Supply and accumulation of metals in two Egyptian desert plant species growing onwadi-fill deposits

Supply and accumulation of metals in two Egyptian desert plant species growing onwadi-fill deposits

Journal of Arid Environments (1996) 32: 421–429 Supply and accumulation of metals in two Egyptian desert plant species growing on wadi-fill deposits ...

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Journal of Arid Environments (1996) 32: 421–429

Supply and accumulation of metals in two Egyptian desert plant species growing on wadi-fill deposits

M.A. Badri*, I.D. Pulford† & I. Springuel* *Faculty of Science, Aswan Campus, South Valley University, Aswan, Egypt †Agricultural, Food and Environmental Chemistry, University of Glasgow, Glasgow G12 8QQ, U.K. (Received 20 August 1993, accepted 2 October 1994) Sandy soils of the wadis in the Eastern Desert of Egypt have a poor retentive capacity for metals. The contents of Ca, Mg, K, Na, Fe, Al, Mn, Co, Ni, Cu and Zn in 37 soils from this area were highly variable. The metal contents of two dominant plant species of the Eastern Desert (Senna alexandrina and Cleome droserifolia) showed no correlation with soil metals. Metals accumulated in the leaves, with Cleome having overall higher contents of Fe, Al, Mn, Co, Ni, Na and Si than Senna, even though Cleome tended to grow in soil with low contents of these metals. ©1996 Academic Press Limited Keywords: indicator geobotany; desert; Egypt; arid climate; desert plants; soil

Introduction Plant life in the southern part of the Eastern Desert of Egypt is confined to the wadis (dry desert rivers) (Kassas & Girgis, 1970). The beds of the wadis are covered by alluvial deposits which are highly sandy in texture. There is, however, some variation in texture down the profile due to the laying down of successive layers of wadi-fill deposits. The physical and chemical properties of these deposits vary both between wadis and within a wadi, depending on the geology of the surrounding rocks and the topography (Springuel et al., 1986). While such deposits act as a soil in that they support plant growth, there has been little pedological development and the deposits retain many of the chemical characteristics of the parent rock. They are highly sandy in texture, have very low organic matter contents and have a very poor ability to hold nutrient cations in a form which is available for utilisation by plants. Macronutrient cations, such as Ca, Mg, K and, Na, may be present as salts which have accumulated due to the arid conditions; micronutrient cations are thought to be associated primarily with the resistant secondary hydrous oxides (Pulford et al., 1992). Plants growing in desert soils are known to accumulate certain elements (El Shazly et al., 1971), and this property is exploited by the use of some plants for medicinal purposes (Boulos, 1983). This property can also be used as the basis of biogeochemical prospecting to indicate 0140–1963/96/040421 + 09 $18.00/0

© 1996 Academic Press Limited

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the presence of high amounts of certain elements in the rocks (Brooks, 1983). This study relates the extractable macro- and microcation contents of wadi soils to the content of these elements in two dominant plant species. Materials and methods Soils were collected from two areas of the Eastern Desert: Wadi Allaqi, which is the major system draining to the west; and Wadi Gimal and Wadi Ghosun, which drain eastwards to the Red Sea (Fig. 1). The geology of Wadi Allaqi is predominantly sedimentary and volcanic rocks, while that of the Red Sea wadis is mainly igneous, especially granite, with some volcanic rock (Abu Al Izz, 1971). Two plant species were chosen; Senna alexandrina Mill. (syn. Cassia senna L.) and Cleome droserifolia (Forssk.) Del. Senna was dominant in Wadi Allaqi and Cleome dominant in the Red Sea area, although some stands in each area contained both species. Both are drought resistant perennials and were collected during their mature flowering and fruiting phases. They are both relatively shallow-rooting species (1–2 m), which is important in this type of study as the metals in the plant tissue must have been derived from the wadi-fill deposits. The soils were air-dried and sieved through a 2 mm sieve. The particle size distribution was measured by a pipette method and pH by combination electrode in a 1:2·5 soils:water suspension. Ca, Mg, K and Na were extracted by 1M ammonium acetate pH 7 by shaking 1 g soil with 50 ml of extractant on an end-over-end shaker for 3 h. The microcations were extracted by acid oxalate (0·175M ammonium oxalate/ 0·1M oxalic acid, pH 3·3) by shaking 2 g soil with 50 ml extractant on an end-over-end shaker for 3 h. In both cases the soil suspensions were filtered through Whatmans No. Mediterranean Sea

