Plant and Soil 152: 225-235, 1993. © 1993 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 9856

Water-soluble organic matter in forest soils H. Interference with plant cation uptake

A.T. KUITERS 1 and W. MULDER Department of Ecology and Ecotoxicology, Faculty of Biology, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands; 1Present address: Institute for Forestry and Nature Research, Kemperbergerweg 67, P.O. Box 9201, 6800 HB Arnhem, The Netherlands

Received 18 November 1992. Accepted in revised form 21 April 1993

Key words: Agrostis capillaris, cation uptake, Silene dioica, soil equilibria, soluble organic matter

Abstract

Soil culture experiments were conducted to examine the effects of water-soluble organic matter (WSOM), isolated from the A n horizon of a forest soil on cation uptake by seedlings of Agrostis capillaris and Silene dioica. In contrast to the large effects on soil equilibria, cation uptake was only slightly affected by WSOM. Solubilized AI, Fe, Cu and Pb, were not found in higher amounts neither in the root nor in the shoot of the test species. Significant effects were only found for Cd, Mg, Mn and Zn, where uptake was either enhanced or decreased by WSOM. An experiment with several standard low molecular weight organic acids affirmed the results obtained with WSOM and supported the hypothesis that cations are taken up predominantly in the free ionic form and not as organic complexes. Results are discussed in relation to the effects often found for fulvic and humic acids.

Introduction

During the last two decades much research effort has been devoted to the interference of soil organic matter with plant cation uptake (Steven- son, 1982; Vaughan and Malcolm, 1985). Soil organic material mainly occurs in an insoluble form tightly bound to soil minerals and acting as a sink for cations due to its high cation exchange capacity. A small part occurs in a soluble form as water-soluble organic matter (WSOM) which largely contributes to the mobility of cations in the soil (Duchaufour, 1991) as was also demon- strated in the first paper in this series, thereby eventually affecting cation uptake by plants.

Most investigations studying the interference of soil organic matter with plant performance and nutrient uptake have been carried out with chemically derived fulvic (FA) and humic acids (HA). These substances have been found to

affect plant growth directly by affecting a variety of processes such as adventitious root formation, root respiration, protein synthesis, or indirectly by effects on ion uptake and translocation within the plant (Elgala et al., 1978; Linehan, 1977; Rauthan and Schnitzer, 1981; Tyler and McBride, 1982; Vaughan and Malcolm, 1985). Recently, hormone-like activity has been found for humic compounds thus facilitating nutrient uptake and transport throughout the plant (Nardi et al., 1989). FA and HA may also interfere with ATPase activity, responsible for energy transduction at the root membrane and necessary for active ion uptake (Maggioni et al., 1987).

WSOM-solutions may partly consist of FA-like components (Linehan and Shepherd, 1979; Vance and David, 1991), but will include also other components, so that complexing properties and behaviour of WSOM solutions in the soil

226 Kuiters and Mulder

may be partly different from that of FA. Com- pounds involved are polysaccharides, poly- phenols, and many low molecular-weight (MW) aliphatic and phenolic acids (Bruckert et Jac- quin, 1969; Kuiters and Denneman, 1987; Yavitt and Fahey, 1986). Furthermore, the structure of FA and HA is affected by the strong alkaline solvent during their extraction (Piccolo and MirabeUa, 1987), thereby changing their affinity towards cations

Therefore in this study, plant growth experi- ments were carried out with WSOM isolated from the humus layer (Ah) of forest soils. In the first paper in this series some characteristics of several WSOM solutions with respect to molecu- lar-size distribution and Cu(II)-complexing prop- erties and their impact on the (im)mobilization of cations from soil matrices were already de- scribed. Here we report the effects of WSOM and of several commonly occurring low MW aliphatic and phenolic complexants on the cation uptake by plants growing in soil culture.

Material and methods

The preparation of WSOM solutions from the humus layer of podzol soils under forest stands of Douglas-fir, European beech and Scots pine and their characterization with respect to molec- ular-size distribution and complexing properties, was already described in the first paper of this series. Several pot experiments were carried out under greenhouse conditions for an assessment of the effects of WSOM and organic acids on plant cation uptake.

