MOVEMENT OF METALS FROM SOIL TO PLANT ROOTS. R. A.

Youssef I and M. Chino 2. 1 -So i l and Water Use Dept.~ Nat iona l Research Centre~ Dokki, Egypt and 2- Agr ic . Chem. Department~ The U n i v e r s i t y of Tokyo~ Tokyo 113, Japan.

Abstract

The rhizobox system offers quantitative information concerning within the pH c~nges=~nd the__range of the rhizosphere. The movement of -~Zn~ U-Mn and Fe across the rhizosphere of soybean was examined, The contributions of the rhizosphere processes to heavy metals solubilization was evaluated. Changes in pH values around the roots of barley and soybean were investigated using the rhizobox. Results indicate that the extent to which the rhizosphere pH can differ from that of the bulk soil depends mainly on the plant species and initial bulk soil pH. Apparently~ soybean has greater ability than barley to solubilize heavy metals in the rhizosphere. The range of pN change was different from that of the heavy metals~ indicating that the solubility of heavy metals is not only influenced by soil pH.

I- INTRODUCTION

Many researchers have been interested in studying the characteristics of the rhizosphere in relation to bulk soil (Nye and Tinker~ 1978; 6reenland~ 1979; Classen and Jungk~ 1982; Barber~ 1984; Helal and Sauerbeck~ 1983; Youssef and Chino~ 1987; Youssef eta!., 1989),

Plant roots alter the rhizosphere and they are not merely a sink for the movement of water and nutrients. They also modify the chemistry and biology of the rhizosphere. Thus they may influence the level of metals released from the soil and the distribution between solution and adsorbed phases. These effects may vary between species and varieties of plants (Marschner and Romheld~ 1983). The behavior of heavy metals in the rhizosphere has received increasing attention (Loneragan~ 1975). So far~ however~ most of the prev ious s tud ies were focused on the d i s t r i b u t i o n of t o t a l contents of metals in the rh izosphere based p a r t i c u l a r l y on r a d i o i s o t o p e work (Barber and Ozanne, 1970). The s o l u b i l i t y of heavy metals across the rh izosphere has not been i n v e s t i g a t e d in d e t a i l .

2- MATERIALS AND METHODS

2~1 Experiment !: Changes in pH and solubilities of heavy metals in the rhizosphere.

The rhizoboxes were filled with a clay loam soil previously treated with fertilizer (0.5 g N as (NH4)2S04~ 0.5 g P205 as NaN2PO4~ and 0.5 g K28 as KCI for 3 kg of soil. Briefly~ the rhizobox system consists of several soil

Water, Air, and Soil Pollution 57-58: 249-258, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands.

250 R . A . YOUSSEF

compartments d i f f e r i n g in t h i ckness , and separated by 500- mesh nylon c l o t h . Soybean (G1ycine max L. v a t . Hawkeye) and b a r l e y (Hordeum vu lqa re L. va r . Dor i rumugi ) seed l i ngs were p l an ted in the c e n t r a l compartment (C.C.) as shown in F igu re 1.

Seeds were germinated and e i g h t of them were t r a n s p l a n t e d i n t o the C.C. of a r h i zobox . The p l a n t s grew in a growth chamber w i t h n a t u r a l l i g h t , a day/ n i g h t tempera tu re regime of 25/20C, and the water content of the s o i l was kept constant a t 50% of water ho ld i ng c a p a c i t y (Youssef and Chino, 1988).

P ~ o n I ~ ~ f C~trol

V ,/ SO --I y i SO ~ (ram)

Fig. 1. Diag~m sh~'ing a p~nt ~ro*v)o£ in lhe cent~] compart- ment of the rhizobox system. S ~ e ~ l soil comp~ments I m m thick arc s h o ~ (Y)

Figure 1. Diagram showing a plant in the central compartment of the rhizobox system. Several soil compartments 1 mm thick are shown (Y).

A f t e r 5 weeks of p l a n t growth, the seed l i ngs were harvested and the rh izobox was d ismant led so as t o peel away the s o i l from the i n t a c t nylon c l o t h . The s o i l f rom each compartment was a i r - d r i e d and analyzed. So i l pH was measured through water suspension ( r a t i o I : 2 . 5 ) . The s o l u b i l i t i e s of Cu and Fe t o ammonium bicarbonate-DTPA (So l tanpour , 1985) as w e l l as the s o l u b i l i t i e s of Zn and Mn t o 1 N ammonium a c e t a t e (V i e t s and L indsay, 1973) were eva lua ted . The c o n c e n t r a t i o n of the meta ls was es t imated by atomic abso rp t i on spec t ropho tomet ry .

2 .2 Experiment 2: Movement of 65Zn in the rh i zosphe re .