30°

EDFU

R. Nile

28°

Cairo

W. ABBAD W. Barramia

NILE Red Sea

26°

W. Shalt

W. Alam Red Sea

W. AL-Gimal

Kom Ombo W. ABU SUBEIRA

24°

Lake Nasser 32°

34°

W. AGAG

Abu Ghosun

.A W

22° 28° 30°

ASWAN

Aswan

36°

L-

W. Um Ashira

Abraq

yn

da

aw H

Lake Nasser

W. Qulleb

W. AL-ALLAQI

0

45 90 Km

Figure 1. The location of the study area in the southern part of Eastern Desert of Egypt.

SUPPLY AND UPTAKE OF METALS IN DESERT SOILS

423

1 paper, and the metals determined by atomic absorption or atomic emission spectrophotometry using a Perkin-Elmer 1100B spectrophotometer. Standard analytical conditions recommended by the manufacturer were used. In the case of measurement of Ca and Mg, chemical interferences were suppressed by addition of an excess of strontium chloride solution. The plant samples were well washed, first with tap water and then deionized water, divided into leaves, stems and roots and dried at 80 °C. The plant tissue (0·1 g) was digested in 10 ml Analar HCl. The digest was diluted slightly with water and filtered through a Whatmans No. 1 filter paper. Silica was trapped in the paper and measured gravimetrically after drying. The filtered acid digest was diluted to 25 ml with deionized water and the metals measured in the diluted digest by atomic absorption or emission.

Results and discussion Soil metals The location of the 37 samples, plant species present, textural properties and pH of the soils are shown in Table 1. The soils were predominantly sandy, with the combined coarse and fine sand fractions being greater than 80% in all but one of the soils. This, plus the obvious lack of humified organic matter, results in the soils having a poor ability to hold cations by ion exchange. All of the soils were of slightly alkaline pH (7·1–8·0), which tends to decrease the solubility of most cations. The two mechanisms by which it is most probable that cations are held in such soils are: as salts, especially sulphates and carbonates, which persist due to the arid climatic conditions; or associated with the resistant secondary hydrous oxides (in particular oxides of Al, Fe and Mn). The first mechanism would be most relevant to the macrocations, while microcations will be associated with the hydrous oxides. Table 2 shows the variation in amounts of Ca, Mg, K and Na in the soils extracted by ammonium acetate, and Fe, Al, Mn, Co, Ni, Cu and Zn extracted by acid oxalate. The data are shown for all 37 soils together, and for the Allaqi soils and Red Sea soils separately. Although the values vary over a wide range, the Allaqi soils had higher mean concentrations of Fe, Al, Mn, Co, Ni and Ca, the Red Sea soils were higher in K and Na, while the concentrations of Cu, Zn and Mg were about the same in the two locations. There are significant correlations between extractable Ca and Mg, and between extractable K and Na, suggesting that in each case the two cations are related; possibly because Ca and Mg are held in the soils by a similar mechanism, as are K and Na (Table 3). Of the microcations, Cu, Zn and Ni are significantly correlated with the oxalate-extractable iron fraction, and cobalt with the oxalate-extractable manganese. None of the metals were correlated with the aluminium oxide fraction (Table 4).