First experiment

Pots of 600 mL were filled with 880 g loamy sand with a pH (CaCl2) of 5.6 and 0.8% Corg. Each pot received seven-day-old seedlings, three and two individuals per pot of Agrostis capillaris L. or Silene dioica L., respectively. After acclimati- zation for one week, pots were weekly watered with a 60 mL solution containing WSOM, iso- lated from the soil under Douglas-fir, in four concentrations equivalent to a weekly addition of 0, 25, 50 and 100/zg C g -1 soil. If necessary, demineralized water was added to keep the soil

moisture at 70% field capacity. Within a period of 30 days, WSOM solutions were added three times. After this period plants were harvested above and below the ground and analyzed for their cation content.

Second experiment

A next experiment was carried out with soils of different pH level to determine the pH depen- dency of WSOM effects on cation uptake. Sand with 0.7% Corg was used and the pH was adjusted to 4.5, 5.3 and 7.4 by addition of suitable amounts of CaCO 3. Soils were alternate- ly wetted and dried during a period of six weeks to stabilize pH. Pots of 600 mL were filled with 900g sand and planted with seven day old seedlings of A. capillaris, two seedlings per pot. WSOM, isolated from the soil under Scots pine, was weekly added in a volume of 60 mL per pot in four concentrations. The small quantity of WSOM available made that the concentrations chosen were somewhat lower relative to the first experiment with a highest concentration of 40/,tg C g 1 soil. Each treatment was replicated three times. Within a period of 30 days, WSOM solutions were added three times. After this period the plants were harvested above and below the ground and analyzed for cation con- tent.

Third experiment

In a subsequent experiment, some low MW aliphatic and phenolic acids, commonly found in soil solutions were applied to determine if similar effects were obtained when low MW complex- ants were used. It has been hypothesized that organic metal complexes can be absorbed and taken up by roots, a process becoming more important with decreasing size of the metal-or- ganic complex (Vaughan and Malcolm, 1985).

Pots of 600 mL were filled with a sandy soil of pH (CaC12) 4.5 with 4.5% Cot ~, 300g per pot. Each pot received three seven-day old seedlings of A. capillaris. After one week of acclimatiza- tion, treatments with the organic acid solutions started. Pots received weekly 40 mL-solutions containing either citric, oxalic, salicylic or protocatechuic acid, in a concentration equiva-

Soil organic matter and plant cation uptake 227

- 1 lent to an addition of 0, 0.5, 2.5 and 10/xmol g soil. Every treatment had five replicates. Within a period of 50 days, the organic acid solutions were added seven times. After this period the plants were harvested above and below the ground and analyzed for their cation content.

Growth conditions and chemical analyses

The experiments were conducted under con- trolled greenhouse conditions at 21-+ 4°C, 75-+ 10% relative humidity and a light/dark period of 16/8 h with a light intensity of 100/xE m-2s 1 At termination of the experiments, plants were harvested and root and shoot materials were separated. Shoot material was rinsed with de- mineralized water. Roots were thoroughly washed to remove adsorbed soil particles. This was followed by a treatment with 0.1 N HCI for 60s to remove cations adsorbed at the root surface and rinsing with demineralized water. Plant material was dried for 48 h at 60°C and dry weight was determined. Ground material was passed through a 2-ram sieve and a 100mg samples were digested overnight in a 10mL mixture of HNO 3 and HC1 (3 : 1, v/v) in a teflon cup. Cation concentrations were determined with flame atomic absorption spectrophotometry or with graphite-furnace AAS.

Statistical analyses were carried out on dry weight and cation contents of root and shoot material. Prior to one- or two-way analysis of variances, within group variances were subjected to Bartlett's test for homogeneity of variances (Sokal and Rohlf, 1981) and if necessary, data were log transformed.

Results

2000 r t A. capillads i S. dioica

1500t I

i ,ooo ia i i1t s0o = . = . _ i l l [ ] [ ] [ ] 1

0 25 50 100 0 25 50 100

WSOM (Ixg C g-1 soil)

Fig. i . Shoot and root biomass (nag) of A. capillaris and S. dioica after growth for 30 days on a soil weekly receiving amounts of WSOM (water-soluble organic matter) isolated from the A h horizon under a forest stand with Douglas-fir.

significant differences found for Zn, Mn, Cd and Na. With A. capillarb, the Zn uptake was decreased and Cd and Mn uptake was enhanced by WSOM. With S.dioica similar effects were observed for Cd and Mn. Zn uptake was in- creased at low WSOM concentrations but de- creased at higher concentrations. In both species Na uptake was increased as a direct result of the small amounts of Na present in the WSOM solutions. No evidence was found that this re- sulted in an enhanced competition with other cations at the root surface. The uptake of K was not affected, although its uptake is normally antagonistic with Na. There was a tendency for Ca and Mg to decrease in the shoot and to increase in the root, although differences were not statistically different. No effects were ob- served for A1, Fe and Cu. Overall, concentra- tions in the shoot were more affected than in the root.

