HEAVY METALS IN SOILS AND VEGETATION OF AN ATLANTIC FOREST IN NORTH OF RIO DE JANEIRO STATE, BRAZIL

Dora M. Villela; Cristina M^a M. Souza; Marcelo T. Nascimento; Luiz E. O. C. de Aragão; Cláudia F. Calasans; Luciano Guedes & Raquel T. S. Sousa (Laboratório de Ciências Ambientais, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, CEP: 28015-620, RJ, Brazil, cristal@cbb.uenf.br)

Abstract

The concentration of Al, Fe, Mn, Cr, Zn, Pb, Cd, Ni and Hg in soils and in leaves of three plant species were studied in two selected areas (unlogged and logged) of an atlantic forest in southern Brazil. Al and Mn concentrations in the soils were higher in the unlogged compared with the logged area, suggesting that weathering is an important process in their origin in this atlantic forest. On the opposite, Pb, Ni and Cd were lower in the unlogged area indicating that the atmospheric input is an important source for those soils. Only Hg and Cd concentrations in the soils presented high values when compared with the world average. The leaves of the three species did not show any consistent differentiation pattern between the two areas. Metrodorea brevifolia and Pachystroma longifolium showed lower uptake efficiency for Pb and Hg than Tillandsia usneiodes. The absence of any relation between the soils and leaves concentrations, associated with the unclear leaf concentration differences between the areas suggests that leaf metal uptake and retention in this forest is complex.

Introduction

There is a continuous exchange of metals between atmosphere, vegetation and soils. In general the main direction for volatile form is from air to plants. The metals are transferred to the vegetation by both wet and dry deposition. On the other hand, leaching or evaporation, depending on their physic-chemical characteristics may release absorbed metals. Elements like Hg and Pb have an important cycling in atmospheric compartment, since these metals have been used in volatile compounds or released by smelting operations, which result in atmospheric contamination. In consequence, the general problem arises of how to distinguish between natural geological weathering and metal enrichment attributable to human activities, mainly in rural areas, which are regarded as nonpoint sources as the metal supply originates from vast regional areas.

In Brazil, primary forests have been successively replaced by agriculture. The practices used in agricultural lands, may increment the heavy metal contents in the environment associated to chemical substances utilized in its activities. Burning the sugar-cane plantation during the dry season is common in the north of Rio de Janeiro State for more than a century. These practices should be able to spread chemical substances even in the surrounding ecosystems.

This study aims to evaluate the heavy metals concentrations in the soils and leaves of an atlantic forest fragment in Rio de Janeiro State and to test the hypothesis that the selective logging may interfere in the heavy metals concentration of soils and leaves in this forest.

Materials and Methods

The study area is a fragment of Tabuleiro forest (Mata do Carvão), a lowland type of Atlantic forest characteristic from the Brazilian coast, in north of Rio de Janeiro State (21⁰24’S, 41⁰04’W), Brazil. It is a semi-deciduous forest with an area of about 1000 ha, rounded by sugar-cane plantation. The mean annual precipitation at Campos dos Goytacazes town (60 km from Mata do Carvão) is 1023 mm, with a very marked dry season (may to august). The mean annual temperature is 24°C. The soil is Podzólico amarelo álico, according to Brazilian, with a sand/clay texture. Two areas were selected for the present study: unlogged area (UA), with no signs of recent logging in the last five years and high density of big trees; logged area (LA), with a lower density of big trees (Nascimento & Da Silva, 1997).

Four bulked surface soil (10 cm depth) samples were collected in each area with a plastic trowel and placed in plastic bags. The samples were oven dried (40⁰ C), grounded and sieved (2mm) for chemical analysis. About 20 mature leaves of three individuals of two tree species (M. brevifolia and P. longifolium) and green leaves of three individuals of one epiphyte species (T. usneiodes) were collected in each study area. The leaves were dried (40⁰ C) and powdered before chemical procedure. Soil and leaves were collected in the dry season (28/08/1997).