A s i m i l a r procedure as t h a t descr ibed in Exper iment. 1. was f o l l o w e d . A f t e r 25 days of p l a n t growth, the C.C. of each rh izobox was separated from o the r compartments. The C.C. in which soybean seed l i ngs were grown was used again f o r the ~ n s t r u c t i o n of a new rh izobox t o put r o o t s in con tac t w i t h

JZn added t o the s o i l of the new r h i z o b o x . In the new

MOVEMENT OF METALS TO PLANT ROOTS 251

rh i zobox , one comp~tment which was 2 mm apar t from the C.C. was l a b e l l e d w i t h - Zn. Tg= th is compartment c o n t a i n i n g 40 g of dry so i l ~ 400 uCi of °UZn a~5 ZnC12 was added and t h e compartment was designated as Zn compartment (Figure ~a . After 5 d the new rhizoboxes were dismantled. The segments of each soil compartment and the soybean ~oaves were analyzed

oJ for.=the total and soluble fraction of Zn. The solubility

~ . . . ~ - _ ~ of Zn to double ac ids which con ta in O.OuM of HCI and o . ~ M ~ H2SO 4) was evaluated (Mehl ich, 1978). The t o t a l amount of -UZn i n O.~=g of dry s o i l from each compartment as we l l as of so l ub l e °UZn was measured using an auto we l l gamma s c i n t i l l a t i o n counter .

SI~BEA -t~--- • 65zo 59, 54

A B C

Figure 2. Schematic presentation of the system for studying the movement of radioisotopes across the rhizosphere. A, B, and C are schematic diagrams of rhizobox system; 1 and 2 indicate the thickness of the soil comportments (I mm and 2 mm, respectively).

2°3 Experiment 3: Movement of 54Mn and 59Fe in the rhizosphere.

A similar procedure as that described in Experiment 2 (F ioure 2b and 2c)was fo l l owed w i th two d i f fe rence~A ( I ) lower ~LoL0ts of tr~ers were used, namely, 32 uCi of Mn as MnCi 2 or 40 uCi of Fe as FeCi 3 (2) the period of contact of the C.C. with radioisotope labelled co@partmen~ was longer ~x~ d). Hmount o+ total and soluble Mn and Fe in each soil compartment and in plant parts was measured by using an auto well gamma scintillation counter.

3- RESULTS

3~! Experiment !: Changes of pH and solubility of heavy

252 R.A. YOUSSEF

metals across the rh izosphere. Bar ley and soybean grown in the c lay loam s o i l wi th bulk

pH of about 7 decreased the rh izosphere pH by as much as 2 u n i t s (Figure 3>. In a d d i t i o n , the same f i g u r e shows the changes of pH by p l an t roo ts grown in sandy and c lay loam s o i l s w i th bulk pH of about 5.0. Roots increased the pH by as much as 2 u n i t s . S i m i l a r t rends were observed in the c lay loam s o i l . These r e s u l t s confirmed prev ious r e s u l t s by Youssef and Chino (1988, 1989a>. So lub le Cu. The d i s t r i b u t i o n of so lub le heavy metals across the rh izosphere of ba r l ey and soybean grown in the c lay loam s o i l shown in Figures 4 t o 7. In these f i g u r e s , the r e l a t i v e values of the s o l u b l e metals of each s o i l compartment are i nd i ca ted assuming t ha t the concent ra t ion of the bulk s o i l (5 mm or more apar t from the C.C.) was 100%. Figure 4 shows t ha t the s o l u b i l i t i e s of Cu in the rh izosphere of soybean and ba r l ey increased s l i g h t l y in the rh izosphere comparing to the bulk s o i l . There were minor d i f f e r e n c e s in the content of s o l u b l e Cu between p lan ts .

]~1

'T

C.C t 2 ~ +I )$+ r a m

Sa~%d - B~rley ~ , Clay loam ~ BarleM- ~----Cla~ loam ~- Bar~

Clny loam - S O ~

Figure 3. Changes of pH across the rh izosphere of ba r l ey and soybean in d i f f e r e n t s o i l s .

MOVEMENT OF METALS TO PLANT ROOTS 253

%

Sulk

140[ 120 100

Cu

BARLEY

I I I I I

C.C 1 2 3 4 !

nuu Di ~ta~c~_ ~ r o m ~oot pl ~n~

F igure 4. R e l a t i o n between the r e l a t i v e percentage of s o l u b l e Cu in the rh i zosphe re of b a r l e y and soybean in the c l a y loam s o i l . B i s the r e l a t i v e c o n c e n t r a t i o n of s o l u b l e metal t o t h a t of the bu lk s o i l (>5 mm) assuming the c o n c e n t r a t i o n of s o l u b l e metal in the bu lk s o i l i s 100%. C.C. c e n t r a l compartment (0 t o 2 mm).