WSOM and plant cation uptake WSOM activity and soil pH

Plant biomass was not affected by the WSOM treatment, although the roots of A. capillaris tended to become smaller with increasing con- centrations of WSOM (Fig. 1). The cation con- centrations in root and shoot are presented in Table 1 and probability values for statistical significance are given Table 2. Only small effects on cation uptake were observed, with statistically

Cation concentrations of A. capillaris after growth on soils of different pH and weekly receiving a certain amount of WSOM are pre- sented in Table 3 and 4 with probability values for statistical significance in Table 5. Plant biomass was largely affected by soil pH (Fig. 2), with highest biomass production at the inter- mediate pH level (5.3). Addition of WSOM

228 Kui ters and M u l d e r

Table 1. Cation concentrations in root and shoot (/xmol g 1) of A. capillaris and S. dioica after growth for 30 days on a soil weekly receiving different amounts of WSOM (water-soluble organic matter). Soil pH was 5.6

WSOM Na K Ca Mg AI Fe Zn Mn Cu Cd (/xg C g -~ )

Roots A. capillar~

0 96 371 40 69 116 54 7.8 0.9 1.1 0.55 25 157 447 47 74 115 62 7.4 0.8 1.2 0.50 50 216 416 52 78 101 51 7.3 1.3 1.2 0.67

100 255 452 57 83 111 60 6.0 1.8 1.2 0.78

S. dioica 0 77 308 79 117 66 20 3.8 1.0 0.38 0.08

25 227 359 81 144 50 17 4.4 1.3 0.35 0.14 50 363 318 66 148 59 17 5.6 1.8 0.40 0.18

100 307 313 60 141 57 17 3.7 1.9 0.41 0.17

Shoots A. capillaris

0 29 1600 262 200 6.2 1.8 8.2 3.2 0.27 0.04 25 48 1675 245 193 7.3 2.6 7.2 3.8 0.28 0.05 50 37 1213 162 106 4.7 2.1 5.4 4.8 0.29 0.07

100 57 1161 139 95 5.5 2.3 4.9 5.5 0.26 0.09

S. dioica 0 53 1478 336 208 6.5 2.2 1.6 1.1 0.25 0.02

25 151 1794 365 198 5.5 2.2 2.9 1.5 0.24 0.03 50 197 1718 329 235 6.6 2.7 3.2 2.2 0.27 0.04

100 190 1443 247 198 7.7 3.7 2.5 2.7 0.27 0.04

Table 2. Statistical significance values for changes in biomass and cation concentrations of root and shoot of A. capillaris and S. dioica after growth for 30 days on a soil weekly receiving WSOM in different amounts. Given are probability values (p) with levels of significance*p < 0.05, **p < 0.01, ***p < 0.001

Variate A. capillaris S. dioica

Roots Shoot Roots Shoot

Biomass 0.167 0.434 0.130 0.200 Na <0.001"** 0.006"* <0.001"** <0.001"* K 0.206 0.171 0.311 0.585 Ca 0.412 0,105 0.049" 0.491 Mg 0.791 0,087 0.057 0.935 A1 0.482 0.426 0.532 0.805 Fe 0.255 0.053 0.748 0.084 Zn 0.822 0.355 0.002"* 0.334 Mn 0.042" 0.002"* 0.002"* <0.001"** Cu 0.991 0.519 0.416 0.589 Cd 0.403 0.005** <0.001"** 0.235

In transformed: A capillaris, roots Mn; shoot Ca, Mg; S. dioica, shoot Na.

sl ightly inc reased b iomass p roduc t ion with

s tat is t ical ly significant va lues for shoot biomass.