The extraction method for total metals was performed using 1g ± 0.001g of dried samples placed in Teflon bomb with acid mixture (HF/HNO₃ for soils and HNO₃/H₂O₂ for leaves) in an oven (110⁰C) during 12h (Krauseet al., 1995). For mercury, the samples were wet digested using an oxidant mixture (HCl/HNO₃, 3:1 for soils and H₂SO₄/HNO₃, 1:1 for leaves) and KMnO₄ 5% (Bastos et al., 1998). Analytical replicates were made for soil (N=2) and leaves (M. brevifolia and P. longifolium N=2; T. usneoides N=3). To certify the quality control, a certified plant sample was analyzed (Standard reference material 1515- NIST), in the same way that an internal soil sample supplied by Biophysics Institute in Rio de Janeiro (Radioisotope Laboratory) was performed. For all studied metals more than 80% was recovery, with variation coefficients for analytical replicates bellow than 10%. All metal determinations were performed by ICP - AES (Varian Liberty Series II), using a VGA accessory for Hg.

Results and Discussion

In general, the magnitude of metal concentrations at Mata do Carvão soils followed the sequence: Al > Fe > Mn > Cr > Zn > Pb > Cu > Ni > Cd > Hg (Table 1).

Table 1: Mean (N=4), variation coefficient (%) and ranges of heavy metals concentration (mg.g¹) in Mata do Carvão soils. The world average values are listed.

Metals	Soil Concentration (mg.g ^-1)
	Unlogged area	Logged area	world average*
Al	8265 ± 37 (3693 – 10215)	7480 ± 16 (5860 - 8748)	---------
Fe	4370 ± 30 (2732 – 5807)	4884 ± 19 (3931 - 6180)	38000
Mn	106 ± 67 (27.3 – 193)	53.3 ± 17 (43.6 - 63.9)	850
Cr	17.6 ± 21 (13.1 – 21.6)	20.7 ± 18 (17.2 - 25.8)	100
Zn	12.0 ± 31 (7.5 - 16.6)	11.1± 26 (9.0 - 15.3)	50
Cu	3.4 ± 17 ( 2.5 - 3.9)	3.5± 23 (2.7 - 4.5)	20
Pb	3.5 ± 57 (0.55 - 4.9)	6.0 ± 49 (3.4 - 8.8)	10
Ni	<DL – 2.8	5.2 ± 15 (4.4 - 6.1)	40
Cd	<DL - 0.11	0.2 ± 82 (0.06 - 0.4)	0.06
Hg	<DL - 0.06	<DL - 0.08	0.03

= detection limit of the instrumental analysis * Fórstner & Wittmann, 1984.

The concentrations of Fe, Cr, Zn, Cu and Hg were similar between the soils of the two areas. Al and Mn concentrations were higher in the UA compared with the LA soils, suggesting that weathering is an important process in their origin in this atlantic forest. On the opposite, Pb, Cd and mainly Ni were lower in the UA, indicating that, at least for Pb and Cd, the atmospheric input is an important source for those soils, what may be facilitated in the LA which has a more opened canopy. The variation coefficient showed a high soil environmental heterogeneity inside the areas, mainly in UA for Al, Fe and Mn and in LA for Pb and Cd. Only Hg and Cd (LA) concentrations presented high values when compared with the world average for soils, what may be due to chemical products used in the surrounding agricultural lands.

Table2: Mean (N=4), variation coefficient (%) and ranges of heavy metals concentration (mg.g¹) in leaves of M. brevifolia and P. longifolium at Mata do Carvão .

Leaf Concentration (mg.g ^-1)

Metals

Unlogged Area

Logged Area

M. brevifolia

P. longifolium

M. brevifolia

P. longifolium

825±54

(645-1231)

907±66

(576 –1968)

837±22

(523-1315)

790 ±25

(545 - 1006)

342 ±22

(231-456)

287±34

(218 –456)

356 ±10

(257-412)

295±14

(227 - 326)

76 ±16

(62-97)

82.2±17

(64.9 –103)

87 ±15

(66.1-110)

93± 16

(77.7 - 114)

<DL - 0.5

<DL - 0.16

<DL - 0.6

18.1±12

(14.5-27.9)

21.6 ±36

(15.1-28.5)

19.6±18

(17.3-26.1)