Soluble Zn. Figure. 5 shows the content of soluble Zn toward the C.C. in rhizosphere soil of both plants. The solubility of Zn increased in the rhizosphere of plants. The ability of soybean roots to influence the soluble Zn was higher than that o~ barley. Soluble Fe. The content of soluble Fe increased toward the C.C. o~ barley and soybean (Figure 6). The percentages o~ increase in the C.C. ~or soybean and barley were 224 and 163 %, respectively. The zone of that increase around soybean r o o t s was much h igher and w ider than t h a t around b a r l e y r o o t s . So lub le Mn. F igu re 7 shows t h a t the s o l u b i l i t y of Mn increased across the rh i zosphe re by about 200% in e i t h e r C.C. of b a r l e y or soybean~ w h i l e a remarkable v a r i a t i o n was observed among o the r s o i l compartments.

254 R.A. YOUSSEF

%

Bulk

160

140

120

IO0

80

Zn

I I I I i I

C.C 1 2 3 4. 7 5 mm Distance + t o m r o o t p l m n m s

Figure 5. Re la t i on between the r e l a t i v e percentage of so lub le Zn and d is tance from p lan t roo ts in the c lay loam s o i l . See the no ta t i on of f i g u r e 4.

%

Bulk

24O

22O

2O0

180

160

14-0

120

100

80

N

i | i i i i

C.C 1 2 3 4- >S Inn l

Figure 6. Re la t i on between the r e l a t i v e percentage of so lub le Fe and the d is tance from p lan t r oo t s in the c lay loam. See the n o t a t i o n of f i g u r e 4.

MOVEMENT OF METALS TO PLANT ROOTS 255

°/o lButk

200

780

160

140

120

100

~n

B A R L E Y ~ "

I i i i I i

C.C 1 2 3 4- > s m m Distance from root olanem

Figure 7. Relation between the relative percentage of soluble Mn and the distance from plant roots in the clay loam soil. See the notation of figure 4.

3.2 Experiment 2: Movement of 65Zn across the rhizosphere.

Table I shows the distribution of soluble 65Zn as well as total in each compartment of the rhizobox of soybean.

Table I. Distribution of 65Zn across the rhizosphere of soybean.

Distance from roots (mm) 65Zn nCi g-1 dry soil

Soluble Total

0-2 2.0 3.0 2-3 6.0 49.0 3-4 75.0 257.0 4-5 a 472.0 1191.0 5-6 64.0 297.0 6-7 6.0 I0.0 >7 1.0 2.0

a This compartment was injected with 65Zn.

No d i s c e r n i b l e t r e n d was observed i n t h e movement of 65Zn.

256 R.A. YOUSSEF

However, it appears that some depletion occurred due to the si~gificant._incorporation of radioactivity to plant (12 nCi g - dw). Zn mov~ in both directions from the (4 to ~ mm)

compartment where Zn was added. The soluble fraction was detected to a.~distance of more 7 mm (bulk soil). The percentages of Zn which moved from the.~abelled compartment 6~ was about 50%. The soluble fraction of Zn was less than 50% of the total amount. 3.3 Experiment ~:~ Movement of 54Mn and 59Fe in the

rhizosphere. 54 59 Table II shows the distribution of Mn and Fe =~cross

the rhizosphere o~soybean. The results indicate that U~Mn is more mobile than Fe. The percentage of the Mn which moved from labelled_compartment (4 to 5 mm) was about 40%. However, the mosz o re remaineo in the labelled soil compartment. The soluble~fraction accounted ~wr about 8% of the total amount of U~Fe. The amount of U~Fe that moved from~_the labelled compartment was less than !% of the total U~e. Therefore, The plant paris contained small amounts of OTFe (Youssef and Chino, 1989) D.

M4 59 Table III shows the ~stribut~n of ~ Mn and Fe in

barley and soybean parts. Mn and Fe in soybean was higher than that of barley. However, there were remarkable variations between plant parts in both species.

Table II. Distribution of 54Mn and 59Fe across the rhizosphere of soybean.

Distance from roots 54Mn b 59Feb

(mm) Soluble Total Soluble Total

0-2 0.3 4.0 ND O. 1 2-3 O. 5 21.0 ND O. 2 3-4 2.8 142.0 0.4 3.0 4-5 (A) 9.9 724.0 80.0 991.O 5-6 5.2 153.0 ND 3.0 6-7 0.3 15.0 ND 0.2 >7 O. 04 1.0 ND O. 02

ND, radioactiv~y was Q~t detected. A~ This compartment was • ". .~ u ~ u 7 •

inDected wi_h Mn or Fe. b= nCi g dry soil.