A d d i t i o n of W S O M to the soils only slightly a f fec ted ca t ion up t ake with statistically lower

va lues found for Fe and Zn in the roots. Na was

i nc rea sed in the roots . No signigicant interac-

t ions were found b e t w e e n W S O M and pH. T h e

shoot ca t ion concen t ra t ions were no t significantly

af fec ted by W S O M . T h e effects of soil p H on

plant cat ion con ten t were large and ref lec ted the

large changes in cat ion solubil i ty, with soil pH.

T h e concen t ra t ions of Cd, Cu, M n and Zn in

Soil organic matter and plant cation uptake 229

Table 3. Cation concentrations in the roots of A. capillaris after growth on soils of different pH, weekly receiving amounts of WSOM during a period of 30 days

WSOM Roots (/x mol g- l) (/xg C g -1 )

Na Ca Mg AI Fe Zn Mn Cu Cd

Soil pH4.5 0 45 24 103 67 22 16 15 1.2 1.4

10 70 24 85 78 21 10 10 1.0 0.9 20 128 24 85 64 20 11 i0 1.0 0.9 40 112 25 100 73 20 i1 13 0.8 0.9

Soil pH 5.3 0 34 24 66 44 18 7.5 8.0 0.6 0.4

10 47 21 59 41 15 6.4 5.4 0.4 0.3 20 60 22 58 35 14 4.9 5.2 0.4 0,3 40 72 23 51 30 15 3.5 4.6 0.5 0.3

Soil pH 7. 4 0 40 44 81 79 22 1.1 0.9 0.3 0.05

10 61 32 85 61 17 1.1 0.7 0.2 0.05 20 51 36 73 53 15 0.8 0.7 0.2 0.05 40 59 42 72 55 17 0.9 1.0 0.3 0.05

Table 4. Cation concentrations in shoots of A. capillaris, after growth on soils of different pH, weekly receiving amounts of WSOM during a period of 30 days

WSOM Shoots (/x mol g- 1 ) (/xg C g 1)

Na Ca Mg AI Fe Zn Mn Cu Cd

Soil pH 4.5 0 5 57 90 9.3 2.1 5.0 11 0.17 0.06

10 5 56 87 8.6 2.3 4.5 11 0.17 0.06 20 3 55 90 6.3 3.3 4.3 12 0.22 0.05 40 7 48 86 5.9 2.1 4.6 13 0.17 0.05

Soil pH 5.3 0 23 105 86 2.2 1.6 4.9 11 0.14 0.03

10 14 101 86 3.0 1.9 5.8 10 0.16 0.05 20 20 91 79 2.5 1.7 4.9 8 0.13 0.05 40 29 81 76 2.7 1.7 3.4 9 0.13 0.03

Soil pH 7.4 0 8 199 60 4.2 2.0 1.7 0.9 0.13 0.005

10 15 193 66 3.8 2.1 1.6 1.3 0.19 0.005 20 25 204 70 4.3 2.0 1.9 1.1 0.14 0.004 40 51 207 67 4.0 2.0 2.0 1.2 0.18 0.004

roo t and shoot largely dec reased with an increase

in soil pH . AI, Fe and Mg had lowest values at

the i n t e r m e d i a t e p H level ( p H 5.3).

L o w M W organic acids and plant cation uptake

A d d i t i o n o f the low M W organic acids to the soil

had no ef fec t on the b iomass o f the A. capillaris seedl ings (Fig. 3), a l though salicylic acid t e n d e d

to inhibit roo t g rowth at the h igher concen-

t rat ions. The organic acids r educed the Mg and

Zn concen t ra t ions in roo t and shoot t issue

(Tables 6 and 7). Ca and Cd were d e c r e a s e d in

the shoots but not in the roots . F o r AI, Fe , Cu

and Pb no clear effects were obse rved . T h e

effects were mos t p r o n o u n c e d for citric acid and

weakes t for p ro toca t echu ic acid. Mn also t e n d e d

to decrease , with the excep t ion of p r o t o c a t e c h u i c

230 Kuiters and Mulder

Table 5. Statistical significance values of changes in biomass and cation concentrations of root and shoot of A. capillaris after growth on soils of different pH weekly receiving amounts of WSOM. Given are probability values (p) with levels of significance *p <0.05, **p <0.01, ***p < 0.001