Plant metals Table 5 shows the maximum, minimum, mean and standard deviation of the mean for the metal contents of Senna alexandrina, and Table 6 the same values for Cleome droserifolia. The metal contents varied widely within each plant part. Analysis of variance shows that there were significant differences between the metal contents of plant parts for all metals measured, except K (Table 7). Examination of the figures in Tables 5 and 6 shows that for both species the leaves contained the highest concentration of metal, with there being less differences between the contents of the stems and roots. Table 7 also shows that there was a significant difference in the contents of Fe, Al, Mn, Co, Ni, Na and Si between the two species. Cleome droserifolia

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Table 1. Sample location, dominant plant species and soil textural and pH properties

Sample number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Location

Plant species

Clay (%)

Silt (%)

Fine sand (%)

Coarse sand (%)

pH

Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Allaqi Red Sea Red Sea Red Sea Red Sea Red Sea Red Sea Red Sea Red Sea

Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Senna Both Senna Senna Senna Senna Senna Senna Senna Senna Senna Both Senna Senna Cleome Cleome Both Cleome Cleome Cleome Cleome Cleome

4·2 2·9 4·0 3·2 4·7 3·5 2·2 2·4 2·8 6·1 5·5 2·4 2·4 1·2 2·3 3·7 3·3 3·3 16·6 1·9 3·4 4·7 6·4 3·9 4·6 3·6 0·3 0·3 1·2 2·3 0·8 19·3 9·7 12·2 6·8 4·5 9·9

2·5 0·2 1·2 2·4 1·9 0·5 2·0 1·4 4·7 9·1 2·0 4·7 10·4 2·8 6·2 2·9 2·7 5·2 2·5 6·5 0·3 0·2 4·2 0·2 0·2 7·3 10·0 10·0 8·8 0·9 9·8 0·0 13·4 0·0 5·9 0·0 0·0

49·8 1·8 6·1 5·1 11·5 7·5 1·5 1·3 1·2 1·6 4·4 13·3 42·7 17·9 18·0 6·4 1·3 18·8 38·5 4·9 8·1 11·3 1·3 5·7 7·6 17·3 49·3 49·3 20·6 4·5 4·0 5·3 2·3 8·9 21·1 16·1 14·8

43·5 95·0 88·6 89·2 81·8 88·5 94·2 94·9 91·3 83·2 88·1 79·6 44·5 78·0 73·3 86·9 92·6 72·7 42·3 86·8 88·1 83·7 88·0 90·1 87·6 71·9 40·4 40·4 69·4 92·3 85·4 75·4 74·6 79·0 66·2 69·3 75·3

7·8 7·6 7·8 7·4 7·4 8·0 7·2 7·4 7·2 7·2 7·8 7·2 7·5 7·6 7·6 7·6 7·4 7·2 7·2 7·4 7·4 7·6 7·2 7·6 7·2 7·1 7·2 7·2 7·1 7·2 7·1 7·2 7·2 7·2 7·2 7·2 7·4

contained significantly more of these metals than Senna alexandrina. These data suggest that the leaves of these desert plants may accumulate metals to some extent. This is particularly the case with Cleome for certain metals. Compared with ‘typical’ values for the metal contents of the dry matter of plant tissue (Mengel & Kirkby, 1978), both species have very high contents of Fe, Ni, Al, Co and Si in their leaves, while Cleome also has a very high amount of Mn. Contents of Ca, Mg, Cu and Zn were within or close to the typical values given by Mengel & Kirkby (1978). Badri & Springuel (1994) found that Cleome droserifolia accumulated higher amounts of metals

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Table 2. Concentration of extractable metals in soil samples (mg kg–1)

Ammonium acetate extractable

Acid oxalate extractable Fe

Al

Mn

Co

All soils (n=37) Max. 3875 347 Min. 41 50

319 35

6·5 1·1

Mean S.D.

127 62

107 56

2·6 1·0

Allaqi soils (n=30) Max. 3875 347 Min. 41 50

319 35

6·5 1·1

Mean S.D.

137 65

117 57

2·8 1·0

4·4 3·9

3·0 2·6

3·5 1·6

10263 5990

Red Sea soils (n=7) Max. 121 103 Min. 71 52

95 35

2·3 1·2

5·6 12 2·5 1·3

3·9 2·1

Mean S.D.