Table III. Distribution of 54Mn and 59Fe in Barley and soybean parts.

plant parts 54Mn (A) 59Fe (A)

Barley Soybean Barley Soybean

Shoots +leaves 1.872 20.9 0.055 0.13 Roots 0.650 1.6 0.057 1.0

A= nCi g-1 dry matter

MOVEMENT OF METALS TO PLANT ROOTS 257

4- DISCUSSION

The rhizobox system which was developed by Youssef and Chino (1987, 1988) for studying the distribution of heavy metals across the rhizosphere, also offers a possibility to examine the effect of different species on the solubilities of Cu, Zn, Fe and Mn. The results indicate that plant species differ considerably in their ability to bring about pH changes (Marschner and Romheld, 1983) o These results confirmed the previous ones (Youssef and Chino, 1989a, b; 1990) and suggest that the extent to which the rhizosphere pH can differ from that of the bulk soil depends mainly on plant species and initial bulk soil pH. Soybean plants appear to be more effective than barley to decrease pH and increase solubility of heavy metals. It is interesting to note that the ability of soybean roots to solubilize Mn is not considerably higher than that of barley. E!gaia and Amberger (!988) stated that the ability of monocots to lower the pH of nutrient medium was lower than that of dicots, being in line with these findings.

Metals move in soil by mass-flow and diffusion (Barber, !984). The modification of the rhizosphere soil by the roots is likely to result in change in the rate of supply of metals to the roots. These modifications may cause changes in supply both by mass flow and diffusion~ The solubility of metals in soil solution is pH dependent~ Results indicate that other mechanisms should be involved to explain the behavior of metals in the rhizosphere.

ACKNOWLEDGMENTS

The authors wish to thank Prof. D. C. Adriano, Savannah River Ecology Laboratory, South Carolina, for his interest in and fruitful discussion on this work.

REFERENCES

Barber, S. Ao: 1984, Soil Nutrient Bioavaiiabiiitv~ John Wiley &Sons~ New York. pp. 259-296.

Barber, S. A. and Ozanne, P. G.: !970, Soil Sci. Soc~ Am. Proc. 34, 635.

Classen, N. and Jungk, A.: 1982, Zo P÷ianzenernaehr. Bodenkd., 145, 313.

Elgala A. M. and Amberger, A. :1988, J. Plant Nutr. ii, 677. Greenland, D. J.: 1979, ~The physics and Chemistry of

the Soil-root interface: Some comments~.in The Soil-Root interface, (ed). J. L. Harley and R. Scott Russel, pp. 83- 98, Academic Press, London.

He!a! N. M and Sauerbeck~ D. R.: 19~x Sni l Bioi n~ochem. 0 0 ~

Loneragan, J. F.: i975~ ~The availability of and absorption of trace elements in soil-plant systems and their relation to the movement and concentration of trace elements in plants', in Trace Elements in Soil- ~° ~ ~ : . . . . . . . . . . . and A . R _ flank--Animal Systems (~d~ n - i n Nichols . . . . .

258 R.A. YOUSSEF

Egan, Academic Press, Inc . , New York. pp.109-134. Marschner, H. and Romheld, V.: 1983, Z__~. Pflanzenernaehr.

Bodenkd., 149, 441. Meh!ich, A. : 1978, Comm. Soil Sci. Plant Anal. 9, 477. Nye, P. H. and Tinker , P. B.: 1978, Solute Movement in The

Soil-Root System. Blackwell, Oxford, London. pp.127-186. Sol tanpour, P.N.: 1985, Comm. Soil Sci. Plant Anal. 16, 323. Viers, F. G. Jr. and Lindsay, W. L.: 1973, ~ Testing

soils for Zinc, Copper, Manganese and Iron', In L.M. Walsh and J.D. Beaton (eds). Soil Testinq and plant analysis. Soil Sci. Soc. Am. Madison, Wis. pp. 153-172.

Youssef, R. A. and Chino, M.: 1987, J. Plant Nutr., I0, 1185. Youssef, R. A. and Chino, M.: 1988, Soil Sci. Plant Nutr.,

3_44, 461. Youssef, R. A. and Chino, M. : 1989 a Soil Sci. Plant Nutr.,

ou, 461. Youssef, R. A. and Chino, M. : 1989 b Soil Sci. Plant

Nutr. 35, 609. Youssef, R. A., Kanazawa, S. and Chino, M.: 1989, Biol.

Fertil. Soils. Z, 341. Youssef, R. A. Chino, M.: 1990, Effects of rhizosphere

processes on the solubilization of manganese as revealed with radioisotope techniques. In Plant Nutrition" Physioloqy and Applications. (ed) M. L. van Beusichem, Academic Publish. Dordrecht. pp. ~9-2~.~..