Variate Roots Shoot

pH WSOM pHxWSOM pH W S O M pHxWSOM

Biomass <0.001"** 0.741 0.998 <0.001"** 0.048* 0.930 Na 0.019" 0.001"** 0.270 0.001"* 0.054 0.599 Ca <0.001"** 0.621 0.940 <0.001"** 0.101 0.415 Mg <0.001"** 0.498 0.729 <0.001"** 0.852 0.585 AI <0.001"** 0.127 0.571 <0.001"** 0.859 0.529 Fe 0.001"* 0.038* 0.861 0.017" 0.542 0.773 Zn <0.001"** 0.033* 0.119 <0.001"** 0.383 0.126 Mn <0.001"** 0.059 0.694 <0.001"** 0.667 0.498 Cu <0.001"** 0.194 0.701 0.010" 0.322 0.068 Cd <0.001"** 0.460 0.892 <0.001"** 0.685 0.627

All root variates were In transformed; shoot variates: biomass, AI, Ca, Cd, Fe, Mn, Na.

-g

i

• i pH 5.3 [lPH 7.4

li," I !

500 pH 4.5

400 .

300

200

lOO 0

100 "

200 0 10 20 40 0 10 20 40 0 10 20 40

WSOM (I.t 9 C g "1 soil) Fig• 2. Shoot and root biomass (mg) of A. capillaris after growth for 30 days on soils of different pH level, weekly receiving amounts of WSOM (water-soluble organic matter) isolated from the A h horizon under a forest stand with Scots pine.

acid where higher concentrations were found in both roots and shoots.

Discuss ion

Although the weekly addition of W S O M to the soils resulted into solubilization of several cat- ions in the soil solution, in particular of AI, Fe, Cu and Pb (see the first paper of this series), the effects on the cation uptake by plants were relatively small• In several of the experiments a lower uptake of Ca, Mg and Zn and a higher up take of Cd and Mn was observed in shoot and /o r root tissue. However , the uptake of A1 Fe, Cu and Pb was hardly affected. No obvious

differences in reaction were observed be tween the mono- and the dicotyledonous test species, although their strategies for cation uptake are partly different (Marschner, 1986)•

Cation uptake by plants is dependent on the concentration in the soil solution, its speciation and on the rate of replenishment f rom the solid phase during restoration of equilibrium condi- tions after uptake (Linehan and Shepherd, 1979). W S O M is known to affect all these vari- ables: a) by maintaining cations in a soluble state at higher concentrations than expected on basis of their solubility products; b) by lowering free cation concentrations in the equilibrium solution due to complexation; c) by mobilizing cations from the solid phase at a higher rate than by

Soil organic matter and plant cation uptake 231

600

400

200

0

200

400

600 0 .5 2.5 10 0 .5 2.5 10 0 .5 2.5 10 0 .5 2.5 10

citric acid oxalic acid salicylic acid protoc, acid

Organic acid (t.tmol g-1 soil)

Fig. 3. Shoot and root biomass (mg) of A. capillaris after growth for 50 days on a soil weekly receiving amounts of an organic acid.

Table 6. Cation concentrations in roots of A. capillaris after growing on a soil weekly receiving amounts of an organic acid during a period of 7 weeks. Soil pH was 4.5

Organic Roots (/xmolg a) acid (/x tool g 1) Ca Mg A1 Fe Zn Mn Cu Cd Pb

Citric acid 0 30 57 12 111 3.1 3.1 0.17 0.006 0.32 0.5 25 38 11 94 2.8 2.8 0.16 0.004 0.29 2.5 34 34 17 133 2.8 3.3 0.18 0.006 0.34

10 33 30 18 108 2.0 2.7 0.18 0.006 0.30

Oxalic acid 0 31 62 19 126 4.4 3.2 0.15 0.006 0.32 0.5 27 48 15 101 3.5 2.8 0.14 0.007 0.27 2.5 33 50 18 118 4.0 3.0 0.18 0.007 0.33

10 31 35 14 83 1.9 2.2 0.15 0.007 0.23

Salicylic acid 0 24 52 17 114 2.5 3.1 O. 16 0.006 0.28 0.5 27 44 18 123 2.8 3.3 O. 19 0.006 0.33 2.5 24 42 14 89 1.9 2.8 0.14 0.008 0.26

10 30 28 17 89 1.6 2.7 0.16 0.005 0.29

Protocatechuic acid 0 20 29 12 80 1.6 2.5 0.13 0.003 0.20 0.5 22 23 12 69 1.3 2.6 0.13 0.002 0.21 2.5 26 45 13 76 1.4 3.5 0.13 0.005 0.25