65 21

1·8 0·42

3·5 1·1

3·0 0·61

301 647

350 712

91 21

84 21

Ni

Cu

Zn

Ca

23 16 10 1·9 1·3 1·8 4·3 3·6

3·1 2·8

K

Na

1555 19

1023 228

78 30

128 255

378 184

23675 181 2950 38

386 19

498 228

78 33

86 86

320 71

8925 4300

97 70

1555 29

1023 323

6807 1539

79 5·7

211 553

627 299

23675 181 2950 38

3·4 1·5

9609 5584

23 16 10 1·9 1·4 1·8

3·2 3·9

Mg

Table 3. Correlation coefficients (r) between concentrations of extractable macrocations in soil (n=37)

Ca Mg K Na

0·84** 0·08 –0·22

Mg

K

– 0·19 –0·05

– – 0·62**

**Significant at p<0·001; others not significant.

Table 4. Correlation coefficients (r) between concentrations of extractable microcations in soils (n=37)

Al Fe Mn Cu Zn Ni Co

0·20 0·54** 0·20 0·28 0·17 0·37

Fe – 0·29 0·77** 0·87** 0·86** 0·37

**Significant at p<0·001; others not significant.

Mn – 0·32 0·21 0·36 0·56**

Cu

– 0·62** 0·72** 0·19

Zn

– 0·66** 0·20

Ni

– 0·51**

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Table 5. Metal contents of the leaves, stems and roots of Senna alexandrina

Metal Leaves Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na Stems Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na Roots Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na

Maximum (mg kg–1)

Minimum (mg kg–1)

Mean (mg kg–1)

S. D. (mg kg–1)

2850 2245 66 12 27 255 63 87690 45033 5824 33220 3624

7·5 7·1 2·2 4·4 4·0 3·4 17 5573 13731 1560 6443 742

644 683 41 8·6 11 39 41 30332 31693 4308 17100 1314

654 535 12 2·1 5·4 65 13 18856 8800 956 5973 648

490 389 106 7·6 13 119 289 30466 31893 5008 22801 1328

6·5 9·5 19 3·2 1·2 0·2 3·2 1110 7212 1059 7624 280

135 131 56 4·5 4·7 12 28 13787 15135 2907 13816 726

119 106 23 1·1 2·7 21 17 7810 5946 1026 3951 273

1012 879 55 7·7 8·2 25 55 39383 26902 4324 20634 3609

7·1 11 0·8 3·5 1·5 0·1 0·8 1680 5748 1124 4900 242

239 217 20 4·8 3·7 10 20 13682 16271 2835 9612 785

223 206 12 1·0 1·6 5·9 12 8748 4548 693 3540 661

than other desert species growing under the same conditions. Although these studies indicate elevated concentrations of some metals above what would be considered the normal range, these plants are not accumulators in the accepted sense. Brooks (1983) summarises the available information on such plants, which tend to be found growing on soils in mineralised areas and on soils derived from ultrabasic rocks, where substrate concentrations are also high. True accumulator species may contain metals in amounts measured in percentages of the dry weight, especially for elements such as Co, Ni, Cu and Zn, which were found here at about one-hundredth of that concentration.

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Table 6. Metal contents of the leaves, stems and roots of Cleome droserifolia

Metal Leaves Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na Stems Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na Roots Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na

Maximum (mg kg–1) 15364 9085 290 34 74 80 57 656850 55611 10337 18284 6399 1118 753 43 6·9 6·4 20 22 40680 24951 4249 25990 1753 3195 1717 74 11 12 19 94 192906 13808 3625 46557 2702

Minimum (mg kg–1)

Mean (mg kg–1)

S. D. (mg kg–1)

12 85 26 4·6 2·9 3·8 17 14980 11220 2594 4553 538

4874 4203 138 18 26 21 31 208024 33371 5697 7681 2619

4742 2389 70 8·8 19 22 11 182738 12573 2470 3945 1627

38 58 5·9 4·3 2·3 6·7 9·1 8586 5461 1409 11325 463

387 275 17 5·4 4·0 12 14 25218 11366 2119 17814 1019

355 229 14 1·0 1·5 5·7 4·5 9889 6755 979 4623 510

74 83 8·7 4·8 4·0 4·4 11 7664 5748 1457 8705 237

1032 647 29 6·9 6·6 11 32 55129 9050 2286 21002 916

1020 600 19 2·5 2·3 4·2 24 54396 2724 829 12673 830

There were no significant correlations between the extractable metal content of the soil and the metal contents of leaves, stems or roots for either species. This suggests either that the amount of metal in the plant did not reflect the soil content, or that the extractants used were not removing the soil pool of metal which was being exploited by the plants. Although Cleome contained very much higher amounts of Fe, Al and Mn than Senna, it was found growing predominantly in the Red Sea wadis, where the soils had lower mean contents of these elements compared with the soils of Wadi Allaqi

