of Ca were: 0.22 dag kg -1 in the first crop and 0.0 dag kg -1 in the second crop.The first crop values were above the values presented by FAO for this element in corn grain, which is 0.05 dag kg -1 .For S, both in the first and in the second crop (0.11 dag kg -1 ), the values are close to the one cited by Mengel & Kirkby (1987), which is 0.17 dag kg -1 , considered ideal for corn.It is observed, therefore, sludge residual effect as for S levels and no residual effect as for Ca levels.
In Figure 2, sewage sludge application on the soil can be seen to result an increase of Zn levels in corn grains in broth crops.Applying 75 t ha -1 of sewage sludge, the levels of Zn in the grains were 40.50 and 27.31 mg kg -1 in first and second crops, respectively.In regards to this element, a residual effect was found when applying sludge compound on the soil.Even though there was an increase in Zn  et al. (2006), also found an increase in Zn values in corn grains with the increase of sewage sludge dose.
In regards to copper in corn grains (Figure 2), the levels of this mineral was not influenced with the application of sewage sludge, being the average values for the first and second crop 1.95 and 1.13 mg kg -1 , respectively.Furthermore, the values were not in the critical toxic level claimed by Sopper (1993).The results were closer to the ones found by Oliveira et al. (2005), in which, after 5 years of application of crescent doses of sewage sludge, did not find any anomaly on Cu levels in corn grains.Also, Rangel et al. (2006) studied heavy metals in corn grains fertilized for 3 consecutive years with sewage sludge.This experiment noted that during the first year the metals level stayed below the limits.Also, successive sewage sludge application was verified to reduce Cu levels in corn grains by the third year.
In relation to Ni, the increase in sewage sludge doses did not increase the level of this mineral in corn grains.The average values found for Ni were 0.76 and 3.67 mg kg -1 in the first and second crops, respectively.Rangel et al. (2006) found that the incorporation of 2 different sources of sewage sludge, which are capable to provide 8 times the Ni required for the crop during 3 consecutive years, has low influence on Ni levels in corn grains.This experiment found an increase in levels of this metal only in second crop and the Ni concentration in the sludge was higher from those allowed by CETESB (1999).In addition, Anjos & Matiazzo (2000) found Ni levels at 388 t ha -1 to be below the determined limit by the analytical applied method even with high doses of applied residual sludge.
The average levels of Pb (Figure 2) in grains Figure 2. Graphics showing the regression equations linking heavy metals levels in corn seeds with applied doses of sewage sludge were 22.84 and 24.08 mg kg -1 in first and second crops, respectively.These values are higher from those considered safe, 0.2 mg kg -1 in dry weight (USEPA 2012).However, the high Pb levels in corn grains are not related to sewage sludge application on the soil, once Pb has not answered to sewage sludge application.Also, treatments that did not receive the compound also reached high Pb levels in the grains.Rangel et al. (2006) found that, after three years applying crescent doses of sewage sludge on the soil, Pb levels in corn grains were higher than in the beginning of the experiment.Nonetheless, the levels stayed below the maximum limit established for grains.
As such, only Pb clearly extrapolated the tolerable concentration level for corn grains without being influenced from sewage sludge or rock phosphate application, but rather due to contaminated soil.The lack of sludge influence in metal levels in corn grains has also been reported by Martins et al. (2003), who applied doses up to 80 t ha -1 of sewage sludge.Divided in 2, 3, and 4 years or in a unique way, these authors reported that even with high concentrations of heavy metals in the soil due to sewage sludge addition, concentrations of these metals in corn grains were not influenced, being below the maximum allowed limits established by the Health Ministry for chemical contaminants in food (BRASIL, 1998).These authors related higher accumulation of metals in corn leaves, stalks and roots opposing to grains and corncob.

CONCLUSION
• Fertilization with rock phosphate increased the levels of N, P, K and Mg in corn grains only in the first crop.This fertilizer did not influence the levels of Zn, Cu, Ni, Pb, Cr and Cd, on both successive crops.

Figure 1 .
Figure 1.Graphics showing the regression equations linking nutrient levels in corn seeds with applied doses of sewage sludge

Table 3 .
Summary of the analysis of variance for nutrients and heavy metals in corn grain in soil fertilized with sewage sludge and reactive rock phosphate.
* Significant at 5% probability and ** Significant at 1% probability, respectively, by t-test ns Not Significant

Table 4 .
Nutrient levels in corn grains in function of fertilization with phosphate rock and sewage sludge compound in two successive cultivation To each variable, average followed by same vertical lower case did not differ statistically, up to 5% probability, by Tukey test.

Table 5 .
Heavy metals level in corn grain in function of fertilization with phosphate and sewage sludge compound in two successive cultivation