10 31 35 16 94 1.6 5.3 0.19 0.003 0.27

so lubi l iza t ion f rom (hydro)oxides , a process

which no rma l ly p roceeds very slowly; d) by

faci l i ta t ing thei r t ranspor t , especial ly at h igher

soil pH . F o r an exp lana t ion of the results ob-

t a ined in the po t expe r imen t s all these effects

should be t aken into cons idera t ion . T h e lack o f

any c lear effect o f W S O M on the up t ake o f A1,

Fe , Cu or Pb a l though solubi l ized f rom the soil

matr ix , can only be u n d e r s t o o d by assuming that

o rganic c o m p l e x e d cat ions are not t aken up by

232 Kui ters and M u l d e r

Table 7. Cation concentrations in shoots of A. capillaris after growing on a soil weekly receiving amounts of an organic acid during a period of 7 weeks

Organic Shoots (/lmol g 1) acid (/x tool g-~) Ca Mg A1 Fe Zn Mn Cu Cd Pb

Citric acid 0 106 120 0.89 4.3 4.8 6.3 0.089 0.017 0.010 0.5 112 111 0.61 2.9 4.2 5.1 0.085 0.015 0.006 2.5 97 122 0.87 3.4 3.3 5.8 0.076 0.014 0.006

10 69 93 1.26 2.5 3.6 4.4 0.073 0.011 0.005

Oxalic acid 0 113 118 1.16 2.6 3.8 5.0 0.085 0.011 0.005 0.5 119 110 2.09 3.4 4.2 6.1 0.077 0.012 0.010 2.5 109 128 0.81 1.8 4.9 5.1 0.097 0.014 0.005

10 73 108 0.99 4.3 3.4 4.2 0.073 0.010 0.009

Salicylic acid 0 129 124 0.85 2.0 3.5 6.3 0.076 0.016 0.005 0.5 126 133 0.88 1.8 3.1 7.7 0.075 0.012 0.004 2.5 125 118 0.79 1.6 3.0 6.0 0.086 0.009 0.004

10 82 98 1.46 2.4 2.5 4.8 0.062 0.008 0.004

Protocatechuic acid 0 130 122 0.88 1.8 3.4 7.5 0.071 0.013 0.006 0.5 117 132 0.67 1.3 3.2 5.8 0.073 0.009 0.005 2.5 105 124 0.33 0.6 3.0 7.9 0.078 0.008 0.007

10 88 119 0.49 2.8 2.7 15.3 0.068 0.013 0.007

plants. This is in contrast to the suggestion that humic substances can be taken up by plant roots and may thereby affect the uptake and transloca- tion of cations within the plant (Vaughan and Malcolm, 1985). Moreover , most organic anions of W S O M responsible for cation complexation and desorpt ion f rom the soil matrix will after complexat ion retain a net negative charge and are not bound substantially to the negatively charged root surface (Dunemann et al., 1991). Up take of cations will only proceed after dis- sociation of the organic complexes near or at the root surfaces. Therefore , the availability of these cations for uptake is not enhanced in the pres- ence of WSOM. This is even irrespective of the molecular size of the complexes as similar results were obtained for several low MW organic acids, where cation uptake was rather reduced than enhanced. In several other investigations with F A and H A as complexants , similar observations were done (Ernst et al., 1987; Linehan and Shepherd, 1979; Tyler and McBride, 1982).

Ca, Cd, Mg, Mn and Zn were the only cations where uptake was affected in the presence of

WSOM. As they have a low affinity for organic complexants (Martell and Smith, 1974), other processes than complexation must have prevailed here. The higher uptake of Cd observed in the first experiment is explained by an increased desorption f rom the soil matrix caused by Na present in small amounts in the W S O M solution. Indeed, with the low MW organic acids, where no Na was added in the t reatments , effects on Cd were absent. The higher uptake of Mn as was observed in the first exper iment after application of W S O M and for protocatechuic acid in the third experiment was presumably the result of an increase of divalent Mn in the soil solution, resulting f rom a redox reaction (see part I). This mechanism has also been repor ted after sewage sludge addition to soils (Hue , 1988).