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Table 7. Analysis of variance applied to plant data using plant parts and plant species as factors

Metal

Plant partsa

Plant speciesa

Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na

0·000 0·000 0·007 0·000 0·000 0·004 0·000 0·000 0·000 0·000 0·2 0·000

0·000 0·000 0·01 0·000 0·000 0·5 0·5 0·000 0·1 0·5 0·2 0·000

a <0·05 significant; <0·01 highly significant.

Table 8. Metal contents of the leaves Senna alexandrina and Cleome droserifolia at the sites where both species were found

Site 17

Site 27

Metal

Senna Cleome (mg kg–1)

Senna Cleome (mg kg–1)

Fe Al Mn Co Ni Cu Zn Si Ca Mg K Na

20 12 303 86 19 26 6·7 4·6 14 2·9 3·8 22 34 31 30615 14980 30886 11220 4496 2875 21391 18284 1246 538

26 1879 19 1936 46 76 4·4 18 4·6 17 4·0 3·8 37 17 30385 125396 19950 27256 1560 2594 8845 4553 3085 943

Site 32 Senna Cleome (mg kg—1) 1031 1010 45 11 10 12 32 87690 44005 5693 20281 3624

1643 5093 137 16 32 18 32 223426 32295 7615 7351 3231

(Table 2). This reinforces the suggestion that Cleome has the ability to accumulate certain elements, especially in its leaves. The data for the three sites (17, 27 and 32) where Senna and Cleome were found growing together provide conflicting evidence of the higher metal uptake by Cleome (Table 8). While there is clear evidence of higher contents of Fe, Al, Mn, Co, Ni and Si in Cleome at sites 27 (Allaqi) and 32 (Red Sea), this was not the case at site 17 (Allaqi). This comparison is difficult to make, however, as these two species were not often found growing together. Further measurements on plants from such sites will be needed in order to confirm the higher metal content in Cleome compared with Senna. The results of this study, and of the previous work by Badri & Springuel (1994), do

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suggest that Cleome droserifolia may have the ability to take up and accumulate metals from desert soils. This work was supported financially by The United Nations Environment Programme and The British Council through the Allaqi Project at the Faculty of Science in Aswan.

References Abu Al Izz, M.S. (1971). Landforms of Egypt. Cairo: The American University Press. 281 pp. Badri, M. & Springuel, I. (1994). Biogeochemical prospecting in the South Eastern Desert of Egypt. Journal of Arid Environments, 28: 257–264. Boulos, L. (1983). Medicinal Plants of North Africa. Algonac, Michigan: Reference Publications Inc. 286 pp. Brooks, R.R. (1983). Biological Methods of Prospecting for Minerals. New York: Wiley Interscience. 313 pp. El Shazly, E.M., Barakat, N., Eissa, E.A., Emara, H.H., Ali, I.S., Shaltout, S. & Sharat, F.S. (1971). The use of Acacia trees in biogeochemical prospecting. Canadian Institute of Mineralogy and Metallurgy, Special volume 11: 426–434. Kassas, M. & Girgis, W.A. (1970). Plant life in the Nubian Desert east of the Nile, Egypt. Bulletin de l’Egypte, 51: 47–71. Mengel, K. & Kirkby, E.A. (1978). Principles of Plant Nutrition. Berne: International Potash Institute. 593 pp. Pulford, I.D., Murphy, K.J., Dickinson, G., Briggs, J.A. & Springuel, I. (1992). Ecological resources for conservation and development in Wadi Allaqi, Egypt. Botanical Journal of the Linnean Society, 108: 131–141. Springuel, I., Mekki, A.M. & Soghir, M. (1986). Vegetation of the upstream parts of the wadis in Southern Eastern Desert, Egypt. Aswan Science Technical Bulletin, 7: 99–120.