21.6±25

(14.8 - 26.4)

1.8 ±10

(1.4-2.6)

2.4 ±19

(2.1 - 3.2)

2.7±56

(2.1- 3.5)

3.1± 18

(2.1 - 3.9)

0.65±52

(0.32-0.91)

<DL - 1.16

0.41±29

(0.25-1.8)

1.3 ±65

(0.38 - 2.33)

<DL -2.7

<DL – 3.5

<DL - 2.5

<DL - 2.8)

<DL- 0.12

<DL- 0.15

<DL - 0.14

<DL - 0.12

0.03±50

(0.01-0.05)

0.03±15

(0.01 - 0.06)

0.04±39

(0.01- 0.1)

0.03±19

(0.01 - 0.05)

<DL= lower than detection limit of the instrumental analysis.

Most of the metal concentrations in the tree leaves were similar between both areas, except Mn and Cu which tended to be lower in UA and Pb which was higher in M. brevifolia and lower in P. longifolium leave of UA. The variability observed for most metals within the areas was quite high for the leaves of the two species in both selected areas. The concentrations obtained for M. brevifolia and P. longifolium showed no consistent differentiation pattern, being in general similar between the species leaves. In general, there was no relation between soil and leaves concentrations.

Higher concentrations of Al, Fe, Mn, Pb and Hg in UA were detected in T. usneoides. A similar pattern verified for soils was observed for Al and Mn, suggesting an internal source for these elements. Considering that most of the canopy species at Mata do Carvão loses their leaves in the dry season, Pb and Hg inputs in the UA is probably associated with agricultural activities developed around the areas, mainly sugar-cane plantations. However, the Hg concentrations is below the background values obtained for T. usneoides in another Rio de Janeiro State area (0.1-0.2 mg.g ^–1, Calasans, 1994). T. usneiodes showed higher uptake efficiency for Pb and Hg, than the other two leaf species, probably as consequence of atmospheric deposition. High metals concentrations in T. usneiodes have been described in the literature for different contaminated environments (Calasans & Malm, 1997).

The absence of a clear relation between the soil and tree leaf concentrations, associated with an inconsistent differentiation of leaf concentration pattern between the areas, suggest that the mechanisms involved in leaf metal uptake and retention in this forest is complex.

Table3: Mean (N=4), variation coefficient (%) and ranges of heavy metals concentration (mg.g¹) in T. usneiodes leaves at Mata do Carvão.

T. usneiodes

Metals (mg.g ^-1)

Unlogged Area

Logged Area

1170 ±88

(100 – 2573)

912 ±88

(215 - 2252)

1194± 41

(825 – 1661)

792 ±81

(54.5 - 1875)

271±65

(81.3 - 587)

112 ±70

(24.9 - 209)

<DL

<DL - 2.3)

11.3 ±12

(9.1 – 12.9)

9.1±44

(4.4 - 13.4)

<DL - 3.8

2.1 ± 63

(0.5 - 4.4)

2.7 ±42

(1.1 – 4.4)

<DL - 2.4

<DL - 1.8

<DL - 2.2

<DL - 0.16

<DL - 0.07

0.07 ±44

(0.02 – 0.12)

<DL - 0.06

<DL= lower than detection limit of the instrumental analysis

References

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Calasans C F, 1994. Utilização de Tillandsia usneoides (Bromeliaceae) como biomonitor da contaminação atmosférica por mercúrio. Master dissertation. IBCCF, Universidade Federal do Rio de Janeiro, Brazil.

Calasans CF, Malm O, 1997. Ciência e Cultura , 49(3): 213 – 216.

Förstner U Wittmann GTW, 1984. Metal Pollution in the Aquatic Environment. Springer Verlag, Berlin, Heidelberg, 486p.

Krause P, Erbslöh B, Niedergesäb R, Pepelnik R, Prange A, 1995. Fresenius Journal of Analytical chemistry, 353: 3 - 11.

Nascimento MT, Da Silva GC, 1997. Effects of selective logging on forest structure and tree diversity in a Brazilian Atlantic forest. Biodiversity in managed forests - concepts and solutions, Sweeden.