The small effects observed with W S O M are in contrast to the large effect sometimes found when FA and H A are used in plant up take experiments. Other experimental conditions may largely explain this difference. Studies with FA and H A are mostly carried out in nutrient solutions (Levesque, 1970; Linehan 1977; 1978;

Soil organic matter and plant cation uptake 233

Table 8. Statistical significance values for changes in cation concentrations in roots and shoot of A. capillaris after growth on a soil weekly receiving amounts of an organic acid. Given are probability values (p) with levels of significance; *p<0.05, **p<0.01, ***p<0.001

Variate Source of variation

Acid Conc Acid x Conc

Ca Root 0.069 0.104 0.810 Shoot 0.041" <0.001"** 0.921

Mg Root <0.001"** <0.001"** 0.003** Shoot 0.230 0.019" 0.553

Al Root 0.036* 0.441 0.049* Shoot <0.001"** 0.092 0.022*

Fe Root 0.036* 0.585 0.366 Shoot 0.007** 0.154 0.358

Zn Root <0.001"** 0.007** 0.168 Shoot 0.004** 0.111 0.615

Mn Root 0.025" 0.373 <0.001'** Shoot <0.001"** 0.207 <0.001"**

Cu Root 0.102 0.528 0.107 Shoot 0.148 0.039* 0.664

Cd Root <0.001"** 0.001"* 0.196 Shoot 0.085 0.143 0.212

Pb Root 0.037* 0.813 0.438 Shoot 0.029* 0.681 0.125

AI, Cd and Pb concentration of the shoot were In trans- formed.

Mirave and Orioli, 1989; Van der Werff and Out, 1981), where under neutral pH conditions in the absence of any synthetic chelator the solubility of many cations is normally low. Introduction of a soluble complexant will under these conditions improve the availability of cations for uptake by plant roots as solubilization of hydroxides occurs at a much slower rate than the dissociation of

soluble organic complexes (Linehan and Shepherd, 1977). Under these conditions cation uptake is mostly enhanced in the presence of FA or HA. Reducing effects are found when cation solubility is high and complexation leads to lower free cation concentrations (Cabrera et al., 1988; Ernst et al., 1987; Gerzabek und Ullah, 1988; Van der Werff and Out, 1981). By carrying out the experiments in soil cultures, organic com- plexants are allowed to chemically interact with mineral and organic soil particles and results are different and often much less pronounced. Simi- lar observations were reported for synthetic chelating agents (Wallace, 1980).

Summarizing it is concluded that water-soluble organic matter, released from litter and humified layers by biological activity, enhances the solubility of those metals and trace metals that have a high affinity towards organic complex- ants, e.g. AI, Fe, Cu and Pb (see the first paper of this series). However effects on uptake by plants are under normal soil conditions negligible as cations are taken up predominantly as free ions. Uptake of cations from the soil solution will temporally disturb equilibrium conditions between free and complexed cations in the solution phase and species in the solid phase until a new equilibrium is reached and the presence of the complexing agents then becomes further irrelevant for the uptake of cations. Only in soils with a deficiency for certain cations it is likely that WSOM may slightly enhance plant cation uptake by increasing cation mobility. This has been observed for certain organic chelators released as root exudates which enhanced the mobility of Fe, Cu, Mn and Zn (Sinclair et al., 1990; Treeby et al., 1989).

The reducing potential of certain organic sub- stances seems more relevant with respect to plant cation availability especially of transition metals such as Mn and Fe. These cations can be reduced to their divalent state, thereby becoming less tightly bound to the soil matrix and being more easily taken up by plant roots as was found in the first and third experiment.

Interference of WSOM with plant cation up- take may also result from a change in membrane permeability of plant roots or by action on membrane ATPase activity. In this respect the surface active properties observed for humic

234 Kuiters and Mulder

compounds are of relevance (Yonebayshi and Hattori, 1987). This may result both into an increased efflux or a decrease in the uptake of ions as has been demonstrated by Maggioni et al. (1987) and Nardi et al. (1989). For certain phenolic components, similar effects have been found (Glass and Dunlop, 1984; Kuiters and Sarink, 1987). This aspect of WSOM activity should get more attention in future studies investigating the interference of WSOM with plant nutrient uptake.

Acknowledgements

This investigation was financially supported by the Netherlands Integrated Soil Research Pro- gramme, project nr. 8936. The authors like to thank prof dr W H O Ernst for critical reading of the manuscript.

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